Air Sampling Strategies
PURPOSE
To assist those responsible for air sampling to make decisions about the sampling strategy appropriate
to their requirements.
SCOPE
This technical information document is designed for use by those responsible for the implementation of
air sampling strategies. It provides information about the selection of sampling methods, including the
numbers of samples appropriate, the duration and timing of sampling etc.
TABLE OF CONTENTS
1
Purpose of Air Sampling
1
2
Factors Influencing Air Sampling Results
1
3
Strategic Choices
3.1 Personal or Static (Area) Sampling
3.2 Duration and Timing of Sampling
Duration of Sampling
Timing of Sampling
3.3 Number of Samples
3
3
4
4
4
5
4
Phased Strategies
4.1 First Level Strategy
4.2 Second Level Strategy
4.3 Third Level Strategy
4.4 Other Considerations
6
6
6
6
6
5
Monitoring Programmes
5.1 Establishing a Programme
Agents to be Sampled
Jobs and Areas to be Sampled
Number of Samples Required
Frequency of Monitoring
The Nature of the Samples
5.2 Selection of Groups for Monitoring
5.3 Frequency of Monitoring
5.4 Review of the Programme
7
7
7
7
7
7
7
8
8
9
1
PURPOSE OF AIR SAMPLING
Levels of airborne contaminants may vary with time and place within a workplace, and the sampling
strategy that is adopted (how to sample, for how long, where etc), will therefore affect the results
obtained. It is essential to choose a strategy that is appropriate to the purpose of the
measurements. Several possible purposes can be identified:
 health risk assessment;
 provision of data for epidemiological purposes;
 determining compliance with Occupational Exposure Limits (OELs);
 investigating or monitoring the performance of control measures;
 investigation of complaints, eg about odour;
 validation or comparison of measurement methods.
In each case, samples may be taken as part of a specific investigation or as part of an ongoing
series of measurements to monitor changes in levels.
While general guidance can be given on the development of a sampling strategy, the final choice
depends on the individual situation. In some cases there may be local legal constraints on the way
samples are collected.
2
FACTORS INFLUENCING AIR SAMPLING RESULTS
Many variables affect the airborne concentrations of hazardous substances in the workplace.
Primary variables include:
 the number of sources from which the contaminant is released;
 the rates of release from each source, which depend on working practices, physicochemical
properties etc;
 the type and position of each source;
 the dispersion or mixing of the contaminant in the air of a workroom, as influenced by local
ventilation and random movements or turbulence in the air;
 the quantity of hazardous substance.
Such factors may produce rapid fluctuations in contaminant concentration or large variations over
very small distances. The variations may be random or systematic.
Systematic changes are those with an identifiable cause such as:
 a change in production rate;
 a change in process conditions;
 a change in the level of ventilation;
 conduct of an intermittent operation, eg charging a vessel, cleaning or maintenance.
Systematic changes can make concentrations vary by several orders of magnitude. They may be a
planned part of the process, or may occur unintentionally. They may last only for a few seconds, or
may be permanent changes. Identifying and controlling the causes of such changes are key tasks.
Figure 1 indicates how concentrations typically vary for an intermittent operation.
Air Sampling Strategies
Page 1
Concentration
Figure 1 Variations in Airborne Concentration Attributable to Intermittent Operations
100
90
80
70
60
Concentration
50
40
30
20
10
0
Time
Time
Random changes cause a residual variation when all systematic causes have been accounted for. They
may arise, for example, from fluctuations in the ventilation rate (draughts) or the rate at which an
employee works. These random variations can be surprisingly large, often amounting to a factor of three
either side of the mean concentration. Figure 2 indicates how random changes affect the measured
concentration on a process that may be considered to be stable.
Figure 2 Variations in Airborne Concentrations Caused by Random Fluctuations in a Stable
Process
Concentration
60
Concentration
50
40
30
20
10
0
Time
Time
As plant and process conditions and general ventilation characteristics may vary from day to day or
display a seasonal pattern, assumptions about long-term exposure patterns should not be made on
the basis of a single estimate of contaminant concentrations at one point in time.
Air Sampling Strategies
Page 2
The situation is made more complex when the actions and behaviour of employees are considered.
An employee's exposure will depend on:
 how close the various sources of exposure are and the length of time spent in the area where
airborne contaminants are present (this may vary from day to day and from one worker to
another, although the jobs may appear to be similar);
 whether the employee has direct influence over the process or the number or nature of the
sources of contaminant release (this can be particularly important where manual handling
operations such as scooping or shovelling dusty materials or unloading or sampling liquids are
concerned).
