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Chapter 30 - Occupational Hygiene
Berenice I. Ferrari Goelzer
Work is essential for life, development and personal fulfilment. Unfortunately,
indispensable activities such as food production, extraction of raw materials,
manufacturing of goods, energy production and services involve processes, operations
and materials which can, to a greater or lesser extent, create hazards to the health of
workers and those in nearby communities, as well as to the general environment.
However, the generation and release of harmful agents in the work environment can
be prevented, through adequate hazard control interventions, which not only protect
workers’ health but also limit the damage to the environment often associated with
industrialization. If a harmful chemical is eliminated from a work process, it will
neither affect the workers nor go beyond, to pollute the environment.
The profession that aims specifically at the prevention and control of hazards arising
from work processes is occupational hygiene. The goals of occupational hygiene
include the protection and promotion of workers’ health, the protection of the
environment and contribution to a safe and sustainable development.
The need for occupational hygiene in the protection of workers’ health cannot be
overemphasized. Even when feasible, the diagnosis and the cure of an occupational
disease will not prevent further occurrences, if exposure to the aetiological agent does
not cease. So long as the unhealthy work environment remains unchanged, its
potential to impair health remains. Only the control of health hazards can break the
vicious circle illustrated in figure 30.1 .
Figure 30.1 Interactions between people and the environment
However, preventive action should start much earlier, not only before the
manifestation of any health impairment but even before exposure actually occurs. The
work environment should be under continuous surveillance so that hazardous agents
and factors can be detected and removed, or controlled, before they cause any ill
effects; this is the role of occupational hygiene.
Furthermore, occupational hygiene may also contribute to a safe and sustainable
development, that is “to ensure that (development) meets the needs of the present
without compromising the ability of the future generations to meet their own needs”
(World Commission on Environment and Development 1987). Meeting the needs of
the present world population without depleting or damaging the global resource base,
and without causing adverse health and environmental consequences, requires
knowledge and means to influence action (WHO 1992a); when related to work
processes this is closely related to occupational hygiene practice.
Occupational health requires a multidisciplinary approach and involves fundamental
disciplines, one of which is occupational hygiene, along with others which include
occupational medicine and nursing, ergonomics and work psychology. A schematic
representation of the scopes of action for occupational physicians and occupational
hygienists is presented in figure 30.2.
Figure 30.2 Scopes of action for occupational physicians and occupational hygienists
It is important that decision makers, managers and workers themselves, as well as all
occupational health professionals, understand the essential role that occupational
hygiene plays in the protection of workers’ health and of the environment, as well as
the need for specialized professionals in this field. The close link between
occupational and environmental health should also be kept in mind, since the
prevention of pollution from industrial sources, through the adequate handling and
disposal of hazardous effluents and waste, should be started at the workplace level.
(See “Evaluation of the work environment”).
Concepts and Definitions
Occupational hygiene
Occupational hygiene is the science of the anticipation, recognition, evaluation and
control of hazards arising in or from the workplace, and which could impair the health
and well-being of workers, also taking into account the possible impact on the
surrounding communities and the general environment.
Definitions of occupational hygiene may be presented in different ways; however,
they all have essentially the same meaning and aim at the same fundamental goal of
protecting and promoting the health and well-being of workers, as well as protecting
the general environment, through preventive actions in the workplace.
Occupational hygienist
An occupational hygienist is a professional able to:
anticipate the health hazards that may result from work processes, operations and
equipment, and accordingly advise on their planning and design
recognize and understand, in the work environment, the occurrence (real or potential) of
chemical, physical and biological agents and other stresses, and their interactions with
other factors, which may affect the health and well-being of workers
understand the possible routes of agent entry into the human body, and the effects that
such agents and other factors may have on health
assess workers’ exposure to potentially harmful agents and factors and to evaluate the
evaluate work processes and methods, from the point of view of the possible generation
and release/propagation of potentially harmful agents and other factors, with a view to
eliminating exposures, or reducing them to acceptable levels
design, recommend for adoption, and evaluate the effectiveness of control strategies,
alone or in collaboration with other professionals to ensure effective and economical
participate in overall risk analysis and management of an agent, process or workplace,
and contribute to the establishment of priorities for risk management
understand the legal framework for occupational hygiene practice in their own country
educate, train, inform and advise persons at all levels, in all aspects of hazard
work effectively in a multidisciplinary team involving other professionals
recognize agents and factors that may have environmental impact, and understand the
need to integrate occupational hygiene practice with environmental protection.
It should be kept in mind that a profession consists not only of a body of knowledge, but also of
a Code of Ethics; national occupational hygiene associations, as well as the International
Occupational Hygiene Association (IOHA), have their own Codes of Ethics (WHO 1992b).
Occupational hygiene is not yet universally recognized as a profession; however, in
many countries, framework legislation is emerging that will lead to its establishment.
Occupational hygiene technician
An occupational hygiene technician is “a person competent to carry out measurements
of the work environment” but not “to make the interpretations, judgements, and
recommendations required from an occupational hygienist”. The necessary level of
competence may be obtained in a comprehensive or limited field (WHO 1992b).
International Occupational Hygiene Association (IOHA)
IOHA was formally established, during a meeting in Montreal, on June 2, 1987. At
present IOHA has the participation of 19 national occupational hygiene associations,
with over nineteen thousand members from seventeen countries.
The primary objective of IOHA is to promote and develop occupational hygiene
throughout the world, at a high level of professional competence, through means that
include the exchange of information among organizations and individuals, the further
development of human resources and the promotion of a high standard of ethical
practice. IOHA activities include scientific meetings and publication of a newsletter.
Members of affiliated associations are automatically members of IOHA; it is also
possible to join as an individual member, for those in countries where there is not yet
a national association.
In addition to an accepted definition of occupational hygiene and of the role of the
occupational hygienist, there is need for the establishment of certification schemes to
ensure acceptable standards of occupational hygiene competence and practice.
Certification refers to a formal scheme based on procedures for establishing and
maintaining knowledge, skills and competence of professionals (Burdorf 1995).
IOHA has promoted a survey of existing national certification schemes (Burdorf
1995), together with recommendations for the promotion of international cooperation
in assuring the quality of professional occupational hygienists, which include the
· “the harmonization of standards on the competence and practice of professional
occupational hygienists”
· “the establishment of an international body of peers to review the quality of
existing certification schemes”.
Other suggestions in this report include items such as: “reciprocity” and “crossacceptance of national designations, ultimately aiming at an umbrella scheme with
one internationally accepted designation”.
The Practice of Occupational Hygiene
The classical steps in occupational hygiene practice are:
the recognition of the possible health hazards in the work environment
· the evaluation of hazards, which is the process of assessing exposure and reaching
conclusions as to the level of risk to human health
· prevention and control of hazards, which is the process of developing and
implementing strategies to eliminate, or reduce to acceptable levels, the occurrence of
harmful agents and factors in the workplace, while also accounting for environmental
The ideal approach to hazard prevention is “anticipated and integrated preventive
action”, which should include:
· occupational health and environmental impact assessments, prior to the design and
installation of any new workplace
· selection of the safest, least hazardous and least polluting technology (“cleaner
environmentally appropriate location
· proper design, with adequate layout and appropriate control technology, including
for the safe handling and disposal of the resulting effluents and waste
· elaboration of guidelines and regulations for training on the correct operation of
processes, including on safe work practices, maintenance and emergency procedures.
The importance of anticipating and preventing all types of environmental pollution
cannot be overemphasized. There is, fortunately, an increasing tendency to consider
new technologies from the point of view of the possible negative impacts and their
prevention, from the design and installation of the process to the handling of the
resulting effluents and waste, in the so-called cradle-to-grave approach.
Environmental disasters, which have occurred in both developed and developing
countries, could have been avoided by the application of appropriate control strategies
and emergency procedures in the workplace.
Economic aspects should be viewed in broader terms than the usual initial cost
consideration; more expensive options that offer good health and environmental
protection may prove to be more economical in the long run. The protection of
workers’ health and of the environment must start much earlier than it usually does.
Technical information and advice on occupational and environmental hygiene should
always be available to those designing new processes, machinery, equipment and
workplaces. Unfortunately such information is often made available much too late,
when the only solution is costly and difficult retrofitting, or worse, when
consequences have already been disastrous.
Recognition of hazards
Recognition of hazards is a fundamental step in the practice of occupational hygiene,
indispensable for the adequate planning of hazard evaluation and control strategies, as
well as for the establishment of priorities for action. For the adequate design of
control measures, it is also necessary to physically characterize contaminant sources
and contaminant propagation paths.
The recognition of hazards leads to the determination of:
which agents may be present and under which circumstances
· the nature and possible extent of associated adverse effects on health and wellbeing.
The identification of hazardous agents, their sources and the conditions of exposure
requires extensive knowledge and careful study of work processes and operations, raw
materials and chemicals used or generated, final products and eventual by-products, as
well as of possibilities for the accidental formation of chemicals, decomposition of
materials, combustion of fuels or the presence of impurities. The recognition of the
nature and potential magnitude of the biological effects that such agents may cause if
overexposure occurs, requires knowledge on and access to toxicological information.
International sources of information in this respect include International Programme
on Chemical Safety (IPCS), International Agency for Research on Cancer (IARC) and
International Register of Potentially Toxic Chemicals, United Nations Environment
Programme (UNEP-IRPTC).
Agents which pose health hazards in the work environment include airborne
contaminants; non-airborne chemicals; physical agents, such as heat and noise;
biological agents; ergonomic factors, such as inadequate lifting procedures and
working postures; and psychosocial stresses.
Occupational hygiene evaluations
Occupational hygiene evaluations are carried out to assess workers’ exposure, as well
as to provide information for the design, or to test the efficiency, of control measures.
Evaluation of workers’ exposure to occupational hazards, such as airborne
contaminants, physical and biological agents, is covered elsewhere in this chapter.
Nevertheless, some general considerations are provided here for a better
understanding of the field of occupational hygiene.
It is important to keep in mind that hazard evaluation is not an end in itself, but must
be considered as part of a much broader procedure that starts with the realization that
a certain agent, capable of causing health impairment, may be present in the work
environment, and concludes with the control of this agent so that it will be prevented
from causing harm. Hazard evaluation paves the way to, but does not replace, hazard
Exposure assessment
Exposure assessment aims at determining how much of an agent workers have been
exposed to, how often and for how long. Guidelines in this respect have been
established both at the national and international level—for example, EN 689,
prepared by the Comité Européen de Normalisation (European Committee for
Standardization) (CEN 1994).
In the evaluation of exposure to airborne contaminants, the most usual procedure is
the assessment of inhalation exposure, which requires the determination of the air
concentration of the agent to which workers are exposed (or, in the case of airborne
particles, the air concentration of the relevant fraction, e.g., the “respirable fraction”)
and the duration of the exposure. However, if routes other than inhalation contribute
appreciably to the uptake of a chemical, an erroneous judgement may be made by
looking only at the inhalation exposure. In such cases, total exposure has to be
assessed, and a very useful tool for this is biological monitoring.
The practice of occupational hygiene is concerned with three kinds of situations:
initial studies to assess workers’ exposure
follow-up monitoring/surveillance
exposure assessment for epidemiological studies.
A primary reason for determining whether there is overexposure to a hazardous agent
in the work environment, is to decide whether interventions are required. This often,
but not necessarily, means establishing whether there is compliance with an adopted
standard, which is usually expressed in terms of an occupational exposure limit. The
determination of the “worst exposure” situation may be enough to fulfil this purpose.
Indeed, if exposures are expected to be either very high or very low in relation to
accepted limit values, the accuracy and precision of quantitative evaluations can be
lower than when the exposures are expected to be closer to the limit values. In fact,
when hazards are obvious, it may be wiser to invest resources initially on controls and
to carry out more precise environmental evaluations after controls have been
Follow-up evaluations are often necessary, particularly if the need existed to install or
improve control measures or if changes in the processes or materials utilized were
foreseen. In these cases, quantitative evaluations have an important surveillance role
· evaluating the adequacy, testing the efficiency or disclosing possible failures in
the control systems
· detecting whether alterations in the processes, such as operating temperature, or in
the raw materials, have altered the exposure situation.
Whenever an occupational hygiene survey is carried out in connection with an
epidemiological study in order to obtain quantitative data on relationships between
exposure and health effects, the exposure must be characterized with a high level of
accuracy and precision. In this case, all exposure levels must be adequately
characterized, since it would not be enough, for example, to characterize only the
worst case exposure situation. It would be ideal, although difficult in practice, to
always keep precise and accurate exposure assessment records since there may be a
future need to have historical exposure data.
In order to ensure that evaluation data is representative of workers’ exposure, and that
resources are not wasted, an adequate sampling strategy, accounting for all possible
sources of variability, must be designed and followed. Sampling strategies, as well as
measurement techniques, are covered in “Evaluation of the work environment”.
Interpretation of results
The degree of uncertainty in the estimation of an exposure parameter, for example, the
true average concentration of an airborne contaminant, is determined through
statistical treatment of the results from measurements (e.g., sampling and analysis).
The level of confidence on the results will depend on the coefficient of variation of
the “measuring system” and on the number of measurements. Once there is an
acceptable confidence, the next step is to consider the health implications of the
exposure: what does it mean for the health of the exposed workers: now? in the near
future? in their working life? will there be an impact on future generations?
The evaluation process is only completed when results from measurements are
interpreted in view of data (sometimes referred to as “risk assessment data”) derived
from experimental toxicology, epidemiological and clinical studies and, in certain
cases, clinical trials. It should be clarified that the term risk assessment has been used
in connection with two types of assessments—the assessment of the nature and extent
of risk resulting from exposure to chemicals or other agents, in general, and the
assessment of risk for a particular worker or group of workers, in a specific workplace
In the practice of occupational hygiene, exposure assessment results are often
compared with adopted occupational exposure limits which are intended to provide
guidance for hazard evaluation and for setting target levels for control. Exposure in
excess of these limits requires immediate remedial action by the improvement of
existing control measures or implementation of new ones. In fact, preventive
interventions should be made at the “action level”, which varies with the country (e.g.,
one-half or one-fifth of the occupational exposure limit). A low action level is the best
assurance of avoiding future problems.
Comparison of exposure assessment results with occupational exposure limits is a
simplification, since, among other limitations, many factors which influence the
uptake of chemicals (e.g., individual susceptibilities, physical activity and body build)
are not accounted for by this procedure. Furthermore, in most workplaces there is
simultaneous exposure to many agents; hence a very important issue is that of
combined exposures and agent interactions, because the health consequences of
exposure to a certain agent alone may differ considerably from the consequences of
exposure to this same agent in combination with others, particularly if there is
synergism or potentiation of effects.
Measurements for control
Measurements with the purpose of investigating the presence of agents and the
patterns of exposure parameters in the work environment can be extremely useful for
the planning and design of control measures and work practices. The objectives of
such measurements include:
source identification and characterization
spotting of critical points in closed systems or enclosures (e.g., leaks)
determination of propagation paths in the work environment
comparison of different control interventions
· verification that respirable dust has settled together with the coarse visible dust,
when using water sprays
checking that contaminated air is not coming from an adjacent area.
Direct-reading instruments are extremely useful for control purposes, particularly
those which can be used for continuous sampling and reflect what is happening in real
time, thus disclosing exposure situations which might not otherwise be detected and
which need to be controlled. Examples of such instruments include: photo-ionization
detectors, infrared analysers, aerosol meters and detector tubes. When sampling to
obtain a picture of the behaviour of contaminants, from the source throughout the
work environment, accuracy and precision are not as critical as they would be for
exposure assessment.
Recent developments in this type of measurement for control purposes include
visualization techniques, one of which is the Picture Mix Exposure—PIMEX (Rosen
1993). This method combines a video image of the worker with a scale showing
airborne contaminant concentrations, which are continuously measured, at the
breathing zone, with a real-time monitoring instrument, thus making it possible to
visualize how the concentration varies while the task is performed. This provides an
excellent tool for comparing the relative efficacy of different control measures, such
as ventilation and work practices, thus contributing to better design.
Measurements are also needed to assess the efficiency of control measures. In this
case, source sampling or area sampling are convenient, alone or in addition to
personal sampling, for the assessment of workers’ exposure. In order to assure
validity, the locations for “before” and “after” sampling (or measurements) and the
techniques used should be the same, or equivalent, in sensitivity, accuracy and
Hazard prevention and control
The primary goal of occupational hygiene is the implementation of appropriate hazard
prevention and control measures in the work environment. Standards and regulations,
if not enforced, are meaningless for the protection of workers’ health, and
enforcement usually requires both monitoring and control strategies. The absence of
legally established standards should not be an obstacle to the implementation of the
necessary measures to prevent harmful exposures or control them to the lowest level
feasible. When serious hazards are obvious, control should be recommended, even
before quantitative evaluations are carried out. It may sometimes be necessary to
change the classical concept of “recognition-evaluation-control” to “recognitioncontrol-evaluation”, or even to “recognition-control”, if capabilities for evaluation of
hazards do not exist. Some examples of hazards in obvious need of action without the
necessity of prior environmental sampling are electroplating carried out in an
unventilated, small room, or using a jackhammer or sand-blasting equipment with no
environmental controls or protective equipment. For such recognized health hazards,
the immediate need is control, not quantitative evaluation.
Preventive action should in some way interrupt the chain by which the hazardous
agent—a chemical, dust, a source of energy—is transmitted from the source to the
worker. There are three major groups of control measures: engineering controls, work
practices and personal measures.
The most efficient hazard prevention approach is the application of engineering
control measures which prevent occupational exposures by managing the work
environment, thus decreasing the need for initiatives on the part of workers or
potentially exposed persons. Engineering measures usually require some process
modifications or mechanical structures, and involve technical measures that eliminate
or reduce the use, generation or release of hazardous agents at their source, or, when
source elimination is not possible, engineering measures should be designed to
prevent or reduce the spread of hazardous agents into the work environment by:
containing them
removing them immediately beyond the source
interfering with their propagation
reducing their concentration or intensity.
Control interventions which involve some modification of the source are the best
approach because the harmful agent can be eliminated or reduced in concentration or
intensity. Source reduction measures include substitution of materials,
substitution/modification of processes or equipment and better maintenance of
When source modifications are not feasible, or are not sufficient to attain the desired
level of control, then the release and dissemination of hazardous agents in the work
environment should be prevented by interrupting their transmission path through
measures such as isolation (e.g., closed systems, enclosures), local exhaust ventilation,
barriers and shields, isolation of workers.
Other measures aiming at reducing exposures in the work environment include
adequate workplace design, dilution or displacement ventilation, good housekeeping
and adequate storage. Labelling and warning signs can assist workers in safe work
practices. Monitoring and alarm systems may be required in a control programme.
Monitors for carbon monoxide around furnaces, for hydrogen sulphide in sewage
work, and for oxygen deficiency in closed spaces are some examples.
Work practices are an important part of control—for example, jobs in which a
worker’s work posture can affect exposure, such as whether a worker bends over his
or her work. The position of the worker may affect the conditions of exposure (e.g.,
breathing zone in relation to contaminant source, possibility of skin absorption).
Lastly, occupational exposure can be avoided or reduced by placing a protective
barrier on the worker, at the critical entry point for the harmful agent in question
(mouth, nose, skin, ear)—that is, the use of personal protective devices. It should be
pointed out that all other possibilities of control should be explored before considering
the use of personal protective equipment, as this is the least satisfactory means for
routine control of exposures, particularly to airborne contaminants.
Other personal preventive measures include education and training, personal hygiene
and limitation of exposure time.
Continuous evaluations, through environmental monitoring and health surveillance,
should be part of any hazard prevention and control strategy.
Appropriate control technology for the work environment must also encompass
measures for the prevention of environmental pollution (air, water, soil), including
adequate management of hazardous waste.
Although most of the control principles hereby mentioned apply to airborne
contaminants, many are also applicable to other types of hazards. For example, a
process can be modified to produce less air contaminants or to produce less noise or
less heat. An isolating barrier can isolate workers from a source of noise, heat or
Far too often prevention dwells on the most widely known measures, such as local
exhaust ventilation and personal protective equipment, without proper consideration
of other valuable control options, such as alternative cleaner technologies, substitution
of materials, modification of processes, and good work practices. It often happens that
work processes are regarded as unchangeable when, in reality, changes can be made
which effectively prevent or at least reduce the associated hazards.
Hazard prevention and control in the work environment requires knowledge and
ingenuity. Effective control does not necessarily require very costly and complicated
measures. In many cases, hazard control can be achieved through appropriate
technology, which can be as simple as a piece of impervious material between the
naked shoulder of a dock worker and a bag of toxic material that can be absorbed
through the skin. It can also consist of simple improvements such as placing a
movable barrier between an ultraviolet source and a worker, or training workers in
safe work practices.
Aspects to be considered when selecting appropriate control strategies and
technology, include the type of hazardous agent (nature, physical state, health effects,
routes of entry into the body), type of source(s), magnitude and conditions of
exposure, characteristics of the workplace and relative location of workstations.
The required skills and resources for the correct design, implementation, operation,
evaluation and maintenance of control systems must be ensured. Systems such as
local exhaust ventilation must be evaluated after installation and routinely checked
thereafter. Only regular monitoring and maintenance can ensure continued efficiency,
since even well-designed systems may lose their initial performance if neglected.
Control measures should be integrated into hazard prevention and control
programmes, with clear objectives and efficient management, involving
multidisciplinary teams made up of occupational hygienists and other occupational
health and safety staff, production engineers, management and workers. Programmes
must also include aspects such as hazard communication, education and training
covering safe work practices and emergency procedures.
Health promotion aspects should also be included, since the workplace is an ideal
setting for promoting healthy life-styles in general and for alerting as to the dangers of
hazardous non-occupational exposures caused, for example, by shooting without
adequate protection, or smoking.
The Links among Occupational Hygiene, Risk Assessment and Risk
Risk assessment
Risk assessment is a methodology that aims at characterizing the types of health
effects expected as a result of a certain exposure to a given agent, as well as providing
estimates on the probability of occurrence of these health effects, at different levels of
exposure. It is also used to characterize specific risk situations. It involves hazard
identification, the establishment of exposure-effect relationships, and exposure
assessment, leading to risk characterization.
The first step refers to the identification of an agent—for example, a chemical—as
causing a harmful health effect (e.g., cancer or systemic poisoning). The second step
establishes how much exposure causes how much of a given effect in how many of
the exposed persons. This knowledge is essential for the interpretation of exposure
assessment data.
Exposure assessment is part of risk assessment, both when obtaining data to
characterize a risk situation and when obtaining data for the establishment of
exposure-effect relationships from epidemiological studies. In the latter case, the
exposure that led to a certain occupational or environmentally caused effect has to be
accurately characterized to ensure the validity of the correlation.
Although risk assessment is fundamental to many decisions which are taken in the
practice of occupational hygiene, it has limited effect in protecting workers’ health,
unless translated into actual preventive action in the workplace.
Risk assessment is a dynamic process, as new knowledge often discloses harmful
effects of substances until then considered relatively harmless; therefore the
occupational hygienist must have, at all times, access to up-to-date toxicological
information. Another implication is that exposures should always be controlled to the
lowest feasible level.
Figure 30.3 is presented as an illustration of different elements of risk assessment.
Figure 30.3 Elements of risk assessment
Risk management in the work environment
It is not always feasible to eliminate all agents that pose occupational health risks
because some are inherent to work processes that are indispensable or desirable;
however, risks can and must be managed.
Risk assessment provides a basis for risk management. However, while risk
assessment is a scientific procedure, risk management is more pragmatic, involving
decisions and actions that aim at preventing, or reducing to acceptable levels, the
occurrence of agents which may pose hazards to the health of workers, surrounding
communities and the environment, also accounting for the socio-economic and public
health context.
Risk management takes place at different levels; decisions and actions taken at the
national level pave the way for the practice of risk management at the workplace
Risk management at the workplace level requires information and knowledge on:
· health hazards and their magnitude, identified and rated according to risk
assessment findings
legal requirements and standards
· technological feasibility, in terms of the available and applicable control
· economic aspects, such as the costs to design, implement, operate and maintain
control systems, and cost-benefit analysis (control costs versus financial benefits
incurred by controlling occupational and environment hazards)
human resources (available and required)
socio-economic and public health context
to serve as a basis for decisions which include:
establishment of a target for control
selection of adequate control strategies and technologies
· establishment of priorities for action in view of the risk situation, as well as of the
existing socio-economic and public health context (particularly important in
developing countries)
and which should lead to actions such as:
identification/search of financial and human resources (if not yet available)
· design of specific control measures, which should be appropriate for the
protection of workers’ health and of the environment, as well as safeguarding as much
as possible the natural resource base
· implementation of control measures, including provisions for adequate operation,
maintenance and emergency procedures
· establishment of a hazard prevention and control programme with adequate
management and including routine surveillance.
Traditionally, the profession responsible for most of these decisions and actions in the
workplace is occupational hygiene.
One key decision in risk management, that of acceptable risk (what effect can be
accepted, in what percentage of the working population, if any at all?), is usually, but
not always, taken at the national policy-making level and followed by the adoption of
occupational exposure limits and the promulgation of occupational health regulations
and standards. This leads to the establishment of targets for control, usually at the
workplace level by the occupational hygienist, who should have knowledge of the
legal requirements. However, it may happen that decisions on acceptable risk have to
be taken by the occupational hygienist at the workplace level—for example, in
situations when standards are not available or do not cover all potential exposures.
All these decisions and actions must be integrated into a realistic plan, which requires
multidisciplinary and multisectorial coordination and collaboration. Although risk
management involves pragmatic approaches, its efficiency should be scientifically
evaluated. Unfortunately risk management actions are, in most cases, a compromise
between what should be done to avoid any risk and the best which can be done in
practice, in view of financial and other limitations.
Risk management concerning the work environment and the general environment
should be well coordinated; not only are there overlapping areas, but, in most
situations, the success of one is interlinked with the success of the other.
Occupational Hygiene Programmes and Services
Political will and decision making at the national level will, directly or indirectly,
influence the establishment of occupational hygiene programmes or services, either at
the governmental or private level. It is beyond the scope of this article to provide
detailed models for all types of occupational hygiene programmes and services;
however, there are general principles that are applicable to many situations and may
contribute to their efficient implementation and operation.
A comprehensive occupational hygiene service should have the capability to carry out
adequate preliminary surveys, sampling, measurements and analysis for hazard
evaluation and for control purposes, and to recommend control measures, if not to
design them.
Key elements of a comprehensive occupational hygiene programme or service are
human and financial resources, facilities, equipment and information systems, well
organized and coordinated through careful planning, under efficient management, and
also involving quality assurance and continuous programme evaluation. Successful
occupational hygiene programmes require a policy basis and commitment from top
management. The procurement of financial resources is beyond the scope of this
Human resources
Adequate human resources constitute the main asset of any programme and should be
ensured as a priority. All staff should have clear job descriptions and responsibilities.
If needed, provisions for training and education should be made. The basic
requirements for occupational hygiene programmes include:
· occupational hygienists—in addition to general knowledge on the recognition,
evaluation and control of occupational hazards, occupational hygienists may be
specialized in specific areas, such as analytical chemistry or industrial ventilation; the
ideal situation is to have a team of well-trained professionals in the comprehensive
practice of occupational hygiene and in all required areas of expertise
laboratory personnel, chemists (depending on the extent of analytical work)
· technicians and assistants, for field surveys and for laboratories, as well as for
instrument maintenance and repairs
information specialists and administrative support.
One important aspect is professional competence, which must not only be achieved
but also maintained. Continuous education, in or outside the programme or service,
should cover, for example, legislation updates, new advances and techniques, and
gaps in knowledge. Participation in conferences, symposia and workshops also
contribute to the maintenance of competence.
Health and safety for staff
Health and safety should be ensured for all staff in field surveys, laboratories and
offices. Occupational hygienists may be exposed to serious hazards and should wear
the required personal protective equipment. Depending on the type of work,
immunization may be required. If rural work is involved, depending on the region,
provisions such as antidote for snake bites should be made. Laboratory safety is a
specialized field discussed elsewhere in this Encyclopaedia.
Occupational hazards in offices should not be overlooked—for example, work with
visual display units and sources of indoor pollution such as laser printers,
photocopying machines and air-conditioning systems. Ergonomic and psychosocial
factors should also be considered.
These include offices and meeting room(s), laboratories and equipment, information
systems and library. Facilities should be well designed, accounting for future needs, as
later moves and adaptations are usually more costly and time consuming.
Occupational hygiene laboratories and equipment
Occupational hygiene laboratories should have in principle the capability to carry out
qualitative and quantitative assessment of exposure to airborne contaminants
(chemicals and dusts), physical agents (noise, heat stress, radiation, illumination) and
biological agents. In the case of most biological agents, qualitative assessments are
enough to recommend controls, thus eliminating the need for the usually difficult
quantitative evaluations.
Although some direct-reading instruments for airborne contaminants may have
limitations for exposure assessment purposes, these are extremely useful for the
recognition of hazards and identification of their sources, the determination of peaks
in concentration, the gathering of data for control measures, and for checking on
controls such as ventilation systems. In connection with the latter, instruments to
check air velocity and static pressure are also needed.
One of the possible structures would comprise the following units:
field equipment (sampling, direct-reading)
analytical laboratory
particles laboratory
physical agents (noise, thermal environment, illumination and radiation)
workshop for maintenance and repairs of instrumentation.
Whenever selecting occupational hygiene equipment, in addition to performance
characteristics, practical aspects have to be considered in view of the expected
conditions of use—for example, available infrastructure, climate, location. These
aspects include portability, required source of energy, calibration and maintenance
requirements, and availability of the required expendable supplies.
Equipment should be purchased only if and when:
there is a real need
skills for the adequate operation, maintenance and repairs are available
· the complete procedure has been developed, since it is of no use, for example, to
purchase sampling pumps without a laboratory to analyse the samples (or an
agreement with an outside laboratory).
Calibration of all types of occupational hygiene measuring and sampling as well as
analytical equipment should be an integral part of any procedure, and the required
equipment should be available.
Maintenance and repairs are essential to prevent equipment from staying idle for long
periods of time, and should be ensured by manufacturers, either by direct assistance or
by providing training of staff.
If a completely new programme is being developed, only basic equipment should be
initially purchased, more items being added as the needs are established and
operational capabilities ensured. However, even before equipment and laboratories are
available and operational, much can be achieved by inspecting workplaces to
qualitatively assess health hazards, and by recommending control measures for
recognized hazards. Lack of capability to carry out quantitative exposure assessments
should never justify inaction concerning obviously hazardous exposures. This is
particularly true for situations where workplace hazards are uncontrolled and heavy
exposures are common.
This includes library (books, periodicals and other publications), databases (e.g. on
CD-ROM) and communications.
Whenever possible, personal computers and CD-ROM readers should be provided, as
well as connections to the INTERNET. There are ever-increasing possibilities for online networked public information servers (World Wide Web and GOPHER sites),
which provide access to a wealth of information sources relevant to workers’ health,
therefore fully justifying investment in computers and communications. Such systems
should include e-mail, which opens new horizons for communications and
discussions, either individually or as groups, thus facilitating and promoting exchange
of information throughout the world.
Timely and careful planning for the implementation, management and periodic
evaluation of a programme is essential to ensure that the objectives and goals are
achieved, while making the best use of the available resources.
Initially, the following information should be obtained and analysed:
nature and magnitude of prevailing hazards, in order to establish priorities
legal requirements (legislation, standards)
available resources
infrastructure and support services.
The planning and organization processes include:
· establishment of the purpose of the programme or service, definition of objectives
and the scope of the activities, in view of the expected demand and the available
allocation of resources
definition of the organizational structure
· profile of the required human resources and plans for their development (if
clear assignment of responsibilities to units, teams and individuals
design/adaptation of the facilities
selection of equipment
operational requirements
establishment of mechanisms for communication within and outside the service
Operational costs should not be underestimated, since lack of resources may seriously
hinder the continuity of a programme. Requirements which cannot be overlooked
· purchase of expendable supplies (including items such as filters, detector tubes,
charcoal tubes, reagents), spare parts for equipment, etc.
maintenance and repairs of equipment
transportation (vehicles, fuel, maintenance) and travel
information update.
Resources must be optimized through careful study of all elements which should be
considered as integral parts of a comprehensive service. A well-balanced allocation of
resources to the different units (field measurements, sampling, analytical laboratories,
etc.) and all the components (facilities and equipment, personnel, operational aspects)
is essential for a successful programme. Moreover, allocation of resources should
allow for flexibility, because occupational hygiene services may have to undergo
adaptations in order to respond to the real needs, which should be periodically
Communication, sharing and collaboration are key words for successful teamwork
and enhanced individual capabilities. Effective mechanisms for communication,
within and outside the programme, are needed to ensure the required multidisciplinary
approach for the protection and promotion of workers’ health. There should be close
interaction with other occupational health professionals, particularly occupational
physicians and nurses, ergonomists and work psychologists, as well as safety
professionals. At the workplace level, this should include workers, production
personnel and managers.
The implementation of successful programmes is a gradual process. Therefore, at the
planning stage, a realistic timetable should be prepared, according to well-established
priorities and in view of the available resources.
Management involves decision-making as to the goals to be achieved and actions
required to efficiently achieve these goals, with participation of all concerned, as well
as foreseeing and avoiding, or recognizing and solving, the problems which may
create obstacles to the completion of the required tasks. It should be kept in mind that
scientific knowledge is no assurance of the managerial competence required to run an
efficient programme.
The importance of implementing and enforcing correct procedures and quality
assurance cannot be overemphasized, since there is much difference between work
done and work well done. Moreover, the real objectives, not the intermediate steps,
should serve as a yardstick; the efficiency of an occupational hygiene programme
should be measured not by the number of surveys carried out, but rather by the
number of surveys that led to actual action to protect workers’ health.
Good management should be able to distinguish between what is impressive and what
is important; very detailed surveys involving sampling and analysis, yielding very
accurate and precise results, may be very impressive, but what is really important are
the decisions and actions that will be taken afterwards.
Quality assurance
The concept of quality assurance, involving quality control and proficiency testing,
refers primarily to activities which involve measurements. Although these concepts
have been more often considered in connection with analytical laboratories, their
scope has to be extended to also encompass sampling and measurements.
Whenever sampling and analysis are required, the complete procedure should be
considered as one, from the point of view of quality. Since no chain is stronger than
the weakest link, it is a waste of resources to use, for the different steps of a same
evaluation procedure, instruments and techniques of unequal levels of quality. The
accuracy and precision of a very good analytical balance cannot compensate for a
pump sampling at a wrong flowrate.
The performance of laboratories has to be checked so that the sources of errors can be
identified and corrected. There is need for a systematic approach in order to keep the
numerous details involved under control. It is important to establish quality assurance
programmes for occupational hygiene laboratories, and this refers both to internal
quality control and to external quality assessments (often called “proficiency testing”).
Concerning sampling, or measurements with direct-reading instruments (including for
measurement of physical agents), quality involves adequate and correct:
· preliminary studies including the identification of possible hazards and the factors
required for the design of the strategy
design of the sampling (or measurement) strategy
· selection and utilization of methodologies and equipment for sampling or
measurements, accounting both for the purpose of the investigation and for quality
performance of the procedures, including time monitoring
handling, transport and storage of samples (if the case).
Concerning the analytical laboratory, quality involves adequate and correct:
design and installation of the facilities
· selection and utilization of validated analytical methods (or, if necessary,
validation of analytical methods)
selection and installation of instrumentation
adequate supplies (reagents, reference samples, etc.).
For both, it is indispensable to have:
clear protocols, procedures and written instructions
routine calibration and maintenance of the equipment
training and motivation of the staff to adequately perform the required procedures
adequate management
internal quality control
external quality assessment or proficiency testing (if applicable).
Furthermore, it is essential to have a correct treatment of the obtained data and
interpretation of results, as well as accurate reporting and record keeping.
Laboratory accreditation, defined by CEN (EN 45001) as “formal recognition that a
testing laboratory is competent to carry out specific tests or specific types of tests” is a
very important control tool and should be promoted. It should cover both the sampling
and the analytical procedures.
Programme evaluation
The concept of quality must be applied to all steps of occupational hygiene practice,
from the recognition of hazards to the implementation of hazard prevention and
control programmes. With this in mind, occupational hygiene programmes and
services must be periodically and critically evaluated, aiming at continuous
Concluding Remarks
Occupational hygiene is essential for the protection of workers’ health and the
environment. Its practice involves many steps, which are interlinked and which have
no meaning by themselves but must be integrated into a comprehensive approach.
Linnéa Lillienberg
A workplace hazard can be defined as any condition that may adversely affect the
well-being or health of exposed persons. Recognition of hazards in any occupational
activity involves characterization of the workplace by identifying hazardous agents
and groups of workers potentially exposed to these hazards. The hazards might be of
chemical, biological or physical origin (see table 30.1). Some hazards in the work
environment are easy to recognize—for example, irritants, which have an immediate
irritating effect after skin exposure or inhalation. Others are not so easy to
recognize—for example, chemicals which are accidentally formed and have no
warning properties. Some agents like metals (e.g., lead, mercury, cadmium,
manganese), which may cause injury after several years of exposure, might be easy to
identify if you are aware of the risk. A toxic agent may not constitute a hazard at low
concentrations or if no one is exposed. Basic to the recognition of hazards are
identification of possible agents at the workplace, knowledge about health risks of
these agents and awareness of possible exposure situations.