In such cases the detail of each operation and each individual's work practices can have an
important influence on the overall exposure pattern.
The main variations that need to be considered may be summarised as follows:
 within shifts: variations in the concentration of the contaminant in the breathing zone of each
person over the period of each shift;
 between shifts: variations in the shift average exposure level of each person;
 between individuals: variations between individuals in their exposure levels, even between
people doing the same kind of work in the same place on the same shift;
 between processes: variations due to the nature of the processes as influenced by operating
procedures, product composition, ambient conditions etc.
Further complications may arise from errors due to the sampling equipment (eg pump flow rates)
and to the analytical methods used. The sampling equipment and the analytical techniques need
to be compatible and it is important that they should have been validated so that the overall
precision of the measurements is known.
Choice of sampling strategy has an influence on the variability of the results obtained. These
variations can be difficult to estimate precisely, although some forms of statistical analysis are
helpful. However, precise estimates may not be necessary, particularly when there is additional
evidence, eg observations by a competent person that will permit a pragmatic interpretation of a
limited number of results. The aim should be to obtain reliable information about the potential risk
to health of exposed persons at a reasonable cost.
3
STRATEGIC CHOICES
3.1
Personal or Static (Area) Sampling
Contaminants are not uniformly distributed in the workplace. They tend to be concentrated around
the sources of emissions. The contaminant cloud thus may be localised around a particular
operation or around a leak from a process plant. If the operation involves manually handling a
material, airborne contaminant may be generated by the worker's activity and may therefore be
localised around the worker.
If the worker's clothing becomes contaminated, eg with powder, the clothing may become a
secondary source of contaminant generation. Secondary sources can also arise from the
disturbance of settled dust deposits.
To measure the exposure of a worker to airborne substances, a personal sample is collected with
the sampler positioned in the person's breathing zone. This is defined as within 30cm of the nose
and mouth. In practice, a sampler is usually fastened to the worker's lapel, as high up as possible
towards the breathing zone. Personal sampling is essential for evaluating compliance with OELs
and regulatory occupational exposure limits.
Air Sampling Strategies
Page 3
When workers are wearing Respiratory Protective Equipment (RPE), personal samples are still
appropriate unless it is physically impracticable to attach the sampler. Take no account of the
protection afforded by RPE in collecting or evaluating the results of personal samples. In this case
the result represents the exposure that would have occurred in the absence of (RPE).
The alternative to personal sampling is static or area sampling, where the position of the sampler is
fixed.
Static sampling permits measurement of background concentrations, as might be
experienced by a visitor walking through an area. If the sampler is positioned appropriately, static
sampling can identify emission sources and check the effectiveness of engineering control
measures.
Static sampling should not generally be used to estimate personal exposure, as the levels
measured in this way may differ by an order of magnitude or more from those measured by
personal sampling. The differences are especially great in situations where the contaminant is
generated by manual operations.
3.2
Duration and Timing of Sampling
Duration of Sampling
Samples may be taken over relatively long periods, such as a full working shift, or over shorter
periods, depending on the effects of the substance(s) under consideration.
The Long-term Exposure Limit (LTEL) is usually referenced over an eight-hour period and is
concerned with the total intake over long periods. It is therefore appropriate for protecting against
the effects of long-term exposure or reducing the risks to an insignificant level.
The Short-term Exposure Limit (STEL) (usually referenced over a 15-minute period) is aimed
primarily at avoiding acute effects, or at least reducing the risk of the occurrence. Specific shortterm exposures are listed in some local legislation for those substances for which there is evidence
of a risk of acute effects occurring as a result of brief exposure.
Note: Some substances have both a long-term and short-term exposure limit. Both the long-term and
short-term exposure limits are expressed as time-weighted average (TWA) concentrations (which
are simply airborne concentrations averaged over a specified period of time).
The ceiling value is a threshold that cannot be exceeded at any time during the work period.
For those substances for which no short-term exposure limit is listed, it is recommended that a
figure of three times the long-term exposure limit be used as a guideline for controlling exposure to
short-term excursions. This is known as the excursion value.
Timing of Sampling
Where short-term limits or excursion values are of concern, the timing of the sample should
normally coincide with the operations or situations likely to cause the highest levels. High
exposures are likely during such tasks as charging materials to a vessel, kegging, weighing out,
cleaning, repair, or maintenance operations. The sample should normally be taken over the
duration of the likely peak exposure, unless:
 the measurement technique used is sufficiently sensitive and it is helpful to use shorter samples
to explore the variations that occur within the period;
 there are health effects attributable to very short exposures, in which case a ceiling value may
be set, and short but continuous measurements are required for comparison with it (this may.be
done with eg continuous reading spectrophotometer or interferometer).