Table 30.1 Hazards of chemical, biological and physical agents
Type of hazard Description
Chemicals enter the body
principally through inhalation, skin
absorption or ingestion. The toxic
effect might be acute, chronic or
Corrosive chemicals actually cause Concentrated acids and alkalis,
tissue destruction at the site of
contact. Skin, eyes and digestive
system are the most commonly
affected parts of the body.
Irritants cause inflammation of
tissues where they are deposited.
Skin irritants may cause reactions
like eczema or dermatitis. Severe
respiratory irritants might cause
Skin: acids, alkalis, solvents, oils
Respiratory: aldehydes, alkaline
dusts, ammonia, nitrogendioxide,
phosgene, chlorine, bromine, ozone
shortness of breath, inflammatory
responses and oedema.
Chemical allergens or sensitizers
can cause skin or respiratory
allergic reactions.
Skin: colophony (rosin),
formaldehyde, metals like
chromium or nickel, some organic
dyes, epoxy hardeners, turpentine
Respiratory: isocyanates, fibrereactive dyes, formaldehyde, many
tropical wood dusts, nickel
Asphyxiants exert their effects by
interfering with the oxygenation of
the tissues. Simple asphyxiants are
inert gases that dilute the available
atmospheric oxygen below the
level required to support life.
Oxygen-deficient atmospheres may
occur in tanks, holds of ships, silos
or mines. Oxygen concentration in
air should never be below 19.5%
by volume. Chemical asphyxiants
prevent oxygen transport and the
normal oxygenation of blood or
prevent normal oxygenation of
Simple asphyxiants: methane,
ethane, hydrogen, helium
Known human carcinogens are
chemicals that have been clearly
demonstrated to cause cancer in
humans. Probable human
carcinogens are chemicals that
have been clearly demonstrated to
cause cancer in animals or the
evidence is not definite in humans.
Soot and coal tars were the first
chemicals suspected to cause
Known: benzene (leukaemia);
vinyl chloride (liver angiosarcoma); 2-naphthylamine,
benzidine (bladder cancer);
asbestos (lung cancer,
mesothelioma); hardwood dust
(nasalor nasal sinus
Reproductive toxicants interfere
with reproductive or sexual
functioning of an individual.
Manganese, carbon disulphide,
monomethyl and ethyl ethers of
ethylene glycol, mercury
Chemical asphyxiants: carbon
monoxide, nitrobenzene,
hydrogencyanide, hydrogen
Probable: formaldehyde, carbon
tetrachloride, dichromates,
Developmental toxicants are agents Organic mercury compounds,
that may cause an adverse effect in carbon monoxide, lead,
offspring of exposed persons; for
thalidomide, solvents
example, birth defects.
Embryotoxic or foetotoxic
chemicals can cause spontaneous
abortions or miscarriages.
Systemic poisons are agents that
cause injury to particular organs or
body systems.
Brain: solvents, lead, mercury,
Peripheral nervous system: nhexane, lead, arsenic, carbon
Blood-forming system: benzene,
ethylene glycol ethers
Kidneys: cadmium, lead, mercury,
chlorinated hydrocarbons
Lungs: silica, asbestos, coal dust
BIOLOGICAL Biological hazards can be defined
as organic dusts originating from
different sources of biological
origin such as virus, bacteria, fungi,
proteins from animals or
substances from plants such as
degradation products of natural
fibres. The aetiological agent might
be derived from a viable organism
or from contaminants or constitute
a specific component in the dust.
Biological hazards are grouped into
infectious and non-infectious
agents. Non-infectious hazards can
be further divided into viable
organisms, biogenic toxins and
biogenic allergens.
Occupational diseases from
infectious agents are relatively
uncommon. Workers at risk
include employees at hospitals,
laboratory workers, farmers,
slaughterhouse workers,
veterinarians, zoo keepers and
cooks. Susceptibility is very
Hepatitis B, tuberculosis, anthrax,
brucella, tetanus, chlamydia
psittaci, salmonella
variable (e.g., persons treated with
immunodepressing drugs will have
a high sensitivity).
Viable organisms include fungi,
Byssinosis, “grain fever”,
organisms and spores and mycotoxins; biogenic
Legionnaire’s disease
biogenic toxins toxins include endotoxins, aflatoxin
and bacteria. The products of
bacterial and fungal metabolism are
complex and numerous and
affected by temperature, humidity
and kind of substrate on which they
grow. Chemically they might
consist of proteins, lipoproteins or
mucopolysaccharides. Examples
are Gram positive and Gram
negative bacteria and moulds.
Workers at risk include cotton mill
workers, hemp and flax workers,
sewage and sludge treatment
workers, grain silo workers.
Biogenic allergens include fungi,
animal-derived proteins, terpenes,
storage mites and enzymes. A
considerable part of the biogenic
allergens in agriculture comes from
proteins from animal skin, hair
from furs and protein from the
faecal material and urine. Allergens
might be found in many industrial
environments, such as fermentation
processes, drug production,
bakeries, paper production, wood
processing (saw mills, production,
manufacturing) as well as in biotechnology (enzyme and vaccine
production, tissue culture) and
spice production. In sensitized
persons, exposure to the allergic
agents may induce allergic
symptoms such as allergic rhinitis,
conjunctivitis or asthma. Allergic
alveolitis is characterized by acute
respiratory symptoms like cough,
chills, fever, headache and pain in
Occupational asthma: wool, furs,
wheat grain, flour, red cedar, garlic
Allergic alveolitis: farmer’s
disease, bagassosis, “bird fancier’s
disease”, humidifier fever,
the muscles, which might lead to
chronic lung fibrosis.
Noise is considered as any
Foundries, woodworking, textile
unwanted sound that may adversely mills, metalworking
affect the health and well-being of
individuals or populations. Aspects
of noise hazards include total
energy of the sound, frequency
distribution, duration of exposure
and impulsive noise. Hearing
acuity is generally affected first
with a loss or dip at 4000 Hz
followed by losses in the frequency
range from 2000 to 6000 Hz. Noise
might result in acute effects like
communication problems,
decreased concentration, sleepiness
and as a consequence interference
with job performance. Exposure to
high levels of noise (usually above
85 dBA) or impulsive noise (about
140 dBC) over a significant period
of time may cause both temporary
and chronic hearing loss.
Permanent hearing loss is the most
common occupational disease in
compensation claims.
Vibration has several parameters in Contract machines, mining loaders,
common with noise—frequency,
fork-lift trucks, pneumatic tools,
amplitude, duration of exposure
chain saws
and whether it is continuous or
intermittent. Method of operation
and skilfulness of the operator
seem to play an important role in
the development of harmful effects
of vibration. Manual work using
powered tools is associated with
symptoms of peripheral circulatory
disturbance known as “Raynaud’s
phenomenon” or “vibrationinduced white fingers” (VWF).
Vibrating tools may also affect the
peripheral nervous system and the
musculo-skeletal system with
reduced grip strength, low back
pain and degenerative back
The most important chronic effect Nuclear reactors, medical and
of ionizing radiation is cancer,
dental x-ray tubes, particle
including leukaemia. Overexposure accelerators, radioisotopes
from comparatively low levels of
radiation have been associated with
dermatitis of the hand and effects
on the haematological system.
Processes or activities which might
give excessive exposure to ionizing
radiation are very restricted and
Non-ionizing radiation consists of
ultraviolet radiation, visible
radiation, infrared, lasers,
electromagnetic fields (microwaves
and radio frequency) and extreme
low frequency radiation. IR
radiation might cause cataracts.
High-powered lasers may cause
eye and skin damage. There is an
increasing concern about exposure
to low levels of electromagnetic
fields as a cause of cancer and as a
potential cause of adverse
reproductive outcomes among
women, especially from exposure
to video display units. The question
about a causal link to cancer is not
yet answered. Recent reviews of
available scientific knowledge
generally conclude that there is no
association between use of VDUs
and adverse reproductive outcome.
Ultraviolet radiation: arc welding
and cutting; UV curing of inks,
glues, paints, etc.; disinfection;
product control
Infrared radiation: furnaces,
Lasers: communications, surgery,
Identification and Classification of Hazards
Before any occupational hygiene investigation is performed the purpose must be
clearly defined. The purpose of an occupational hygiene investigation might be to
identify possible hazards, to evaluate existing risks at the workplace, to prove
compliance with regulatory requirements, to evaluate control measures or to assess
exposure with regard to an epidemiological survey. This article is restricted to
programmes aimed at identification and classification of hazards at the workplace.
Many models or techniques have been developed to identify and evaluate hazards in
the working environment. They differ in complexity, from simple checklists,
preliminary industrial hygiene surveys, job-exposure matrices and hazard and
operability studies to job exposure profiles and work surveillance programmes (Renes
1978; Gressel and Gideon 1991; Holzner, Hirsh and Perper 1993; Goldberg et al.
1993; Bouyer and Hémon 1993; Panett, Coggon and Acheson 1985; Tait 1992). No
single technique is a clear choice for everyone, but all techniques have parts which are
useful in any investigation. The usefulness of the models also depends on the purpose
of the investigation, size of workplace, type of production and activity as well as
complexity of operations.
Identification and classification of hazards can be divided into three basic elements:
workplace characterization, exposure pattern and hazard evaluation.
Workplace characterization
A workplace might have from a few employees up to several thousands and have
different activities (e.g., production plants, construction sites, office buildings,
hospitals or farms). At a workplace different activities can be localized to special
areas such as departments or sections. In an industrial process, different stages and
operations can be identified as production is followed from raw materials to finished
Detailed information should be obtained about processes, operations or other activities
of interest, to identify agents utilized, including raw materials, materials handled or
added in the process, primary products, intermediates, final products, reaction
products and by-products. Additives and catalysts in a process might also be of
interest to identify. Raw material or added material which has been identified only by
trade name must be evaluated by chemical composition. Information or safety data
sheets should be available from manufacturer or supplier.
Some stages in a process might take place in a closed system without anyone exposed,
except during maintenance work or process failure. These events should be
recognized and precautions taken to prevent exposure to hazardous agents. Other
processes take place in open systems, which are provided with or without local
exhaust ventilation. A general description of the ventilation system should be
provided, including local exhaust system.
When possible, hazards should be identified in the planning or design of new plants or
processes, when changes can be made at an early stage and hazards might be
anticipated and avoided. Conditions and procedures that may deviate from the
intended design must be identified and evaluated in the process state. Recognition of
hazards should also include emissions to the external environment and waste
materials. Facility locations, operations, emission sources and agents should be
grouped together in a systematic way to form recognizable units in the further analysis
of potential exposure. In each unit, operations and agents should be grouped according
to health effects of the agents and estimation of emitted amounts to the work
Exposure patterns
The main exposure routes for chemical and biological agents are inhalation and
dermal uptake or incidentally by ingestion. The exposure pattern depends on
frequency of contact with the hazards, intensity of exposure and time of exposure.
Working tasks have to be systematically examined. It is important not only to study
work manuals but to look at what actually happens at the workplace. Workers might
be directly exposed as a result of actually performing tasks, or be indirectly exposed
because they are located in the same general area or location as the source of
exposure. It might be necessary to start by focusing on working tasks with high
potential to cause harm even if the exposure is of short duration. Non-routine and
intermittent operations (e.g., maintenance, cleaning and changes in production cycles)
have to be considered. Working tasks and situations might also vary throughout the
Within the same job title exposure or uptake might differ because some workers wear
protective equipment and others do not. In large plants, recognition of hazards or a
qualitative hazard evaluation very seldom can be performed for every single worker.
Therefore workers with similar working tasks have to be classified in the same
exposure group. Differences in working tasks, work techniques and work time will
result in considerably different exposure and have to be considered. Persons working
outdoors and those working without local exhaust ventilation have been shown to
have a larger day-to-day variability than groups working indoors with local exhaust
ventilation (Kromhout, Symanski and Rappaport 1993). Work processes, agents
applied for that process/job or different tasks within a job title might be used, instead
of the job title, to characterize groups with similar exposure. Within the groups,
workers potentially exposed must be identified and classified according to hazardous
agents, routes of exposure, health effects of the agents, frequency of contact with the
hazards, intensity and time of exposure. Different exposure groups should be ranked
according to hazardous agents and estimated exposure in order to determine workers
at greatest risk.
Qualitative hazard evaluation
Possible health effects of chemical, biological and physical agents present at the
workplace should be based on an evaluation of available epidemiological,
toxicological, clinical and environmental research. Up-to-date information about
health hazards for products or agents used at the workplace should be obtained from
health and safety journals, databases on toxicity and health effects, and relevant
scientific and technical literature.
Material Safety Data Sheets (MSDSs) should if necessary be updated. Data Sheets
document percentages of hazardous ingredients together with the Chemical Abstracts
Service chemical identifier, the CAS-number, and threshold limit value (TLV), if any.
They also contain information about health hazards, protective equipment, preventive
actions, manufacturer or supplier, and so on. Sometimes the ingredients reported are
rather rudimentary and have to be supplemented with more detailed information.
Monitored data and records of measurements should be studied. Agents with TLVs
provide general guidance in deciding whether the situation is acceptable or not,
although there must be allowance for possible interactions when workers are exposed
to several chemicals. Within and between different exposure groups, workers should
be ranked according to health effects of agents present and estimated exposure (e.g.,
from slight health effects and low exposure to severe health effects and estimated high
exposure). Those with the highest ranks deserve highest priority. Before any
prevention activities start it might be necessary to perform an exposure monitoring
programme. All results should be documented and easily attainable. A working
scheme is illustrated in figure 30.3.
In occupational hygiene investigations the hazards to the outdoor environment (e.g.,
pollution and greenhouse effects as well as effects on the ozone layer) might also be
Chemical, Biological and Physical Agents
Hazards might be of chemical, biological or physical origin. In this section and
in table 30.1 a brief description of the various hazards will be given together with
examples of environments or activities where they will be found (Casarett 1980;
International Congress on Occupational Health 1985; Jacobs 1992; Leidel, Busch and
Lynch 1977; Olishifski 1988; Rylander 1994). More detailed information will be
found elsewhere in this Encyclopaedia.
Chemical agents
Chemicals can be grouped into gases, vapours, liquids and aerosols (dusts, fumes,
Gases are substances that can be changed to liquid or solid state only by the combined
effects of increased pressure and decreased temperature. Handling gases always
implies risk of exposure unless they are processed in closed systems. Gases in
containers or distribution pipes might accidentally leak. In processes with high
temperatures (e.g., welding operations and exhaust from engines) gases will be
Vapours are the gaseous form of substances that normally are in the liquid or solid
state at room temperature and normal pressure. When a liquid evaporates it changes to
a gas and mixes with the surrounding air. A vapour can be regarded as a gas, where
the maximal concentration of a vapour depends on the temperature and the saturation
pressure of the substance. Any process involving combustion will generate vapours or
gases. Degreasing operations might be performed by vapour phase degreasing or soak
cleaning with solvents. Work activities like charging and mixing liquids, painting,
spraying, cleaning and dry cleaning might generate harmful vapours.
Liquids may consist of a pure substance or a solution of two or more substances (e.g.,
solvents, acids, alkalis). A liquid stored in an open container will partially evaporate
into the gas phase. The concentration in the vapour phase at equilibrium depends on
the vapour pressure of the substance, its concentration in the liquid phase, and the
temperature. Operations or activities with liquids might give rise to splashes or other
skin contact, besides harmful vapours.
Dusts consist of inorganic and organic particles, which can be classified as inhalable,
thoracic or respirable, depending on particle size. Most organic dusts have a biological
origin. Inorganic dusts will be generated in mechanical processes like grinding,
sawing, cutting, crushing, screening or sieving. Dusts may be dispersed when dusty
material is handled or whirled up by air movements from traffic. Handling dry
materials or powder by weighing, filling, charging, transporting and packing will
generate dust, as will activities like insulation and cleaning work.
Fumes are solid particles vaporized at high temperature and condensed to small
particles. The vaporization is often accompanied by a chemical reaction such as
oxidation. The single particles that make up a fume are extremely fine, usually less
than 0.1 µm, and often aggregate in larger units. Examples are fumes from welding,
plasma cutting and similar operations.
Mists are suspended liquid droplets generated by condensation from the gaseous state
to the liquid state or by breaking up a liquid into a dispersed state by splashing,
foaming or atomizing. Examples are oil mists from cutting and grinding operations,
acid mists from electroplating, acid or alkali mists from pickling operations or paint
spray mists from spraying operations.
Lori A. Todd
Hazard Surveillance and Survey Methods
Occupational surveillance involves active programmes to anticipate, observe,
measure, evaluate and control exposures to potential health hazards in the workplace.
Surveillance often involves a team of people that includes an occupational hygienist,
occupational physician, occupational health nurse, safety officer, toxicologist and
engineer. Depending upon the occupational environment and problem, three
surveillance methods can be employed: medical, environmental and biological.
Medical surveillance is used to detect the presence or absence of adverse health
effects for an individual from occupational exposure to contaminants, by performing
medical examinations and appropriate biological tests. Environmental surveillance is
used to document potential exposure to contaminants for a group of employees, by
measuring the concentration of contaminants in the air, in bulk samples of materials,
and on surfaces. Biological surveillance is used to document the absorption of
contaminants into the body and correlate with environmental contaminant levels, by
measuring the concentration of hazardous substances or their metabolites in the blood,
urine or exhaled breath of workers.
Medical Surveillance
Medical surveillance is performed because diseases can be caused or exacerbated by
exposure to hazardous substances. It requires an active programme with professionals
who are knowledgeable about occupational diseases, diagnoses and treatment.
Medical surveillance programmes provide steps to protect, educate, monitor and, in
some cases, compensate the employee. It can include pre-employment screening
programmes, periodic medical examinations, specialized tests to detect early changes
and impairment caused by hazardous substances, medical treatment and extensive
record keeping. Pre-employment screening involves the evaluation of occupational
and medical history questionnaires and results of physical examinations.
Questionnaires provide information concerning past illnesses and chronic diseases
(especially asthma, skin, lung and heart diseases) and past occupational exposures.
There are ethical and legal implications of pre-employment screening programmes if
they are used to determine employment eligibility. However, they are fundamentally
important when used to (1) provide a record of previous employment and associated
exposures, (2) establish a baseline of health for an employee and (3) test for
hypersusceptibility. Medical examinations can include audiometric tests for hearing
loss, vision tests, tests of organ function, evaluation of fitness for wearing respiratory
protection equipment, and baseline urine and blood tests. Periodic medical
examinations are essential for evaluating and detecting trends in the onset of adverse
health effects and may include biological monitoring for specific contaminants and
the use of other biomarkers.