Air Sampling Strategies
Page 4
If the process under study is continuous and exposure appears to be uniform (ie there are no
identifiable peak exposure periods), short-term samples may be taken at any time. Timing of such
samples should be random, as a systematic approach (such as sampling at the start of each shift)
may introduce unintentional bias.
It should be noted that sensitisation is often associated with peak exposures rather than long term
levels.
Where 8-hour limits are set, it is usually appropriate to sample over a whole working shift,
regardless of the variations in exposure that occur during the shift. This is always the preferred
option for testing compliance with an 8-hour OEL. Alternatively, a number of short samples could
be taken either sequentially over the shift or at random during it. A calculation can then be applied
to derive the time weighted average exposure over the shift.
If, however, exposure is largely confined to a short period of time (e.g. when charging a vessel) it
may still be appropriate to sample only while that operation is in progress even though an 8-hour
OEL is set. The exposure measured over the duration of the operation is a useful guide to the
need for, or efficacy of, engineering controls. An 8-hour exposure figure can be calculated from the
results if assumptions can be made about the level of exposure for the rest of the shift.
The longer the duration of a sample, the greater the degree of averaging or smoothing that results.
Consequently the less variable and the more consistent the results will be. However, information
will be lost about the variations that occurred within the sampling period. Timing and duration of
samples therefore depend on whether systematic changes in airborne concentrations can be
foreseen and what the implications of those changes might be for the level of risk or for the control
measures required.
The type of sampling method and sample time that can be used will also be dependent on the
analytical method. Before starting any measurement it is essential to ensure that the Limit of
Quantitation (LOQ) of the analytical method is suitable for your purposes. If this is not the case
either the sampling method or analytical method will need to be adjusted.
3.3
Number of Samples
A single sample cannot provide complete information on the variations in exposure level, whether
within shifts or between shifts. Also, while systematic effects can be taken into consideration in the
interpretation of a single result, random effects cannot. If a sample was repeated under apparently
similar conditions, the result might differ by as much as a factor of ten because of random
fluctuations. Thus a single result is never a sufficient basis for a decision.
An average concentration, obtained by taking a number of samples, will be more consistent and
more reliable than individual results. The more samples that are taken the better, though in
practice a stage is reached where the cost of taking further samples is not justified by the increased
accuracy. Mathematically, the relationship between the number of samples and the confidence
limits that can be placed on the airborne concentration is complex. The following rules of thumb
can be given:
 Generally not less than 6-10 samples should be taken to characterise exposure, supported by
competent observations of the factors that may have influenced the concentrations measured;
This may not always be practicable, but under no circumstances should decisions ever be
based on less than three replicate samples:
 For in-depth studies, 20-40 replicate samples are required, which will provide an estimate of
mean airborne concentration to within about 25% of the true value.
Air Sampling Strategies
Page 5
4
PHASED STRATEGIES
An efficient way of obtaining the required information, without having to collect an excessive
number of samples, is to adopt a phased approach – a three-level strategy:
4.1
First Level Strategy
This is a basic survey using relatively crude techniques to get an initial idea of exposure levels and
sources. The techniques may include:
 screening tests such as indicator tubes;
 measurements with direct-reading instruments;
 static samples;
 short-period personal samples;
 a limited number of full-shift personal samples.
Usually, a basic survey focuses on worst-case situations. If the results are well below, or well
above, the relevant exposure limits they may be sufficient to allow a decision to be made. If not,
they provide a basis for designing a more detailed study.
4.2
Second Level Strategy
This is a more detailed survey, usually involving the accurate measurement of personal exposures
over a full shift or the duration of the task, on a number of occasions sufficient to characterise fully
any variability in the exposure levels. Often it is best to identify a homogeneous group of workers,
ie a group with similar tasks and exposures. Samples may then be taken on a number of
individuals chosen at random from the group. The mean (ie simple arithmetical average) result and
some measure of distribution of the results then characterises the exposure of the group. In
addition, the spread or distribution of results will indicate whether the exposures are really
homogeneous or whether certain individuals or activities should be scrutinised more carefully.
4.3
Third Level Strategy
This would involve statistical studies, with rigorous protocols to ensure maximum reliability in the
results and comparability of data with other studies. The design and interpretation of such studies
requires considerable expertise and is beyond the scope of this document.
4.4
Other Considerations
In addition to any measurements for airborne contamination the need for surface contamination
measurements (ie wipe sampling) will need to be considered at each strategy level.