Environmental and Biological Surveillance
Environmental and biological surveillance starts with an occupational hygiene survey
of the work environment to identify potential hazards and contaminant sources, and
determine the need for monitoring. For chemical agents, monitoring could involve air,
bulk, surface and biological sampling. For physical agents, monitoring could include
noise, temperature and radiation measurements. If monitoring is indicated, the
occupational hygienist must develop a sampling strategy that includes which
employees, processes, equipment or areas to sample, the number of samples, how long
to sample, how often to sample, and the sampling method. Industrial hygiene surveys
vary in complexity and focus depending upon the purpose of the investigation, type
and size of establishment, and nature of the problem.
There are no rigid formulas for performing surveys; however, thorough preparation
prior to the on-site inspection significantly increases effectiveness and efficiency.
Investigations that are motivated by employee complaints and illnesses have an
additional focus of identifying the cause of the health problems. Indoor air quality
surveys focus on indoor as well as outdoor sources of contamination. Regardless of
the occupational hazard, the overall approach to surveying and sampling workplaces
is similar; therefore, this chapter will use chemical agents as a model for the
Routes of Exposure
The mere presence of occupational stresses in the workplace does not automatically
imply that there is a significant potential for exposure; the agent must reach the
worker. For chemicals, the liquid or vapour form of the agent must make contact with
and/or be absorbed into the body to induce an adverse health effect. If the agent is
isolated in an enclosure or captured by a local exhaust ventilation system, the
exposure potential will be low, regardless of the chemical’s inherent toxicity.
The route of exposure can impact the type of monitoring performed as well as the
hazard potential. For chemical and biological agents, workers are exposed through
inhalation, skin contact, ingestion and injection; the most common routes of
absorption in the occupational environment are through the respiratory tract and the
skin. To assess inhalation, the occupational hygienist observes the potential for
chemicals to become airborne as gases, vapours, dusts, fumes or mists.
Skin absorption of chemicals is important primarily when there is direct contact with
the skin through splashing, spraying, wetting or immersion with fat-soluble
hydrocarbons and other organic solvents. Immersion includes body contact with
contaminated clothing, hand contact with contaminated gloves, and hand and arm
contact with bulk liquids. For some substances, such as amines and phenols, skin
absorption can be as rapid as absorption through the lungs for substances that are
inhaled. For some contaminants such as pesticides and benzidine dyes, skin
absorption is the primary route of absorption, and inhalation is a secondary route.
Such chemicals can readily enter the body through the skin, increase body burden and
cause systemic damage. When allergic reactions or repeated washing dries and cracks
the skin, there is a dramatic increase in the number and type of chemicals that can be
absorbed into the body. Ingestion, an uncommon route of absorption for gases and
vapours, can be important for particulates, such as lead. Ingestion can occur from
eating contaminated food, eating or smoking with contaminated hands, and coughing
and then swallowing previously inhaled particulates.
Injection of materials directly into the bloodstream can occur from hypodermic
needles inadvertently puncturing the skin of health care workers in hospitals, and from
high-velocity projectiles released from high-pressure sources and directly contacting
the skin. Airless paint sprayers and hydraulic systems have pressures high enough to
puncture the skin and introduce substances directly into the body.
The Walk-Through Inspection
The purpose of the initial survey, called the walk-through inspection, is to
systematically gather information to judge whether a potentially hazardous situation
exists and whether monitoring is indicated. An occupational hygienist begins the
walk-through survey with an opening meeting that can include representatives of
management, employees, supervisors, occupational health nurses and union
representatives. The occupational hygienist can powerfully impact the success of the
survey and any subsequent monitoring initiatives by creating a team of people who
communicate openly and honestly with one another and understand the goals and
scope of the inspection. Workers must be involved and informed from the beginning
to ensure that cooperation, not fear, dominates the investigation.
During the meeting, requests are made for process flow diagrams, plant layout
drawings, past environmental inspection reports, production schedules, equipment
maintenance schedules, documentation of personal protection programmes, and
statistics concerning the number of employees, shifts and health complaints. All
hazardous materials used and produced by an operation are identified and quantified.
A chemical inventory of products, by-products, intermediates and impurities is
assembled and all associated Material Safety Data Sheets are obtained. Equipment
maintenance schedules, age and condition are documented because the use of older
equipment may result in higher exposures due to the lack of controls.
After the meeting, the occupational hygienist performs a visual walk-through survey
of the workplace, scrutinizing the operations and work practices, with the goal of
identifying potential occupational stresses, ranking the potential for exposure,
identifying the route of exposure and estimating the duration and frequency of
exposure. Examples of occupational stresses are given in figure 30.4 .
Figure 30.4 Occupational stresses
The occupational hygienist uses the walk-through inspection to observe the workplace
and have questions answered. Examples of observations and questions are given
in figure 30.5 .
Figure 30.5 Observations and questions to ask on a walk-through survey
In addition to the questions shown in figure 30.5 , questions should be asked that
uncover what is not immediately obvious. Questions could address:
non-routine tasks and schedules for maintenance and cleaning activities
recent process changes and chemical substitutions
recent physical changes in the work environment
changes in job functions
recent renovations and repairs.
Non-routine tasks can result in significant peak exposures to chemicals that are
difficult to predict and measure during a typical workday. Process changes and
chemical substitutions may alter the release of substances into the air and affect
subsequent exposure. Changes in the physical layout of a work area can alter the
effectiveness of an existing ventilation system. Changes in job functions can result in
tasks performed by inexperienced workers and increased exposures. Renovations and
repairs may introduce new materials and chemicals into the work environment which
off-gas volatile organic chemicals or are irritants.
Indoor Air Quality Surveys
Indoor air quality surveys are distinct from traditional occupational hygiene surveys
because they are typically encountered in non-industrial workplaces and may involve
exposures to mixtures of trace quantities of chemicals, none of which alone appears
capable of causing illness (Ness 1991). The goal of indoor air quality surveys is
similar to occupational hygiene surveys in terms of identifying sources of
contamination and determining the need for monitoring. However, indoor air quality
surveys are always motivated by employee health complaints. In many cases, the
employees have a variety of symptoms including headaches, throat irritation, lethargy,
coughing, itching, nausea and non-specific hypersensitivity reactions that disappear
when they go home. When health complaints do not disappear after the employees
leave work, non-occupational exposures should be considered as well. Nonoccupational exposures include hobbies, other jobs, urban air pollution, passive
smoking and indoor exposures in the home. Indoor air quality surveys frequently use
questionnaires to document employee symptoms and complaints and link them to job
location or job function within the building. The areas with the highest incidence of
symptoms are then targeted for further inspection.
Sources of indoor air contaminants that have been documented in indoor air quality
surveys include:
inadequate ventilation (52%)
contamination from inside of the building (17%)
contamination from outside of the building (11%)
microbial contamination (5%)
contamination from the building materials (3%)
unknown causes (12%).
For indoor air quality investigations, the walk-through inspection is essentially a
building and environmental inspection to determine potential sources of
contamination both inside and outside of the building. Inside building sources include:
1. building construction materials such as insulation, particleboard, adhesives and
human occupants that can release chemicals from metabolic activities
human activities such as smoking
equipment such as copy machines
ventilation systems that can be contaminated with micro-organisms.
Observations and questions that can be asked during the survey are listed in figure
30.6 .
Figure 30.6 Observations and questions for an indoor air quality walk-through survey
Sampling and Measurement Strategies
Occupational exposure limits
After the walk-through inspection is completed, the occupational hygienist must
determine whether sampling is necessary; sampling should be performed only if the
purpose is clear. The occupational hygienist must ask, “What will be made of the
sampling results and what questions will the results answer?” It is relatively easy to
sample and obtain numbers; it is far more difficult to interpret them.
Air and biological sampling data are usually compared to recommended or mandated
occupational exposure limits (OELs). Occupational exposure limits have been
developed in many countries for inhalation and biological exposure to chemical and
physical agents. To date, out of a universe of over 60,000 commercially used
chemicals, approximately 600 have been evaluated by a variety of organizations and
countries. The philosophical bases for the limits are determined by the organizations
that have developed them. The most widely used limits, called threshold limit values
(TLVs), are those issued in the United States by the American Conference of
Governmental Industrial Hygienists (ACGIH). Most of the OELs used by the
Occupational Safety and Health Administration (OSHA) in the United States are
based upon the TLVs. However, the National Institute for Occupational Safety and
Health (NIOSH) of the US Department of Health and Human Services has suggested
their own limits, called recommended exposure limits (RELs).
For airborne exposures, there are three types of TLVs: an eight-hour time-weightedaverage exposure, TLV-TWA, to protect against chronic health effects; a fifteenminute average short-term exposure limit, TLV-STEL, to protect against acute health
effects; and an instantaneous ceiling value, TLV-C, to protect against asphyxiants or
chemicals that are immediately irritating. Guidelines for biological exposure levels are
called biological exposure indices (BEIs). These guidelines represent the
concentration of chemicals in the body that would correspond to inhalation exposure
of a healthy worker at a specific concentration in air. Outside of the United States as
many as 50 countries or groups have established OELs, many of which are identical to
the TLVs. In Britain, the limits are called the Health and Safety Executive
Occupational Exposure Standards (OES), and in Germany OELs are called Maximum
Workplace Concentrations (MAKs).
OELs have been set for airborne exposures to gases, vapours and particulates; they do
not exist for airborne exposures to biological agents. Therefore, most investigations of
bioaerosol exposure compare indoor with outdoor concentrations. If the
indoor/outdoor profile and concentration of organisms is different, an exposure
problem may exist. There are no OELs for skin and surface sampling, and each case
must be evaluated separately. In the case of surface sampling, concentrations are
usually compared with acceptable background concentrations that were measured in
other studies or were determined in the current study. For skin sampling, acceptable
concentrations are calculated based upon toxicity, rate of absorption, amount absorbed
and total dose. In addition, biological monitoring of a worker may be used to
investigate skin absorption.
Sampling strategy
An environmental and biological sampling strategy is an approach to obtaining
exposure measurements that fulfils a purpose. A carefully designed and effective
strategy is scientifically defensible, optimizes the number of samples obtained, is costeffective and prioritizes needs. The goal of the sampling strategy guides decisions
concerning what to sample (selection of chemical agents), where to sample (personal,
area or source sample), whom to sample (which worker or group of workers), sample
duration (real-time or integrated), how often to sample (how many days), how many
samples, and how to sample (analytical method). Traditionally, sampling performed
for regulatory purposes involves brief campaigns (one or two days) that concentrate
on worst-case exposures. While this strategy requires a minimum expenditure of
resources and time, it often captures the least amount of information and has little
applicability to evaluating long-term occupational exposures. To evaluate chronic
exposures so that they are useful for occupational physicians and epidemiological
studies, sampling strategies must involve repeated sampling over time for large
numbers of workers.
The goal of environmental and biological sampling strategies is either to evaluate
individual employee exposures or to evaluate contaminant sources. Employee
monitoring may be performed to:
evaluate individual exposures to chronic or acute toxicants
respond to employee complaints about health and odours
create a baseline of exposures for a long-term monitoring programme
determine whether exposures comply with governmental regulations
evaluate the effectiveness of engineering or process controls
evaluate acute exposures for emergency response
evaluate exposures at hazardous waste sites
evaluate the impact of work practices on exposure
evaluate exposures for individual job tasks
investigate chronic illnesses such as lead and mercury poisoning
investigate the relationship between occupational exposure and disease
carry out an epidemiological study.
Source and ambient air monitoring may be performed to:
· establish a need for engineering controls such as local exhaust ventilation systems
and enclosures
evaluate the impact of equipment or process modifications
evaluate the effectiveness of engineering or process controls
evaluate emissions from equipment or processes
evaluate compliance after remediation activities such as asbestos and lead removal
respond to indoor air, community illness and odour complaints
evaluate emissions from hazardous waste sites
investigate an emergency response
carry out an epidemiological study.
When monitoring employees, air sampling provides surrogate measures of dose
resulting from inhalation exposure. Biological monitoring can provide the actual dose
of a chemical resulting from all absorption routes including inhalation, ingestion,
injection and skin. Thus, biological monitoring can more accurately reflect an
individual’s total body burden and dose than air monitoring. When the relationship
between airborne exposure and internal dose is known, biological monitoring can be
used to evaluate past and present chronic exposures.
Goals of biological monitoring are listed in figure 30.7 .
Figure 30.7 Goals of biological monitoring
Biological monitoring has its limitations and should be performed only if it
accomplishes goals that cannot be accomplished with air monitoring alone (Fiserova-
Bergova 1987). It is invasive, requiring samples to be taken directly from workers.
Blood samples generally provide the most useful biological medium to monitor;
however, blood is taken only if non-invasive tests such as urine or exhaled breath are
not applicable. For most industrial chemicals, data concerning the fate of chemicals
absorbed by the body are incomplete or non-existent; therefore, only a limited number
of analytical measurement methods are available, and many are not sensitive or
Biological monitoring results may be highly variable between individuals exposed to
the same airborne concentrations of chemicals; age, health, weight, nutritional status,
drugs, smoking, alcohol consumption, medication and pregnancy can impact uptake,
absorption, distribution, metabolism and elimination of chemicals.
What to sample
Most occupational environments have exposures to multiple contaminants. Chemical
agents are evaluated both individually and as multiple simultaneous assaults on
workers. Chemical agents can act independently within the body or interact in a way
that increases the toxic effect. The question of what to measure and how to interpret
the results depends upon the biological mechanism of action of the agents when they
are within the body. Agents can be evaluated separately if they act independently on
altogether different organ systems, such as an eye irritant and a neurotoxin. If they act
on the same organ system, such as two respiratory irritants, their combined effect is
important. If the toxic effect of the mixture is the sum of the separate effects of the
individual components, it is termed additive. If the toxic effect of the mixture is
greater than the sum of the effects of the separate agents, their combined effect is
termed synergistic. Exposure to cigarette smoking and inhalation of asbestos fibres
gives rise to a much greater risk of lung cancer than a simple additive effect.
Sampling all the chemical agents in a workplace would be both expensive and not
necessarily defensible. The occupational hygienist must prioritize the laundry list of
potential agents by hazard or risk to determine which agents receive the focus.
Factors involved in ranking chemicals include:
whether the agents interact independently, additively or synergistically
inherent toxicity of the chemical agent
quantities used and generated
number of people potentially exposed
anticipated duration and concentration of the exposure
confidence in the engineering controls
anticipated changes in the processes or controls
occupational exposure limits and guidelines.
Where to sample
To provide the best estimate of employee exposure, air samples are taken in the
breathing zone of the worker (within a 30 cm radius of the head), and are called
personal samples. To obtain breathing zone samples, the sampling device is placed
directly on the worker for the duration of the sampling. If air samples are taken near
the worker, outside of the breathing zone, they are called area samples. Area samples
tend to underestimate personal exposures and do not provide good estimates of
inhalation exposure. However, area samples are useful for evaluating contaminant
sources and measuring ambient levels of contaminants. Area samples can be taken
while walking through the workplace with a portable instrument, or with fixed
sampling stations. Area sampling is routinely used at asbestos abatement sites for
clearance sampling and for indoor air investigations.
Whom to sample
Ideally, to evaluate occupational exposure, each worker would be individually
sampled for multiple days over the course of weeks or months. However, unless the
workplace is small (<10 employees), it is usually not feasible to sample all the
workers. To minimize the sampling burden in terms of equipment and cost, and
increase the effectiveness of the sampling programme, a subset of employees from the
workplace is sampled, and their monitoring results are used to represent exposures for
the larger work force.
To select employees who are representative of the larger work force, one approach is
to classify employees into groups with similar expected exposures, called
homogeneous exposure groups (HEGs) (Corn 1985). After the HEGs are formed, a
subset of workers is randomly selected from each group for sampling. Methods for
determining the appropriate sample sizes assume a lognormal distribution of
exposures, an estimated mean exposure, and a geometric standard deviation of 2.2 to
2.5. Prior sampling data might allow a smaller geometric standard deviation to be
used. To classify employees into distinct HEGs, most occupational hygienists observe
workers at their jobs and qualitatively predict exposures.
There are many approaches to forming HEGs; generally, workers may be classified by
job task similarity or work area similarity. When both job and work area similarity are
used, the method of classification is called zoning (see figure 30.8). Once airborne,
chemical and biological agents can have complex and unpredictable spatial and
temporal concentration patterns throughout the work environment. Therefore,
proximity of the source relative to the employee may not be the best indicator of
exposure similarity. Exposure measurements made on workers initially expected to
have similar exposures may show that there is more variation between workers than
predicted. In these cases, the exposure groups should be reconstructed into smaller
sets of workers, and sampling should continue to verify that workers within each
group actually have similar exposures (Rappaport 1995).
Figure 30.8 Factors involved in creating HEGs using zoning
Exposures can be estimated for all the employees, regardless of job title or risk, or it
can be estimated only for employees who are assumed to have the highest exposures;
this is called worst-case sampling. The selection of worst-case sampling employees
may be based upon production, proximity to the source, past sampling data, inventory
and chemical toxicity. The worst-case method is used for regulatory purposes and
does not provide a measure of long-term mean exposure and day-to-day variability.
Task-related sampling involves selecting workers with jobs that have similar tasks
that occur on a less than daily basis.
There are many factors that enter into exposure and can affect the success of HEG
classification, including the following:
1. Employees rarely perform the same work even when they have the same job
description, and rarely have the same exposures.
Employee work practices can significantly alter exposure.
3. Workers who are mobile throughout the work area may be unpredictably
exposed to several contaminant sources throughout the day.
4. Air movement in a workplace can unpredictably increase the exposures of
workers who are located a considerable distance from a source.
Exposures may be determined not by the job tasks but by the work environment.
Sample duration
The concentrations of chemical agents in air samples are either measured directly in
the field, obtaining immediate results (real-time or grab), or are collected over time in
the field on sampling media or in sampling bags and are measured in a laboratory
(integrated) (Lynch 1995). The advantage of real-time sampling is that results are
obtained quickly onsite, and can capture measurements of short-term acute exposures.
However, real-time methods are limited because they are not available for all
contaminants of concern and they may not be analytically sensitive or accurate
enough to quantify the targeted contaminants. Real-time sampling may not be
applicable when the occupational hygienist is interested in chronic exposures and
requires time-weighted-average measurements to compare with OELs.
Real-time sampling is used for emergency evaluations, obtaining crude estimates of
concentration, leak detection, ambient air and source monitoring, evaluating
engineering controls, monitoring short-term exposures that are less than 15 minutes,
monitoring episodic exposures, monitoring highly toxic chemicals (carbon monoxide),
explosive mixtures and process monitoring. Real-time sampling methods can capture
changing concentrations over time and provide immediate qualitative and quantitative
information. Integrated air sampling is usually performed for personal monitoring,
area sampling and for comparing concentrations to time-weighted-average OELs. The
advantages of integrated sampling are that methods are available for a wide variety of
contaminants; it can be used to identify unknowns; accuracy and specificity is high
and limits of detection are usually very low. Integrated samples that are analysed in a
laboratory must contain enough contaminant to meet minimum detectable analytical
requirements; therefore, samples are collected over a predetermined time period.
In addition to analytical requirements of a sampling method, sample duration should
be matched to the sampling purpose. For source sampling, duration is based upon the
process or cycle time, or when there are anticipated peaks of concentrations. For peak
sampling, samples should be collected at regular intervals throughout the day to
minimize bias and identify unpredictable peaks. The sampling period should be short
enough to identify peaks while also providing a reflection of the actual exposure
For personal sampling, duration is matched to the occupational exposure limit, task
duration or anticipated biological effect. Real-time sampling methods are used for
assessing acute exposures to irritants, asphyxiants, sensitizers and allergenic agents.
Chlorine, carbon monoxide and hydrogen sulphide are examples of chemicals that can
exert their effects quickly and at relatively low concentrations.
Chronic disease agents such as lead and mercury are usually sampled for a full shift
(seven hours or more per sample), using integrated sampling methods. To evaluate
full shift exposures, the occupational hygienist uses either a single sample or a series
of consecutive samples that cover the entire shift. The sampling duration for
exposures that occur for less than a full shift are usually associated with particular
tasks or processes. Construction workers, indoor maintenance personnel and
maintenance road crews are examples of jobs with exposures that are tied to tasks.
How many samples and how often to sample?
Concentrations of contaminants can vary minute to minute, day to day and season to
season, and variability can occur between individuals and within an individual.
Exposure variability affects both the number of samples and the accuracy of the
results. Variations in exposure can arise from different work practices, changes in
pollutant emissions, the volume of chemicals used, production quotas, ventilation,
temperature changes, worker mobility and task assignments. Most sampling
campaigns are performed for a couple of days in a year; therefore, the measurements
obtained are not representative of exposure. The period over which samples are
collected is very short compared with the unsampled period; the occupational
hygienist must extrapolate from the sampled to the unsampled period. For long-term
exposure monitoring, each worker selected from a HEG should be sampled multiple
times over the course of weeks or months, and exposures should be characterized for
all shifts. While the day shift may be the busiest, the night shift may have the least
supervision and there may be lapses in work practices.
Measurement Techniques
Active and passive sampling
Contaminants are collected on sampling media either by actively pulling an air sample
through the media, or by passively allowing the air to reach the media. Active
sampling uses a battery-powered pump, and passive sampling uses diffusion or
gravity to bring the contaminants to the sampling media. Gases, vapours, particulates
and bioaerosols are all collected by active sampling methods; gases and vapours can
also be collected by passive diffusion sampling.
For gases, vapours and most particulates, once the sample is collected the mass of the
contaminant is measured, and concentration is calculated by dividing the mass by the
volume of sampled air. For gases and vapours, concentration is expressed as parts per
million (ppm) or mg/m3, and for particulates concentration is expressed as
mg/m3 (Dinardi 1995).
In integrated sampling, air sampling pumps are critical components of the sampling
system because concentration estimates require knowledge of the volume of sampled
air. Pumps are selected based upon desired flowrate, ease of servicing and calibration,
size, cost and suitability for hazardous environments. The primary selection criterion
is flowrate: low-flow pumps (0.5 to 500 ml/min) are used for sampling gases and
vapours; high-flow pumps (500 to 4,500 ml/min) are used for sampling particulates,
bioaerosols and gases and vapours. To insure accurate sample volumes, pumps must
be accurately calibrated. Calibration is performed using primary standards such as
manual or electronic soap-bubble meters, which directly measure volume, or
secondary methods such as wet test meters, dry gas meters and precision rotameters
that are calibrated against primary methods.
Gases and vapours: sampling media
Gases and vapours are collected using porous solid sorbent tubes, impingers, passive
monitors and bags. Sorbent tubes are hollow glass tubes that have been filled with a
granular solid that enables adsorption of chemicals unchanged on its surface. Solid
sorbents are specific for groups of compounds; commonly used sorbents include
charcoal, silica gel and Tenax. Charcoal sorbent, an amorphous form of carbon, is
electrically nonpolar, and preferentially adsorbs organic gases and vapours. Silica gel,
an amorphous form of silica, is used to collect polar organic compounds, amines and
some inorganic compounds. Because of its affinity for polar compounds, it will adsorb
water vapour; therefore, at elevated humidity, water can displace the less polar
chemicals of interest from the silica gel. Tenax, a porous polymer, is used for
sampling very low concentrations of nonpolar volatile organic compounds.
The ability to accurately capture the contaminants in air and avoid contaminant loss
depends upon the sampling rate, sampling volume, and the volatility and
concentration of the airborne contaminant. Collection efficiency of solid sorbents can
be adversely affected by increased temperature, humidity, flowrate, concentration,
sorbent particle size and number of competing chemicals. As collection efficiency
decreases chemicals will be lost during sampling and concentrations will be
underestimated. To detect chemical loss, or breakthrough, solid sorbent tubes have
two sections of granular material separated by a foam plug. The front section is used
for sample collection and the back section is used to determine breakthrough.
Breakthrough has occurred when at least 20 to 25% of the contaminant is present in
the back section of the tube. Analysis of contaminants from solid sorbents requires
extraction of the contaminant from the medium using a solvent. For each batch of
sorbent tubes and chemicals collected, the laboratory must determine the desorption
efficiency, the efficiency of removal of chemicals from the sorbent by the solvent. For
charcoal and silica gel, the most commonly used solvent is carbon disulphide. For
Tenax, the chemicals are extracted using thermal desorption directly into a gas
Impingers are usually glass bottles with an inlet tube that allows air to be drawn into
the bottle through a solution that collects the gases and vapours by absorption either
unchanged in solution or by a chemical reaction. Impingers are used less and less in
workplace monitoring, especially for personal sampling, because they can break, and
the liquid media can spill onto the employee. There are a variety of types of
impingers, including gas wash bottles, spiral absorbers, glass bead columns, midget
impingers and fritted bubblers. All impingers can be used to collect area samples; the
most commonly used impinger, the midget impinger, can be used for personal
sampling as well.
Passive, or diffusion monitors are small, have no moving parts and are available for
both organic and inorganic contaminants. Most organic monitors use activated
charcoal as the collection medium. In theory, any compound that can be sampled by a
charcoal sorbent tube and pump can be sampled using a passive monitor. Each
monitor has a uniquely designed geometry to give an effective sampling rate.
Sampling starts when the monitor cover is removed and ends when the cover is
replaced. Most diffusion monitors are accurate for eight-hour time-weighted-average
exposures and are not appropriate for short-term exposures.
Sampling bags can be used to collect integrated samples of gases and vapours. They
have permeability and adsorptive properties that enable storage for a day with
minimal loss. Bags are made of Teflon (polytetrafluoroethylene) and Tedlar
Sampling media: particulate materials
Occupational sampling for particulate materials, or aerosols, is currently in a state of
flux; traditional sampling methods will eventually be replaced by particle size
selective (PSS) sampling methods. Traditional sampling methods will be discussed
first, followed by PSS methods.
The most commonly used media for collecting aerosols are fibre or membrane filters;
aerosol removal from the air stream occurs by collision and attachment of the particles
to the surface of the filters. The choice of filter medium depends upon the physical
and chemical properties of the aerosols to be sampled, the type of sampler and the
type of analysis. When selecting filters, they must be evaluated for collection
efficiency, pressure drop, hygroscopicity, background contamination, strength and
pore size, which can range from 0.01 to 10 µm. Membrane filters are manufactured in
a variety of pore sizes and are usually made from cellulose ester, polyvinylchloride or
polytetrafluoroethylene. Particle collection occurs at the surface of the filter;
therefore, membrane filters are usually used in applications where microscopy will be
performed. Mixed cellulose ester filters can be easily dissolved with acid and are
usually used for collection of metals for analysis by atomic absorption. Nucleopore
filters (polycarbonate) are very strong and thermally stable, and are used for sampling
and analysing asbestos fibres using transmission electron microscopy. Fibre filters are
usually made of fibreglass and are used to sample aerosols such as pesticides and lead.
For occupational exposures to aerosols, a known volume of air can be sampled
through the filters, the total increase in mass (gravimetric analysis) can be measured
(mg/m3 air), the total number of particles can be counted (fibres/cc) or the aerosols
can be identified (chemical analysis). For mass calculations, the total dust that enters
the sampler or only the respirable fraction can be measured. For total dust, the
increase in mass represents exposure from deposition in all parts of the respiratory
tract. Total dust samplers are subject to error due to high winds passing across the
sampler and improper orientation of the sampler. High winds, and filters facing
upright, can result in collection of extra particles and overestimation of exposure.
For respirable dust sampling, the increase in mass represents exposure from
deposition in the gas exchange (alveolar) region of the respiratory tract. To collect
only the respirable fraction, a preclassifier called a cyclone is used to alter the
distribution of airborne dust presented to the filter. Aerosols are drawn into the
cyclone, accelerated and whirled, causing the heavier particles to be thrown out to the
edge of the air stream and dropped to a removal section at the bottom of the cyclone.
The respirable particles that are less than 10 µm remain in the air stream and are
drawn up and collected on the filter for subsequent gravimetric analysis.
Sampling errors encountered when performing total and respirable dust sampling
result in measurements that do not accurately reflect exposure or relate to adverse
health effects. Therefore, PSS has been proposed to redefine the relationship between
particle size, adverse health impact and sampling method. In PSS sampling, the
measurement of particles is related to the sizes that are associated with specific health
effects. The International Organization for Standardization (ISO) and the ACGIH
have proposed three particulate mass fractions: inhalable particulate mass (IPM),
thoracic particulate mass (TPM) and respirable particulate mass (RPM). IPM refers to
particles that can be expected to enter through the nose and mouth, and would replace
the traditional total mass fraction. TPM refers to particles that can penetrate the upper
respiratory system past the larynx. RPM refers to particles that are capable of
depositing in the gas-exchange region of the lung, and would replace the current
respirable mass fraction. The practical adoption of PSS sampling requires the
development of new aerosol sampling methods and PSS-specific occupational
exposure limits.
Sampling media: biological materials
There are few standardized methods for sampling biological material or bioaerosols.
Although sampling methods are similar to those used for other airborne particulates,
viability of most bioaerosols must be preserved to ensure laboratory culturability.
Therefore, they are more difficult to collect, store and analyse. The strategy for
sampling bioaerosols involves collection directly on semisolid nutrient agar or plating
after collection in fluids, incubation for several days and identification and
quantification of the cells that have grown. The mounds of cells that have multiplied
on the agar can be counted as colony-forming units (CFU) for viable bacteria or fungi,
and plaque-forming units (PFU) for active viruses. With the exception of spores,
filters are not recommended for bioaerosol collection because dehydration causes cell
Viable aerosolized micro-organisms are collected using all-glass impingers (AGI-30),
slit samplers and inertial impactors. Impingers collect bioaerosols in liquid and the slit
sampler collects bioaerosols on glass slides at high volumes and flowrates. The
impactor is used with one to six stages, each containing a Petri dish, to allow for
separation of particles by size.
Interpretation of sampling results must be done on a case-by-case basis because there
are no occupational exposure limits. Evaluation criteria must be determined prior to
sampling; for indoor air investigations, in particular, samples taken outside of the
building are used as a background reference. A rule of thumb is that concentrations
should be ten times background to suspect contamination. When using culture plating
techniques, concentrations are probably underestimated because of losses of viability
during sampling and incubation.
Skin and surface sampling
There are no standard methods for evaluating skin exposure to chemicals and
predicting dose. Surface sampling is performed primarily to evaluate work practices
and identify potential sources of skin absorption and ingestion. Two types of surface
sampling methods are used to assess dermal and ingestion potential: direct methods,
which involve sampling the skin of a worker, and indirect methods, which involve
wipe sampling surfaces.
Direct skin sampling involves placing gauze pads on the skin to absorb chemicals,
rinsing the skin with solvents to remove contaminants and using fluorescence to
identify skin contamination. Gauze pads are placed on different parts of the body and
are either left exposed or are placed under personal protective equipment. At the end
of the workday the pads are removed and are analysed in the laboratory; the
distribution of concentrations from different parts of the body are used to identify skin
exposure areas. This method is inexpensive and easy to perform; however, the results
are limited because gauze pads are not good physical models of the absorption and
retention properties of skin, and measured concentrations are not necessarily
representative of the entire body.
Skin rinses involve wiping the skin with solvents or placing hands in plastic bags
filled with solvents to measure the concentration of chemicals on the surface. This
method can underestimate dose because only the unabsorbed fraction of chemicals is
Fluorescence monitoring is used to identify skin exposure for chemicals that naturally
fluoresce, such as polynuclear aromatics, and to identify exposures for chemicals in
which fluorescent compounds have been intentionally added. The skin is scanned with
an ultraviolet light to visualize contamination. This visualization provides workers
with evidence of the effect of work practices on exposure; research is underway to
quantify the fluorescence intensity and relate it to dose.
Indirect wipe sampling methods involve the use of gauze, glass fibre filters or
cellulose paper filters, to wipe the insides of gloves or respirators, or the tops of
surfaces. Solvents may be added to increase collection efficiency. The gauze or filters
are then analysed in the laboratory. To standardize the results and enable comparison
between samples, a square template is used to sample a 100 cm2 area.
Biological media
Blood, urine and exhaled air samples are the most suitable specimens for routine
biological monitoring, while hair, milk, saliva and nails are less frequently used.
Biological monitoring is performed by collecting bulk blood and urine samples in the
workplace and analysing them in the laboratory. Exhaled air samples are collected in
Tedlar bags, specially designed glass pipettes or sorbent tubes, and are analysed in the
field using direct-reading instruments, or in the laboratory. Blood, urine and exhaled
air samples are primarily used to measure the unchanged parent compound (same
chemical that is sampled in workplace air), its metabolite or a biochemical change
(intermediate) that has been induced in the body. For example, the parent compound
lead is measured in blood to evaluate lead exposure, the metabolite mandelic acid is
measured in urine for both styrene and ethyl benzene, and carboxyhaemoglobin is the
intermediate measured in blood for both carbon monoxide and methylene chloride
exposure. For exposure monitoring, the concentration of an ideal determinant will be
highly correlated with intensity of exposure. For medical monitoring, the
concentration of an ideal determinant will be highly correlated with target organ
The timing of specimen collection can impact the usefulness of the measurements;
samples should be collected at times which most accurately reflect exposure. Timing
is related to the excretion biological half-life of a chemical, which reflects how
quickly a chemical is eliminated from the body; this can vary from hours to years.
Target organ concentrations of chemicals with short biological half-lives closely
follow the environmental concentration; target organ concentrations of chemicals with
long biological half-lives fluctuate very little in response to environmental exposures.
For chemicals with short biological half-lives, less than three hours, a sample is taken
immediately at the end of the workday, before concentrations rapidly decline, to
reflect exposure on that day. Samples may be taken at any time for chemicals with
long half-lives, such as polychlorinated biphenyls and lead.
Real-time monitors
Direct-reading instruments provide real-time quantification of contaminants; the
sample is analysed within the equipment and does not require off-site laboratory
analysis (Maslansky and Maslansky 1993). Compounds can be measured without first
collecting them on separate media, then shipping, storing and analysing them.
Concentration is read directly from a meter, display, strip chart recorder and data
logger, or from a colour change. Direct-reading instruments are primarily used for
gases and vapours; a few instruments are available for monitoring particulates.
Instruments vary in cost, complexity, reliability, size, sensitivity and specificity. They
include simple devices, such as colorimetric tubes, that use a colour change to indicate
concentration; dedicated instruments that are specific for a chemical, such as carbon
monoxide indicators, combustible gas indicators (explosimeters) and mercury vapour
meters; and survey instruments, such as infrared spectrometers, that screen large
groups of chemicals. Direct-reading instruments use a variety of physical and
chemical methods to analyse gases and vapours, including conductivity, ionization,
potentiometry, photometry, radioactive tracers and combustion.
Commonly used portable direct-reading instruments include battery-powered gas
chromatographs, organic vapour analysers and infrared spectrometers. Gas
chromatographs and organic vapour monitors are primarily used for environmental
monitoring at hazardous waste sites and for community ambient air monitoring. Gas
chromatographs with appropriate detectors are specific and sensitive, and can quantify
chemicals at very low concentrations. Organic vapour analysers are usually used to
measure classes of compounds. Portable infrared spectrometers are primarily used for
occupational monitoring and leak detection because they are sensitive and specific for
a wide range of compounds.
Small direct-reading personal monitors are available for a few common gases
(chlorine, hydrogen cyanide, hydrogen sulphide, hydrazine, oxygen, phosgene,
sulphur dioxide, nitrogen dioxide and carbon monoxide). They accumulate
concentration measurements over the course of the day and can provide a direct
readout of time-weighted-average concentration as well as provide a detailed
contaminant profile for the day.
Colorimetric tubes (detector tubes) are simple to use, cheap and available for a wide
variety of chemicals. They can be used to quickly identify classes of air contaminants
and provide ballpark estimates of concentrations that can be used when determining
pump flow rates and volumes. Colorimetric tubes are glass tubes filled with solid
granular material which has been impregnated with a chemical agent that can react
with a contaminant and create a colour change. After the two sealed ends of a tube are
broken open, one end of the tube is placed in a hand pump. The recommended volume
of contaminated air is sampled through the tube by using a specified number of pump
strokes for a particular chemical. A colour change or stain is produced on the tube,
usually within two minutes, and the length of the stain is proportional to
concentration. Some colorimetric tubes have been adapted for long duration sampling,
and are used with battery-powered pumps that can run for at least eight hours. The
colour change produced represents a time-weighted-average concentration.
Colorimetric tubes are good for both qualitative and quantitative analysis; however,
their specificity and accuracy is limited. The accuracy of colorimetric tubes is not as
high as that of laboratory methods or many other real-time instruments. There are
hundreds of tubes, many of which have cross-sensitivities and can detect more than
one chemical. This can result in interferences that modify the measured
Direct-reading aerosol monitors cannot distinguish between contaminants, are usually
used for counting or sizing particles, and are primarily used for screening, not to
determine TWA or acute exposures. Real-time instruments use optical or electrical
properties to determine total and respirable mass, particle count and particle size.
Light-scattering aerosol monitors, or aerosol photometers, detect the light scattered by
particles as they pass through a volume in the equipment. As the number of particles
increases, the amount of scattered light increases and is proportional to mass. Lightscattering aerosol monitors cannot be used to distinguish between particle types;
however, if they are used in a workplace where there are a limited number of dusts
present, the mass can be attributed to a particular material. Fibrous aerosol monitors
are used to measure the airborne concentration of particles such as asbestos. Fibres are
aligned in an oscillating electric field and are illuminated with a helium neon laser; the
resulting pulses of light are detected by a photomultiplier tube. Light-attenuating
photometers measure the extinction of light by particles; the ratio of incident light to
measured light is proportional to concentration.
Analytical Techniques
There are many available methods for analysing laboratory samples for contaminants.
Some of the more commonly used techniques for quantifying gases and vapours in air
include gas chromatography, mass spectrometry, atomic absorption, infrared and UV
spectroscopy and polarography.
Gas chromatography is a technique used to separate and concentrate chemicals in
mixtures for subsequent quantitative analysis. There are three main components to the
system: the sample injection system, a column and a detector. A liquid or gaseous
sample is injected using a syringe, into an air stream that carries the sample through a
column where the components are separated. The column is packed with materials
that interact differently with different chemicals, and slows down the movement of the
chemicals. The differential interaction causes each chemical to travel through the
column at a different rate. After separation, the chemicals go directly into a detector,
such as a flame ionization detector (FID), photo-ionization detector (PID) or electron
capture detector (ECD); a signal proportional to concentration is registered on a chart
recorder. The FID is used for almost all organics including: aromatics, straight chain
hydrocarbons, ketones and some chlorinated hydrocarbons. Concentration is measured
by the increase in the number of ions produced as a volatile hydrocarbon is burned by
a hydrogen flame. The PID is used for organics and some inorganics; it is especially
useful for aromatic compounds such as benzene, and it can detect aliphatic, aromatic
and halogenated hydrocarbons. Concentration is measured by the increase in the
number of ions produced when the sample is bombarded by ultraviolet radiation. The
ECD is primarily used for halogen-containing chemicals; it gives a minimal response
to hydrocarbons, alcohols and ketones. Concentration is measured by the current flow
between two electrodes caused by ionization of the gas by radioactivity.
The mass spectrophotometer is used to analyse complex mixtures of chemicals
present in trace amounts. It is often coupled with a gas chromatograph for the
separation and quantification of different contaminants.
Atomic absorption spectroscopy is primarily used for the quantification of metals such
as mercury. Atomic absorption is the absorption of light of a particular wavelength by
a free, ground-state atom; the quantity of light absorbed is related to concentration.
The technique is highly specific, sensitive and fast, and is directly applicable to
approximately 68 elements. Detection limits are in the sub-ppb to low-ppm range.
Infrared analysis is a powerful, sensitive, specific and versatile technique. It uses the
absorption of infrared energy to measure many inorganic and organic chemicals; the
amount of light absorbed is proportional to concentration. The absorption spectrum of
a compound provides information enabling its identification and quantification.
UV absorption spectroscopy is used for analysis of aromatic hydrocarbons when
interferences are known to be low. The amount of absorption of UV light is directly
proportional to concentration.
Polarographic methods are based upon the electrolysis of a sample solution using an
easily polarized electrode and a nonpolarizable electrode. They are used for
qualitative and quantitative analysis of aldehydes, chlorinated hydrocarbons and
James Stewart
After a hazard has been recognized and evaluated, the most appropriate interventions
(methods of control) for a particular hazard must be determined. Control methods
usually fall into three categories:
engineering controls
administrative controls
personal protective equipment.
As with any change in work processes, training must be provided to ensure the
success of the changes.
Engineering controls are changes to the process or equipment that reduce or eliminate
exposures to an agent. For example, substituting a less toxic chemical in a process or
installing exhaust ventilation to remove vapours generated during a process step, are
examples of engineering controls. In the case of noise control, installing soundabsorbing materials, building enclosures and installing mufflers on air exhaust outlets
are examples of engineering controls. Another type of engineering control might be
changing the process itself. An example of this type of control would be removal of
one or more degreasing steps in a process that originally required three degreasing
steps. By removing the need for the task that produced the exposure, the overall
exposure for the worker has been controlled. The advantage of engineering controls is
the relatively small involvement of the worker, who can go about the job in a more
controlled environment when, for instance, contaminants are automatically removed
from the air. Contrast this to the situation where the selected method of control is a
respirator to be worn by the worker while performing the task in an “uncontrolled”
workplace. In addition to the employer actively installing engineering controls on
existing equipment, new equipment can be purchased that contains the controls or
other more effective controls. A combination approach has often been effective (i.e.,
installing some engineering controls now and requiring personal protective equipment
until new equipment arrives with more effective controls that will eliminate the need
for personal protective equipment). Some common examples of engineering controls
ventilation (both general and local exhaust ventilation)
isolation (place a barrier between the worker and the agent)
substitution (substitute less toxic, less flammable material, etc.)
change the process (eliminate hazardous steps).
The occupational hygienist must be sensitive to the worker’s job tasks and must solicit
worker participation when designing or selecting engineering controls. Placing
barriers in the workplace, for example, could significantly impair a worker’s ability to
perform the job and may encourage “work arounds”. Engineering controls are the
most effective methods of reducing exposures. They are also, often, the most
expensive. Since engineering controls are effective and expensive it is important to
maximize the involvement of the workers in the selection and design of the controls.
This should result in a greater likelihood that the controls will reduce exposures.
Administrative controls involve changes in how a worker accomplishes the necessary
job tasks—for example, how long they work in an area where exposures occur, or
changes in work practices such as improvements in body positioning to reduce
exposures. Administrative controls can add to the effectiveness of an intervention but
have several drawbacks:
1. Rotation of workers may reduce overall average exposure for the workday but it
provides periods of high short-term exposure for a larger number of workers. As more
becomes known about toxicants and their modes of action, short-term peak exposures
may represent a greater risk than would be calculated based on their contribution to
average exposure.
2. Changing work practices of workers can present a significant enforcement and
monitoring challenge. How work practices are enforced and monitored determines
whether or not they will be effective. This constant management attention is a
significant cost of administrative controls.
Personal protective equipment consists of devices provided to the worker and required
to be worn while performing certain (or all) job tasks. Examples include respirators,
chemical goggles, protective gloves and faceshields. Personal protective equipment is
commonly used in cases where engineering controls have not been effective in
controlling the exposure to acceptable levels or where engineering controls have not
been found to be feasible (for cost or operational reasons). Personal protective
equipment can provide significant protection to workers if worn and used correctly. In
the case of respiratory protection, protection factors (ratio of concentration outside the
respirator to that inside) can be 1,000 or more for positive-pressure supplied air
respirators or ten for half-face air-purifying respirators. Gloves (if selected
appropriately) can protect hands for hours from solvents. Goggles can provide
effective protection from chemical splashes.
Intervention: Factors to Consider
Often a combination of controls is used to reduce the exposures to acceptable levels.
Whatever methods are selected, the intervention must reduce the exposure and
resulting hazard to an acceptable level. There are, however, many other factors that
need to be considered when selecting an intervention. For example:
effectiveness of the controls
ease of use by the employee
cost of the controls
adequacy of the warning properties of the material
acceptable level of exposure
frequency of exposure
route(s) of exposure
regulatory requirements for specific controls.
Effectiveness of controls
Effectiveness of controls is obviously a prime consideration when taking action to
reduce exposures. When comparing one type of intervention to another, the level of
protection required must be appropriate for the challenge; too much control is a waste
of resources. Those resources could be used to reduce other exposures or exposures of
other employees. On the other hand, too little control leaves the worker exposed to
unhealthy conditions. A useful first step is to rank the interventions according to their
effectiveness, then use this ranking to evaluate the significance of the other factors.
Ease of use
For any control to be effective the worker must be able to perform his or her job tasks
with the control in place. For example, if the control method selected is substitution,
then the worker must know the hazards of the new chemical, be trained in safe
handling procedures, understand proper disposal procedures, and so on. If the control
is isolation—placing an enclosure around the substance or the worker—the enclosure
must allow the worker to do his or her job. If the control measures interfere with the
tasks of the job, the worker will be reluctant to use them and may find ways to
accomplish the tasks that could result in increased, not decreased, exposures.
Every organization has limits on resources. The challenge is to maximize the use of
those resources. When hazardous exposures are identified and an intervention strategy
is being developed, cost must be a factor. The “best buy” many times will not be the
lowest- or highest-cost solutions. Cost becomes a factor only after several viable
methods of control have been identified. Cost of the controls can then be used to
select the controls that will work best in that particular situation. If cost is the
determining factor at the outset, poor or ineffective controls may be selected, or
controls that interfere with the process in which the employee is working. It would be
unwise to select an inexpensive set of controls that interfere with and slow down a
manufacturing process. The process then would have a lower throughput and higher
cost. In very short time the “real” costs of these “low cost” controls would become
enormous. Industrial engineers understand the layout and overall process; production
engineers understand the manufacturing steps and processes; the financial analysts
understand the resource allocation problems. Occupational hygienists can provide a
unique insight into these discussions due to their understanding of the specific
employee’s job tasks, the employee’s interaction with the manufacturing equipment as
well as how the controls will work in a particular setting. This team approach
increases the likelihood of selecting the most appropriate (from a variety of
perspectives) control.
Adequacy of warning properties
When protecting a worker against an occupational health hazard, the warning
properties of the material, such as odour or irritation, must be considered. For
example, if a semiconductor worker is working in an area where arsine gas is used,
the extreme toxicity of the gas poses a significant potential hazard. The situation is
compounded by arsine’s very poor warning properties—the workers cannot detect the
arsine gas by sight or smell until it is well above acceptable levels. In this case,
controls that are marginally effective at keeping exposures below acceptable levels
should not be considered because excursions above acceptable levels cannot be
detected by the workers. In this case, engineering controls should be installed to
isolate the worker from the material. In addition, a continuous arsine gas monitor
should be installed to warn workers of the failure of the engineering controls. In
situations involving high toxicity and poor warning properties, preventive
occupational hygiene is practised. The occupational hygienist must be flexible and
thoughtful when approaching an exposure problem.
Acceptable level of exposure
If controls are being considered to protect a worker from a substance such as acetone,
where the acceptable level of exposure may be in the range of 800 ppm, controlling to
a level of 400 ppm or less may be achieved relatively easily. Contrast the example of
acetone control to control of 2-ethoxyethanol, where the acceptable level of exposure
may be in the range of 0.5 ppm. To obtain the same per cent reduction (0.5 ppm to
0.25 ppm) would probably require different controls. In fact, at these low levels of
exposure, isolation of the material may become the primary means of control. At high
levels of exposure, ventilation may provide the necessary reduction. Therefore, the
acceptable level determined (by the government, company, etc.) for a substance can
limit the selection of controls.
Frequency of exposure
When assessing toxicity the classic model uses the following relationship:
Dose, in this case, is the amount of material being made available for absorption. The
previous discussion focused on minimizing (lowering) the concentration portion of
this relationship. One might also reduce the time spent being exposed (the underlying
reason for administrative controls). This would similarly reduce the dose. The issue
here is not the employee spending time in a room, but how often an operation (task) is
performed. The distinction is important. In the first example, the exposure is
controlled by removing the workers when they are exposed to a selected amount of
toxicant; the intervention effort is not directed at controlling the amount of toxicant
(in many situations there may be a combination approach). In the second case, the
frequency of the operation is being used to provide the appropriate controls, not to
determine a work schedule. For example, if an operation such as degreasing is
performed routinely by an employee, the controls may include ventilation, substitution
of a less toxic solvent or even automation of the process. If the operation is performed
rarely (e.g., once per quarter) personal protective equipment may be an option
(depending on many of the factors described in this section). As these two examples
illustrate, the frequency with which an operation is performed can directly affect the
selection of controls. Whatever the exposure situation, the frequency with which a
worker performs the tasks must be considered and factored into the control selection.
Route of exposure obviously is going to affect the method of control. If a respiratory
irritant is present, ventilation, respirators, and so on, would be considered. The
challenge for the occupational hygienist is identifying all routes of exposure. For
example, glycol ethers are used as a carrier solvent in printing operations. Breathingzone air concentrations can be measured and controls implemented. Glycol ethers,
however, are rapidly absorbed through intact skin. The skin represents a significant
route of exposure and must be considered. In fact, if the wrong gloves are chosen, the
skin exposure may continue long after the air exposures have decreased (due to the
employee continuing to use gloves that have experienced breakthrough). The
hygienist must evaluate the substance—its physical properties, chemical and
toxicological properties, and so on—to determine what routes of exposure are possible
and plausible (based on the tasks performed by the employee).
In any discussion of controls, one of the factors that must be considered is the
regulatory requirements for controls. There may well be codes of practice, regulations,
and so on, that require a specific set of controls. The occupational hygienist has
flexibility above and beyond the regulatory requirements, but the minimum mandated
controls must be installed. Another aspect of the regulatory requirements is that the
mandated controls may not work as well or may conflict with the best judgement of
the occupational hygienist. The hygienist must be creative in these situations and find
solutions that satisfy the regulatory as well as best practice goals of the organization.
Training and Labelling
Regardless of what form of intervention is eventually selected, training and other
forms of notification must be provided to ensure that the workers understand the
interventions, why they were selected, what reductions in exposure are expected, and
the role of the workers in achieving those reductions. Without the participation and
understanding of the workforce, the interventions will likely fail or at least operate at
reduced efficiency. Training builds hazard awareness in the workforce. This new
awareness can be invaluable to the occupational hygienist in identifying and reducing
previously unrecognized exposures or new exposures.
Training, labelling and related activities may be part of a regulatory compliance
scheme. It would be prudent to check the local regulations to ensure that whatever
type of training or labelling is undertaken satisfies the regulatory as well as
operational requirements.
In this short discussion on interventions, some general considerations have been
presented to stimulate thought. In practice, these rules become very complex and often
have significant ramifications for employee and company health. The occupational
hygienist’s professional judgement is essential in selecting the best controls. Best is a
term with many different meanings. The occupational hygienist must become adept at
working in teams and soliciting input from the workers, management and technical
Dick Heederik
Workplace exposure assessment is concerned with identifying and evaluating agents
with which a worker may come in contact, and exposure indices can be constructed to
reflect the amount of an agent present in the general environment or in inhaled air, as
well as to reflect the amount of agent that is actually inhaled, swallowed or otherwise
absorbed (the intake). Other indices include the amount of agent that is resorbed (the
uptake) and the exposure at the target organ. Dose is a pharmacological or
toxicological term used to indicate the amount of a substance administered to a
subject. Dose rate is the amount administered per unit of time. The dose of a
workplace exposure is difficult to determine in a practical situation, since physical and
biological processes, like inhalation, uptake and distribution of an agent in the human
body cause exposure and dose to have complex, non-linear relationships. The
uncertainty about the actual level of exposure to agents also makes it difficult to
quantify relationships between exposure and health effects.
For many occupational exposures there exists a time window during which the
exposure or dose is most relevant to the development of a particular health-related
problem or symptom. Hence, the biologically relevant exposure, or dose, would be
that exposure which occurs during the relevant time window. Some exposures to
occupational carcinogens are believed to have such a relevant time window of
exposure. Cancer is a disease with a long latency period, and hence it could be that the
exposure which is related to the ultimate development of the disease took place many
years before the cancer actually manifested itself. This phenomenon is counterintuitive, since one would have expected that cumulative exposure over a working
lifetime would have been the relevant parameter. The exposure at the time of
manifestation of disease may not be of particular importance.
The pattern of exposure—continuous exposure, intermittent exposure and exposure
with or without sharp peaks—may be relevant as well. Taking exposure patterns into
account is important for both epidemiological studies and for environmental
measurements which may be used to monitor compliance with health standards or for
environmental control as part of control and prevention programmes. For example, if
a health effect is caused by peak exposures, such peak levels must be monitorable in
order to be controlled. Monitoring which provides data only about long-term average
exposures is not useful since the peak excursion values may well be masked by
averaging, and certainly cannot be controlled as they occur.
The biologically relevant exposure or dose for a certain endpoint is often not known
because the patterns of intake, uptake, distribution and elimination, or the mechanisms
of biotransformation, are not understood in sufficient detail. Both the rate at which an
agent enters and leaves the body (the kinetics) and the biochemical processes for
handling the substance (biotransformation) will help determine the relationships
between exposure, dose and effect.
Environmental monitoring is the measurement and assessment of agents at the
workplace to evaluate ambient exposure and related health risks. Biological
monitoring is the measurement and assessment of workplace agents or their
metabolites in tissue, secreta or excreta to evaluate exposure and assess health risks.
Sometimes biomarkers, such as DNA-adducts, are used as measures of exposure.
Biomarkers may also be indicative of the mechanisms of the disease process itself, but
this is a complex subject, which is covered more fully in the chapter Biological
Monitoring and later in the discussion here.
A simplification of the basic model in exposure-response modelling is as follows:
exposure → uptake → distribution, elimination, transformation → target dose
→ physiopathology → effect
Depending on the agent, exposure-uptake and exposure-intake relationships can be
complex. For many gases, simple approximations can be made, based on the
concentration of the agent in the air during the course of a working day and on the
amount of air that is inhaled. For dust sampling, deposition patterns are also related to
particle size. Size considerations may also lead to a more complex relationship. The
chapter Respiratory System provides more detail on the aspect of respiratory toxicity.
Exposure and dose assessment are elements of quantitative risk assessment. Health
risk assessment methods often form the basis upon which exposure limits are
established for emission levels of toxic agents in the air for environmental as well as
for occupational standards. Health risk analysis provides an estimate of the probability
(risk) of occurrence of specific health effects or an estimate of the number of cases
with these health effects. By means of health risk analysis an acceptable concentration
of a toxicant in air, water or food can be provided, given an a priori chosen acceptable
magnitude of risk. Quantitative risk analysis has found an application in cancer
epidemiology, which explains the strong emphasis on retrospective exposure
assessment. But applications of more elaborate exposure assessment strategies can be
found in both retrospective as well as prospective exposure assessment, and exposure
assessment principles have found applications in studies focused on other endpoints as
well, such as benign respiratory disease (Wegman et al. 1992; Post et al. 1994). Two
directions in research predominate at this moment. One uses dose estimates obtained
from exposure monitoring information, and the other relies on biomarkers as measures
of exposure.
Exposure Monitoring and Prediction of Dose
Unfortunately, for many exposures few quantitative data are available for predicting
the risk for developing a certain endpoint. As early as 1924, Haber postulated that the
severity of the health effect (H) is proportional to the product of exposure
concentration (X) and time of exposure (T):
Haber’s law, as it is called, formed the basis for development of the concept that timeweighted average (TWA) exposure measurements—that is, measurements taken and
averaged over a certain period of time—would be a useful measure for the exposure.
This assumption about the adequacy of the time-weighted average has been
questioned for many years. In 1952, Adams and co-workers stated that “there is no
scientific basis for the use of the time-weighted average to integrate varying exposures
…” (in Atherly 1985). The problem is that many relations are more complex than the
relationship that Haber’s law represents. There are many examples of agents where
the effect is more strongly determined by concentration than by length of time. For
example, interesting evidence from laboratory studies has shown that in rats exposed
to carbon tetrachloride, the pattern of exposure (continuous versus intermittent and
with or without peaks) as well as the dose can modify the observed risk of the rats
developing liver enzyme level changes (Bogers et al. 1987). Another example is bioaerosols, such as a-amylase enzyme, a dough improver, which can cause allergic
diseases in people who work in the bakery industry (Houba et al. 1996). It is unknown
whether the risk of developing such a disease is mainly determined by peak
exposures, average exposure, or cumulative level of exposure. (Wong 1987;
Checkoway and Rice 1992). Information on temporal patterns is not available for
most agents, especially not for agents that have chronic effects.
The first attempts to model exposure patterns and estimate dose were published in the
1960s and 1970s by Roach (1966; 1977). He showed that the concentration of an
agent reaches an equilibrium value at the receptor after an exposure of infinite
duration because elimination counterbalances the uptake of the agent. In an eight-hour
exposure, a value of 90% of this equilibrium level can be reached if the half-life of the
agent at the target organ is smaller than approximately two-and-a-half hours. This
illustrates that for agents with a short half-life, the dose at the target organ is
determined by an exposure shorter than an eight-hour period. Dose at the target organ
is a function of the product of exposure time and concentration for agents with a long
half-life. A similar but more elaborate approach has been applied by Rappaport
(1985). He showed that intra-day variability in exposure has a limited influence when
dealing with agents with long half-lives. He introduced the term dampening at the
The information presented above has mainly been used to draw conclusions on
appropriate averaging times for exposure measurements for compliance purposes.
Since Roach’s papers it is common knowledge that for irritants, grab samples with
short averaging times have to be taken, while for agents with long half-lives, such as
asbestos, long-term average of cumulative exposure has to be approximated. One
should however realize that the dichotomization into grab sample strategies and eighthour time average exposure strategies as adopted in many countries for compliance
purposes is an extremely crude translation of the biological principles discussed
An example of improving an exposure assessment strategy based on pharmocokinetic
principles in epidemiology can be found in a paper of Wegman et al. (1992). They
applied an interesting exposure assessment strategy by using continuous monitoring
devices to measure personal dust exposure peak levels and relating these to acute
reversible respiratory symptoms occurring every 15 minutes.A conceptual problem in
this kind of study, extensively discussed in their paper, is the definition of a healthrelevant peak exposure. The definition of a peak will, again, depend on biological
considerations. Rappaport (1991) gives two requirements for peak exposures to be of
aetiological relevance in the disease process: (1) the agent is eliminated rapidly from
the body and (2) there is a non-linear rate of biological damage during a peak
exposure. Non-linear rates of biological damage may be related to changes in uptake,
which in turn are related to exposure levels, host susceptibility, synergy with other
exposures, involvement of other disease mechanisms at higher exposures or threshold
levels for disease processes.
These examples also show that pharmacokinetic approaches can lead elsewhere than
to dose estimates. The results of pharmacokinetic modelling can also be used to
explore the biological relevance of existing indices of exposure and to design new
health-relevant exposure assessment strategies.
Pharmacokinetic modelling of the exposure may also generate estimates of the actual
dose at the target organ. For instance in the case of ozone, an acute irritant gas,
models have been developed which predict the tissue concentration in the airways as a
function of the average ozone concentration in the airspace of the lung at a certain
distance from the trachea, the radius of the airways, the average air velocity, the
effective dispersion, and the ozone flux from air to lung surface (Menzel 1987; Miller
and Overton 1989). Such models can be used to predict ozone dose in a particular
region of the airways, dependent on environmental ozone concentrations and
breathing patterns.
In most cases estimates of target dose are based on information on the exposure
pattern over time, job history and pharmacokinetic information on uptake,
distribution, elimination and transformation of the agent. The whole process can be
described by a set of equations which can be mathematically solved. Often
information on pharmacokinetic parameters is not available for humans, and
parameter estimates based on animal experiments have to be used. There are several
examples by now of the use of pharmacokinetic modelling of exposure in order to
generate dose estimates. The first references to modelling of exposure data into dose
estimates in the literature go back to the paper of Jahr (1974).
Although dose estimates have generally not been validated and have found limited
application in epidemiological studies, the new generation of exposure or dose indices
is expected to result in optimal exposure-response analyses in epidemiological studies
(Smith 1985, 1987). A problem not yet tackled in pharmacokinetic modelling is that
large interspecies differences exist in kinetics of toxic agents, and therefore effects of
intra-individual variation in pharmacokinetic parameters are of interest (Droz 1992).
Biomonitoring and Biomarkers of Exposure
Biological monitoring offers an estimate of dose and therefore is often considered
superior to environmental monitoring. However, the intra-individual variability of
biomonitoring indices can be considerable. In order to derive an acceptable estimate
of a worker’s dose, repeated measurements have to be taken, and sometimes the
measurement effort can become larger than for environmental monitoring.
This is illustrated by an interesting study on workers producing boats made of plastic
reinforced with glass fibre (Rappaport et al. 1995). The variability of styrene exposure
was assessed by measuring styrene in air repeatedly. Styrene in exhaled air of exposed
workers was monitored, as well as sister chromatid exchanges (SCEs). They showed
that an epidemiological study using styrene in the air as a measure of exposure would
be more efficient, in terms of numbers of measurements required, than a study using
the other indices of exposure. For styrene in air three repeats were required to estimate
the long-term average exposure with a given precision. For styrene in exhaled air, four
repeats per worker were necessary, while for the SCEs 20 repeats were necessary. The
explanation for this observation is the signal-to-noise ratio, determined by the day-today and between-worker variability in exposure, which was more favourable for
styrene in air than for the two biomarkers of exposure. Thus, although the biological
relevance of a certain exposure surrogate might be optimal, the performance in an
exposure-response analysis can still be poor because of a limited signal-to-noise ratio,
leading to misclassification error.
Droz (1991) applied pharmacokinetic modelling to study advantages of exposure
assessment strategies based on air sampling compared to biomonitoring strategies
dependent on the half-life of the agent. He showed that biological monitoring is also
greatly affected by biological variability, which is not related to variability of the
toxicological test. He suggested that no statistical advantage exists in using biological
indicators when the half-life of the agent considered is smaller than about ten hours.
Although one might tend to decide to measure the environmental exposure instead of
a biological indicator of an effect because of variability in the variable measured,
additional arguments can be found for choosing a biomarker, even when this would
lead to a greater measurement effort, such as when a considerable dermal exposure is
present. For agents like pesticides and some organic solvents, dermal exposure can be
of greater relevance that the exposure through the air. A biomarker of exposure would
include this route of exposure, while measuring of dermal exposure is complex and
results are not easily interpretable (Boleij et al. 1995). Early studies among
agricultural workers using “pads” to assess dermal exposure showed remarkable
distributions of pesticides over the body surface, depending on the tasks of the
worker. However, because little information is available on skin uptake, exposure
profiles cannot yet be used to estimate an internal dose.
Biomarkers can also have considerable advantages in cancer epidemiology. When a
biomarker is an early marker of the effect, its use could result in reduction of the
follow-up period. Although validation studies are required, biomarkers of exposure or
individual susceptibility could result in more powerful epidemiological studies and
more precise risk estimates.
Time Window Analysis
Parallel to the development of pharmacokinetic modelling, epidemiologists have
explored new approaches in the data analysis phase such as “time frame analysis” to
relate relevant exposure periods to endpoints, and to implement effects of temporal
patterns in the exposure or peak exposures in occupational cancer epidemiology
(Checkoway and Rice 1992). Conceptually this technique is related to
pharmacokinetic modelling since the relationship between exposure and outcome is
optimized by putting weights on different exposure periods, exposure patterns and
exposure levels. In pharmacokinetic modelling these weights are believed to have a
physiological meaning and are estimated beforehand. In time frame analysis the
weights are estimated from the data on the basis of statistical criteria. Examples of this
approach are given by Hodgson and Jones (1990), who analysed the relationship
between radon gas exposure and lung cancer in a cohort of UK tin miners, and by
Seixas, Robins and Becker (1993), who analysed the relationship between dust
exposure and respiratory health in a cohort of US coal miners. A very interesting
study underlining the relevance of time window analysis is the one by Peto et al.
They showed that mesothelioma death rates appeared to be proportional to some
function of time since first exposure and cumulative exposure in a cohort of insulation
workers. Time since first exposure was of particular relevance because this variable
was an approximation of the time required for a fibre to migrate from its place of
deposition in the lungs to the pleura. This example shows how kinetics of deposition
and migration determine the risk function to a large extent. A potential problem with
time frame analysis is that it requires detailed information on exposure periods and
exposure levels, which hampers its application in many studies of chronic disease
Concluding Remarks
In conclusion, the underlying principles of pharmacokinetic modelling and time frame
or time window analysis are widely recognized. Knowledge in this area has mainly
been used to develop exposure assessment strategies. More elaborate use of these
approaches, however, requires a considerable research effort and has to be developed.
The number of applications is therefore still limited. Relatively simple applications,
such as the development of more optimal exposure assessment strategies dependent
on the endpoint, have found wider use. An important issue in the development of
biomarkers of exposure or effect is validation of these indices. It is often assumed that
a measurable biomarker can predict health risk better than traditional methods.
However, unfortunately, very few validation studies substantiate this assumption.
Dennis J. Paustenbach
The History of Occupational Exposure Limits
Over the past 40 years, many organizations in numerous countries have proposed
occupational exposure limits (OELs) for airborne contaminants. The limits or
guidelines that have gradually become the most widely accepted both in the United
States and in most other countries are those issued annually by the American
Conference of Governmental Industrial Hygienists (ACGIH), which are termed
threshold limit values (TLVs) (LaNier 1984; Cook 1986; ACGIH 1994).
The usefulness of establishing OELs for potentially harmful agents in the working
environment has been demonstrated repeatedly since their inception (Stokinger 1970;
Cook 1986; Doull 1994). The contribution of OELs to the prevention or minimization
of disease is now widely accepted, but for many years such limits did not exist, and
even when they did, they were often not observed (Cook 1945; Smyth 1956;
Stokinger 1981; LaNier 1984; Cook 1986).
It was well understood as long ago as the fifteenth century, that airborne dusts and
chemicals could bring about illness and injury, but the concentrations and lengths of
exposure at which this might be expected to occur were unclear (Ramazinni 1700).
As reported by Baetjer (1980), “early in this century when Dr. Alice Hamilton began
her distinguished career in occupational disease, no air samples and no standards were
available to her, nor indeed were they necessary. Simple observation of the working
conditions and the illness and deaths of the workers readily proved that harmful
exposures existed. Soon however, the need for determining standards for safe
exposure became obvious.”
The earliest efforts to set an OEL were directed to carbon monoxide, the toxic gas to
which more persons are occupationally exposed than to any other (for a chronology of
the development of OELs, see figure 30.9 . The work of Max Gruber at the Hygienic
Institute at Munich was published in 1883. The paper described exposing two hens
and twelve rabbits to known concentrations of carbon monoxide for up to 47 hours
over three days; he stated that “the boundary of injurious action of carbon monoxide
lies at a concentration in all probability of 500 parts per million, but certainly (not less
than) 200 parts per million”. In arriving at this conclusion, Gruber had also inhaled
carbon monoxide himself. He reported no symptoms or uncomfortable sensations
after three hours on each of two consecutive days at concentrations of 210 parts per
million and 240 parts per million (Cook 1986).
Figure 30.9 Chronology of occupational exposure levels (OELS)
The earliest and most extensive series of animal experiments on exposure limits were
those conducted by K.B. Lehmann and others under his direction. In a series of
publications spanning 50 years they reported on studies on ammonia and hydrogen
chloride gas, chlorinated hydrocarbons and a large number of other chemical
substances (Lehmann 1886; Lehmann and Schmidt-Kehl 1936).
Kobert (1912) published one of the earlier tables of acute exposure limits.
Concentrations for 20 substances were listed under the headings: (1) rapidly fatal to
man and animals, (2) dangerous in 0.5 to one hour, (3) 0.5 to one hour without serious
disturbances and (4) only minimal symptoms observed. In his paper “Interpretations
of permissible limits”, Schrenk (1947) notes that the “values for hydrochloric acid,
hydrogen cyanide, ammonia, chlorine and bromine as given under the heading ‘only
minimal symptoms after several hours’ in the foregoing Kobert paper agree with
values as usually accepted in present-day tables of MACs for reported exposures”.
However, values for some of the more toxic organic solvents, such as benzene, carbon
tetrachloride and carbon disulphide, far exceeded those currently in use (Cook 1986).
One of the first tables of exposure limits to originate in the United States was that
published by the US Bureau of Mines (Fieldner, Katz and Kenney 1921). Although its
title does not so indicate, the 33 substances listed are those encountered in
workplaces. Cook (1986) also noted that most of the exposure limits through the
1930s, except for dusts, were based on rather short animal experiments. A notable
exception was the study of chronic benzene exposure by Leonard Greenburg of the
US Public Health Service, conducted under the direction of a committee of the
National Safety Council (NSC 1926). An acceptable exposure for human beings based
on long-term animal experiments was derived from this work.
According to Cook (1986), for dust exposures, permissible limits established before
1920 were based on exposures of workers in the South African gold mines, where the
dust from drilling operations was high in crystalline free silica. In 1916, an exposure
limit of 8.5 million particles per cubic foot of air (mppcf) for the dust with an 80 to
90% quartz content was set (Phthisis Prevention Committee 1916). Later, the level
was lowered to 5 mppcf. Cook also reported that, in the United States, standards for
dust, also based on exposure of workers, were recommended by Higgins and coworkers following a study at the south-western Missouri zinc and lead mines in 1917.
The initial level established for high quartz dusts was ten mppcf, appreciably higher
than was established by later dust studies conducted by the US Public Health Service.
In 1930, the USSR Ministry of Labour issued a decree that included maximum
allowable concentrations for 12 industrial toxic substances.
The most comprehensive list of occupational exposure limits up to 1926 was for 27
substances (Sayers 1927). In 1935 Sayers and Dalle Valle published physiological
responses to five concentrations of 37 substances, the fifth being the maximum
allowable concentration for prolonged exposure. Lehmann and Flury (1938) and
Bowditch et al. (1940) published papers that presented tables with a single value for
repeated exposures to each substance.
Many of the exposure limits developed by Lehmann were included in a monograph
initially published in 1927 by Henderson and Haggard (1943), and a little later in
Flury and Zernik’s Schadliche Gase (1931). According to Cook (1986), this book was
considered the authoritative reference on effects of injurious gases, vapours and dusts
in the workplace until Volume II of Patty’s Industrial Hygiene and Toxicology (1949)
was published.
The first lists of standards for chemical exposures in industry, called maximum
allowable concentrations (MACs), were prepared in 1939 and 1940 (Baetjer 1980).
They represented a consensus of opinion of the American Standard Association and a
number of industrial hygienists who had formed the ACGIH in 1938. These
“suggested standards” were published in 1943 by James Sterner. A committee of the
ACGIH met in early 1940 to begin the task of identifying safe levels of exposure to
workplace chemicals, by assembling all the data which would relate the degree of
exposure to a toxicant to the likelihood of producing an adverse effect (Stokinger
1981; LaNier 1984). The first set of values were released in 1941 by this committee,
which was composed of Warren Cook, Manfred Boditch (reportedly the first hygienist
employed by industry in the United States), William Fredrick, Philip Drinker,
Lawrence Fairhall and Alan Dooley (Stokinger 1981).
In 1941, a committee (designated as Z-37) of the American Standards Association,
which later became the American National Standards Institute, developed its first
standard of 100 ppm for carbon monoxide. By 1974 the committee had issued
separate bulletins for 33 exposure standards for toxic dusts and gases.
At the annual meeting of the ACGIH in 1942, the newly appointed Subcommittee on
Threshold Limits presented in its report a table of 63 toxic substances with the
“maximum allowable concentrations of atmospheric contaminants” from lists
furnished by the various state industrial hygiene units. The report contains the
statement, “The table is not to be construed as recommended safe concentrations. The
material is presented without comment” (Cook 1986).
In 1945 a list of 132 industrial atmospheric contaminants with maximum allowable
concentrations was published by Cook, including the then current values for six states,
as well as values presented as a guide for occupational disease control by federal
agencies and maximum allowable concentrations that appeared best supported by the
references on original investigations (Cook 1986).
At the 1946 annual meeting of ACGIH, the Subcommittee on Threshold Limits
presented their second report with the values of 131 gases, vapours, dusts, fumes and
mists, and 13 mineral dusts. The values were compiled from the list reported by the
subcommittee in 1942, from the list published by Warren Cook in Industrial Medicine
(1945) and from published values of the Z-37 Committee of the American Standards
Association. The committee emphasized that the “list of M.A.C. values is presented
… with the definite understanding that it be subject to annual revision.”
Intended use of OELs
The ACGIH TLVs and most other OELs used in the United States and some other
countries are limits which refer to airborne concentrations of substances and represent
conditions under which “it is believed that nearly all workers may be repeatedly
exposed day after day without adverse health effects” (ACGIH 1994). (See table
30.2 .) In some countries the OEL is set at a concentration which will protect virtually
everyone. It is important to recognize that unlike some exposure limits for ambient air
pollutants, contaminated water, or food additives set by other professional groups or
regulatory agencies, exposure to the TLV will not necessarily prevent discomfort or
injury for everyone who is exposed (Adkins et al. 1990). The ACGIH recognized long
ago that because of the wide range in individual susceptibility, a small percentage of
workers may experience discomfort from some substances at concentrations at or
below the threshold limit and that a smaller percentage may be affected more
seriously by aggravation of a pre-existing condition or by development of an
occupational illness (Cooper 1973; ACGIH 1994). This is clearly stated in the
introduction to the ACGIH’s annual booklet Threshold Limit Values for Chemical
Substances and Physical Agents and Biological Exposure Indices (ACGIH 1994).
Table 30.2 Occupational exposure limits (OELs) in various countries (as of 1986)
Type of standard
OELs are essentially the same as those of the 1978 ACGIH TLVs. The
principal difference from the ACGIH list is that, for the 144 substances (of
the total of 630) for which no STELs are listed by ACGIH, the values used
for the Argentina TWAs are entered also under this heading.
The National Health and Medical Research Council (NHMRC) adopted a
revised edition of the Occupational Health Guide Threshold Limit Values
(1990-91) in 1992. The OELs have no legal status in Australia, except
where specifically incorporated into law by reference. The ACGIHTLVs
are published in Australia as an appendix to the occupational health
guides, revised with the ACGIH revisions in odd-numbered years.
The values recommended by the Expert Committee of the Worker
Protection Commission for Appraisal of MAC (maximal acceptable
concentration) Values in cooperation with the General Accident
Prevention Institute of the Chemical Workers Trade Union, is considered
obligatory by the Federal Ministry for Social Administration. They are
applied by the Labour Inspectorate under the Labour Protection Law.
The Administration of Hygiene and Occupational Medicine of the
Ministry of Employment and of Labour uses the TLVs of the ACGIH as a
The TLVs of the ACGIH have been used as the basis for the occupational
health legislation of Brazil since 1978. As the Brazilian work week is
usually 48 hours, the values of the ACGIH were adjusted in conformity
with a formula developed for this purpose. The ACGIH list was adopted
only for those air contaminants which at the time had nationwide
application. The Ministry of Labour has brought the limits up to date with
establishment of values for additional contaminants in accordance with
recommendations from the Fundacentro Foundation of Occupational
Safety and Medicine.
Canada (and
Each province has its own regulations:
OELs are under the Occupational Health and Safety Act, Chemical Hazard
Regulation, which requires the employer to ensure that workers are not
exposed above the limits.
British Columbia
The Industrial Health and Safety Regulations set legal requirements for
most of British Columbia industry, which refer to the current schedule of
TLVs for atmospheric contaminants published by the ACGIH.
The Department of Environment and Workplace Safety and Health is
responsible for legislation and its administration concerning the OELs.
The guidelines currently used to interpret risk to health are the ACGIH
TLVs with the exception that carcinogens are given a zero exposure level
“so far as is reasonably practicable”.
New Brunswick
The applicable standards are those published in the latest ACGIH issue
and, in case of an infraction, it is the issue in publication at the time of
infraction that dictates compliance.
The Northwest Territories Safety Division of the Justice and Service
Department regulates workplace safety for non-federal employees under
the latest edition of the ACGIH TLVs.
Nova Scotia
The list of OELs is the same as that of the ACGIH as published in 1976
and its subsequent amendments and revisions.
Regulations for a number of hazardous substances are enforced under the
Occupational Health and Safety Act, published each in a separate booklet
that includes the permissible exposure level and codes for respiratory
equipment, techniques for measuring airborne concentrations and medical
surveillance approaches.
Permissible exposure levels are similar to the ACGIH TLVs and
compliance with the permissible exposure levels for workplace air
contaminants is required.
The maximum concentration of eleven substances having the capacity of
causing acute, severe or fatal effects cannot be exceeded for even a
moment. The values in the Chile standard are those of the ACGIH TLVs
to which a factor of 0.8 is applied in view of the 48-hour week.
OELs include values for 542 chemical substances and 20 particulates. It is
legally required that these not be exceeded as time-weighted averages.
Data from the ACGIH are used in the preparation of the Danish standards.
About 25 per cent of the values are different from those of ACGIH with
nearly all of these being somewhat more stringent.
Ecuador does not have a list of permissible exposure levels incorporated in
its legislation. The TLVs of the ACGIH are used as a guide for good
industrial hygiene practice.
OELs are defined as concentrations that are deemed to be hazardous to at
least some workers on long-term exposure. Whereas the ACGIH has as
their philosophy that nearly all workers may be exposed to substances
below the TLV without adverse effect, the viewpoint in Finland is that
where exposures are above the limiting value, deleterious effects on health
may occur.
The MAC value is “the maximum permissible concentration of a chemical
compound present in the air within a working area (as gas, vapour,
particulate matter) which, according to current knowledge, generally does
not impair the health of the employee nor cause undue annoyance. Under
these conditions, exposure can be repeated and of long duration over a
daily period of eight hours, constituting an average work week of 40 hours
(42 hours per week as averaged over four successive weeks for firms
having four work shifts).… Scientifically based criteria for health
protection, rather than their technical or economical feasibility, are
The latest TLVs of the ACGIH are normally used. However, the ACGIH
list is not incorporated in the national laws or regulations.
MAC values are taken largely from the list of the ACGIH, as well as from
the Federal Republic of Germany and NIOSH. The MAC is defined as
“that concentration in the workplace air which, according to present
knowledge, after repeated long-term exposure even up to a whole working
life, in general does not harm the health of workers or their offspring.”
The 1970 TLVs of the ACGIH are used, except 50 ppm for vinyl chloride
and 0.15 mg/m3 for lead, inorganic compounds, fume and dust.
The former USSR established many of its limits with the goal of
eliminating any possibility for even reversible effects. Such subclinical
and fully reversible responses to workplace exposures have, thus far, been
considered too restrictive to be useful in the United States and in most
other countries. In fact, due to the economic and engineering difficulties in
achieving such low levels of air contaminants in the workplace, there is
little indication that these limits have actually been achieved in countries
which have adopted them. Instead, the limits appear to serve more as
idealized goals rather than limits which manufacturers are legally bound or
morally committed to achieve.
United States
At least six groups recommend exposure limits for the workplace: the
TLVs of the ACGIH, the Recommended Exposure Limits (RELs)
suggested by the National Institute for Occupational Safety and Health
(NIOSH), the Workplace Environment Exposure Limits (WEEL)
developed by the American Industrial Hygiene Association (AIHA),
standards for workplace air contaminants suggested by the Z-37
Committee of the American National Standards Institute (EAL), the
proposed workplace guides of the American Public Health Association
(APHA 1991), and recommendations by local, state or regional
governments. In addition, permissible exposure limits (PELs), which are
regulations that must be met in the workplace because they are law, have
been promulgated by the Department of Labor and are enforced by the
Occupational Safety and Health Administration (OSHA).
Source: Cook 1986.
This limitation, although perhaps less than ideal, has been considered a practical one
since airborne concentrations so low as to protect hypersusceptibles have traditionally
been judged infeasible due to either engineering or economic limitations. Until about
1990, this shortcoming in the TLVs was not considered a serious one. In light of the
dramatic improvements since the mid-1980s in our analytical capabilities, personal
monitoring/sampling devices, biological monitoring techniques and the use of robots
as a plausible engineering control, we are now technologically able to consider more
stringent occupational exposure limits.
The background information and rationale for each TLV are published periodically in
the Documentation of the Threshold Limit Values (ACGIH 1995). Some type of
documentation is occasionally available for OELs set in other countries. The rationale
or documentation for a particular OEL should always be consulted before interpreting
or adjusting an exposure limit, as well as the specific data that were considered in
establishing it (ACGIH 1994).
TLVs are based on the best available information from industrial experience and
human and animal experimental studies—when possible, from a combination of these
sources (Smith and Olishifski 1988; ACGIH 1994). The rationale for choosing
limiting values differs from substance to substance. For example, protection against
impairment of health may be a guiding factor for some, whereas reasonable freedom
from irritation, narcosis, nuisance or other forms of stress may form the basis for
others. The age and completeness of the information available for establishing
occupational exposure limits also varies from substance to substance; consequently,
the precision of each TLV is different. The most recent TLV and its documentation
(or its equivalent) should always be consulted in order to evaluate the quality of the
data upon which that value was set.
Even though all of the publications which contain OELs emphasize that they were
intended for use only in establishing safe levels of exposure for persons in the
workplace, they have been used at times in other situations. It is for this reason that all
exposure limits should be interpreted and applied only by someone knowledgeable of
industrial hygiene and toxicology. The TLV Committee (ACGIH 1994) did not intend
that they be used, or modified for use:
as a relative index of hazard or toxicity
in the evaluation of community air pollution
· for estimating the hazards of continuous, uninterrupted exposures or other
extended work periods
as proof or disproof of an existing disease or physical condition
· for adoption by countries whose working conditions differ from those of the
United States.
The TLV Committee and other groups which set OELs warn that these values should
not be “directly used” or extrapolated to predict safe levels of exposure for other
exposure settings. However, if one understands the scientific rationale for the
guideline and the appropriate approaches for extrapolating data, they can be used to
predict acceptable levels of exposure for many different kinds of exposure scenarios
and work schedules (ACGIH 1994; Hickey and Reist 1979).
Philosophy and approaches in setting exposure limits
TLVs were originally prepared to serve only for the use of industrial hygienists, who
could exercise their own judgement in applying these values. They were not to be
used for legal purposes (Baetjer 1980). However, in 1968 the United States WalshHealey Public Contract Act incorporated the 1968 TLV list, which covered about 400
chemicals. In the United States, when the Occupational Safety and Health Act
(OSHA) was passed it required all standards to be national consensus standards or
established federal standards.
Exposure limits for workplace air contaminants are based on the premise that,
although all chemical substances are toxic at some concentration when experienced
for a period of time, a concentration (e.g., dose) does exist for all substances at which
no injurious effect should result no matter how often the exposure is repeated. A
similar premise applies to substances whose effects are limited to irritation, narcosis,
nuisance or other forms of stress (Stokinger 1981; ACGIH 1994).
This philosophy thus differs from that applied to physical agents such as ionizing
radiation, and for some chemical carcinogens, since it is possible that there may be no
threshold or no dose at which zero risk would be expected (Stokinger 1981). The issue
of threshold effects is controversial, with reputable scientists arguing both for and
against threshold theories (Seiler 1977; Watanabe et al. 1980, Stott et al. 1981;
Butterworth and Slaga 1987; Bailer et al. 1988; Wilkinson 1988; Bus and Gibson
1994). With this in mind, some occupational exposure limits proposed by regulatory
agencies in the early 1980s were set at levels which, although not completely without
risk, posed risks that were no greater than classic occupational hazards such as
electrocution, falls, and so on. Even in those settings which do not use industrial
chemicals, the overall workplace risks of fatal injury are about one in one thousand.
This is the rationale that has been used to justify selecting this theoretical cancer risk
criterion for setting TLVs for chemical carcinogens (Rodricks, Brett and Wrenn 1987;
Travis et al. 1987).
Occupational exposure limits established both in the United States and elsewhere are
derived from a wide variety of sources. The 1968 TLVs (those adopted by OSHA in
1970 as federal regulations) were based largely on human experience. This may come
as a surprise to many hygienists who have recently entered the profession, since it
indicates that, in most cases, the setting of an exposure limit has come after a
substance has been found to have toxic, irritational or otherwise undesirable effects on
humans. As might be anticipated, many of the more recent exposure limits for
systemic toxins, especially those internal limits set by manufacturers, have been based
primarily on toxicology tests conducted on animals, in contrast to waiting for
observations of adverse effects in exposed workers (Paustenbach and Langner 1986).
However, even as far back as 1945, animal tests were acknowledged by the TLV
Committee to be very valuable and they do, in fact, constitute the second most
common source of information upon which these guidelines have been based
(Stokinger 1970).
Several approaches for deriving OELs from animal data have been proposed and put
into use over the past 40 years. The approach used by the TLV Committee and others
is not markedly different from that which has been used by the US Food and Drug
Administration (FDA) in establishing acceptable daily intakes (ADI) for food
additives. An understanding of the FDA approach to setting exposure limits for food
additives and contaminants can provide good insight to industrial hygienists who are
involved in interpreting OELs (Dourson and Stara 1983).
Discussions of methodological approaches which can be used to establish workplace
exposure limits based exclusively on animal data have also been presented (Weil
1972; WHO 1977; Zielhuis and van der Kreek 1979a, 1979b; Calabrese 1983;
Dourson and Stara 1983; Leung and Paustenbach 1988a; Finley et al. 1992;
Paustenbach 1995). Although these approaches have some degree of uncertainty, they
seem to be much better than a qualitative extrapolation of animal test results to
Approximately 50% of the 1968 TLVs were derived from human data, and
approximately 30% were derived from animal data. By 1992, almost 50% were
derived primarily from animal data. The criteria used to develop the TLVs may be
classified into four groups: morphological, functional, biochemical and miscellaneous
(nuisance, cosmetic). Of those TLVs based on human data, most are derived from
effects observed in workers who were exposed to the substance for many years.
Consequently, most of the existing TLVs have been based on the results of workplace
monitoring, compiled with qualitative and quantitative observations of the human
response (Stokinger 1970; Park and Snee 1983). In recent times, TLVs for new
chemicals have been based primarily on the results of animal studies rather than
human experience (Leung and Paustenbach 1988b; Leung et al. 1988).
It is noteworthy that in 1968 only about 50% of the TLVs were intended primarily to
prevent systemic toxic effects. Roughly 40% were based on irritation and about two
per cent were intended to prevent cancer. By 1993, about 50% were meant to prevent
systemic effects, 35% to prevent irritation, and five per cent to prevent cancer. Figure
30.10 provides a summary of the data often used in developing OELs.
Figure 30.10 Data often used in developing an occupational exposure
Limits for irritants
Prior to 1975, OELs designed to prevent irritation were largely based on human
experiments. Since then, several experimental animal models have been developed
(Kane and Alarie 1977; Alarie 1981; Abraham et al. 1990; Nielsen 1991). Another
model based on chemical properties has been used to set preliminary OELs for
organic acids and bases (Leung and Paustenbach 1988).
Limits for carcinogens
In 1972, the ACGIH Committee began to distinguish between human and animal
carcinogens in its TLV list. According to Stokinger (1977), one reason for this
distinction was to assist the stakeholders in discussions (union representatives,
workers and the public) in focusing on those chemicals with more probable workplace
Do the TLVs Protect Enough Workers?
Beginning in 1988, concerns were raised by numerous persons regarding the adequacy
or health protectiveness of TLVs. The key question raised was, what percentage of the
working population is truly protected from adverse health effects when exposed to the
Castleman and Ziem (1988) and Ziem and Castleman (1989) argued both that the
scientific basis of the standards was inadequate and that they were formulated by
hygienists with vested interests in the industries being regulated.
These papers engendered an enormous amount of discussion, both supportive of and
opposed to the work of the ACGIH (Finklea 1988; Paustenbach 1990a, 1990b, 1990c;
Tarlau 1990).
A follow-up study by Roach and Rappaport (1990) attempted to quantify the safety
margin and scientific validity of the TLVs. They concluded that there were serious
inconsistencies between the scientific data available and the interpretation given in the
1976 Documentation by the TLV Committee. They also note that the TLVs were
probably reflective of what the Committee perceived to be realistic and attainable at
the time. Both the Roach and Rappaport and the Castleman and Ziem analyses have
been responded to by the ACGIH, who have insisted on the inaccuracy of the
Although the merit of the Roach and Rappaport analysis, or for that matter, that of
Ziem and Castleman, will be debated for a number of years, it is clear that the process
by which TLVs and other OELs will be set will probably never be as it was between
1945 and 1990. It is likely that in coming years, the rationale, as well as the degree of
risk inherent in a TLV, will be more explicitly described in the documentation for
each TLV. Also, it is certain that the definition of “virtually safe” or “insignificant
risk” with respect to workplace exposure will change as the values of society change
(Paustenbach 1995, 1997).
The degree of reduction in TLVs or other OELs that will undoubtedly occur in the
coming years will vary depending on the type of adverse health effect to be prevented
(central nervous system depression, acute toxicity, odour, irritation, developmental
effects, or others). It is unclear to what degree the TLV committee will rely on various
predictive toxicity models, or what risk criteria they will adopt, as we enter the next
Standards and Nontraditional Work Schedules
The degree to which shift work affects a worker’s capabilities, longevity, mortality,
and overall well-being is still not well understood. So-called nontraditional work
shifts and work schedules have been implemented in a number of industries in an
attempt to eliminate, or at least reduce, some of the problems caused by normal shift
work, which consists of three eight-hour work shifts per day. One kind of work
schedule which is classified as nontraditional is the type involving work periods
longer than eight hours and varying (compressing) the number of days worked per
week (e.g., a 12-hours-per-day, three-day workweek). Another type of nontraditional
work schedule involves a series of brief exposures to a chemical or physical agent
during a given work schedule (e.g., a schedule where a person is exposed to a
chemical for 30 minutes, five times per day with one hour between exposures). The
last category of nontraditional schedule is that involving the “critical case” wherein
persons are continuously exposed to an air contaminant (e.g., spacecraft, submarine).
Compressed workweeks are a type of nontraditional work schedule that has been used
primarily in non-manufacturing settings. It refers to full-time employment (virtually
40 hours per week) which is accomplished in less than five days per week. Many
compressed schedules are currently in use, but the most common are: (a) four-day
workweeks with ten-hour days; (b) three-day workweeks with 12-hour days; (c) 41/2–day workweeks with four nine-hour days and one four-hour day (usually Friday);
and (d) the five/four, nine plan of alternating five-day and four-day workweeks of
nine-hour days (Nollen and Martin 1978; Nollen 1981).
Of all workers, those on nontraditional schedules represent only about 5% of the
working population. Of this number, only about 50,000 to 200,000 Americans who
work nontraditional schedules are employed in industries where there is routine
exposure to significant levels of airborne chemicals. In Canada, the percentage of
chemical workers on nontraditional schedules is thought to be greater (Paustenbach
One Approach to Setting International OELs
As noted by Lundberg (1994), a challenge facing all national committees is to identify
a common scientific approach to setting OELs. Joint international ventures are
advantageous to the parties involved since writing criteria documents is both a timeand cost-consuming process (Paustenbach 1995).
This was the idea when the Nordic Council of Ministers in 1977 decided to establish
the Nordic Expert Group (NEG). The task of the NEG was to develop scientificallybased criteria documents to be used as a common scientific basis of OELs by the
regulatory authorities in the five Nordic countries (Denmark, Finland, Iceland,
Norway and Sweden). The criteria documents from the NEG lead to the definition of
a critical effect and dose-response/dose-effect relationships. The critical effect is the
adverse effect that occurs at the lowest exposure. There is no discussion of safety
factors and a numerical OEL is not proposed. Since 1987, criteria documents are
published by the NEG concurrently in English on a yearly basis.
Lundberg (1994) has suggested a standardized approach that each county would use.
He suggested building a document with the following characteristics:
· A standardized criteria document should reflect the up-to-date knowledge as
presented in the scientific literature.
· The literature used should preferably be peer-reviewed scientific papers but at
least be available publicly. Personal communications should be avoided. An openness
toward the general public, particularly workers, decreases the kind of suspiciousness
that recently has been addressed toward documentation from the ACGIH.
· The scientific committee should consist of independent scientists from academia
and government. If the committee should include scientific representatives from the
labour market, both employers and employees should be represented.
· All relevant epidemiological and experimental studies should be thoroughly
scrutinized by the scientific committee, especially “key studies” that present data on
the critical effect. All observed effects should be described.
· Environmental and biological monitoring possibilities should be pointed out. It is
also necessary to thoroughly scrutinize these data, including toxicokinetic data.
· Data permitting, the establishment of dose-response and dose-effect relationships
should be stated. A no observable effect level (NOEL) or lowest observable effect
level (LOEL) for each observed effect should be stated in the conclusion. If necessary,
reasons should be given as to why a certain effect is the critical one. The toxicological
significance of an effect is thereby considered.
· Specifically, mutagenic, carcinogenic and teratogenic properties should be pointed
out as well as allergic and immunological effects.
· A reference list for all studies described should be given. If it is stated in the
document that only relevant studies have been used, there is no need to give a list of
references not used or why. On the other hand, it could be of interest to list those
databases that have been used in the literature search.
There are in practice only minor differences in the way OELs are set in the various
countries that develop them. It should, therefore, be relatively easy to agree upon the
format of a standardized criteria document containing the key information. From this
point, the decision as to the size of the margin of safety that is incorporated in the
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