If biological monitoring is to be done as part of a survey, careful consideration will have to be given
as to when measurements are made. The relationship between exposure to the substance and its
expression in biological media needs to be taken into account. Consultation with an occupational
health physician is essential.
Air Sampling Strategies
Page 6
5
MONITORING PROGRAMMES
5.1
Establishing a Programme
Monitoring involves taking samples on a regular basis in order to detect trends that may arise from
changes in working practices or deterioration of control measures. The changes may be gradual or
stepwise. A decision to create a monitoring programme should be based on the initial chemical risk
assessment and a detailed survey of the working environment.
Typically in a monitoring programme, a batch of samples is taken at regular intervals. The results
may be analysed statistically to identify trends. A monitoring programme should have a written
protocol that defines:
 the agents that will be sampled;
 the jobs and areas to be sampled;
 the number of samples required;
 the frequency of monitoring;
 the nature of the samples to be collected.
Agents to be Sampled
A chemical risk assessment should have been performed to identify those agents requiring
measurement. Monitoring is particularly likely to be required for substances where chronic
exposure can produce serious irreversible long-term health effects. Examples of such substances
are carcinogens, sensitisers and reproductive toxins.
Jobs and Areas to be Sampled
Advice on the selection of groups for monitoring is contained in section 5.2 below.
Number of Samples Required
Considerations are similar to those identified in section 3.3. Ten replicate samples per group would
be typical.
Frequency of Monitoring
This will depend on the proximity of the exposure level to the OEL, the likelihood of failure of control
measures, the severity of any health effects that may be produced and the time that would be
needed for the plant to respond to adverse findings. More detailed advice is given in section 5.3
below.
The Nature of the Samples
A decision needs to be made about the nature of the samples to be collected, ie personal, area, full
shift, short term, etc. See section 3.1.
A monitoring plan may be designed to produce data that accurately reflect the exposure of the
population while working under representative operating conditions and job tasks. Programmes
may also include additional sampling in order to determine compliance with legislation that requires
worst-case sampling. The worst-case results should be used to determine control priorities and
strategies.
Air Sampling Strategies
Page 7
Central to the concept of monitoring is that there should be an intervention level set. Once a
significant upward trend in the average exposure pattern is detected there is time to remedy any
defects in the plant before any individual exposures exceed the limit. The response time of the
plant (ie the time taken to implement corrective measures) may range from a few minutes to
several months. The longer the response time, and the greater the variability in the data, the lower
will be the intervention level.
In some cases monitoring is by continuous-reading instruments permanently installed in the plant.
Often these will be linked to alarm systems. Continuous monitors are used where failure of control
for even a short period could have serious effects (eg when handling highly irritant gases such as
chlorine).
5.2
Selection of Groups for Monitoring
The procedure for selection of an employee or group of employees for exposure monitoring is
complex. However, some general guidelines can be given.
It is rarely practicable to sample every individual worker when the exposed population is large.
One approach is to sample workers randomly from within the whole exposed population. However,
from a statistical standpoint this requires a large number of samples.
An alternative approach would be to sub-divide the exposed population into homogeneous groups.
From the statistical point of view, the advantage of grouping is that the variability of full shift
exposure levels is smaller for well-defined groups than for the exposed workforce as a whole.
Groupings also have the practical advantage that resources can be concentrated on those groups
at maximum risk or with the highest exposure patterns.
These homogeneous groups can be selected on the basis of:
 location;
 job description;
 tasks;
 chemicals;
 process;
 a combination of these factors, depending on the interactions between substance, workplace,
and exposure factors.
Proper definition of the homogeneous group is an important precursor to establishing a monitoring
programme. Difficulties may arise when employees are exposed to multiple substances, and when
exposure events are intermittent and unpredictable.
5.3
Frequency of Monitoring
The decision about frequency of monitoring needs to be made according to the specific needs of
each workplace. Some suggested baselines are given in Table 1, in terms of the mean level of
exposure compared to the OEL. The mean (or average) level of exposure is used as this provides
a more reliable estimate than individual values. The suggested frequencies increase as the
exposure gets closer to the OEL. These suggestions will need to be adjusted to take into account:
 the likelihood of failures of control measures;
 the severity of any effects that might result from overexposure;
 the time required to correct a defect once it is identified.
Air Sampling Strategies
Page 8
Table 1 Suggested Frequency of Monitoring
Mean (average) Exposure Level (ExpA,
arbitrary units)
Suggested Frequency
ExpA <0.1 times OEL
Not required
Average level 0.1-0.25 times OEL
Every 2 years
Average level 0.25-0.5 times OEL
Every 6-12 months
Average level > 0.5 times OEL
Frequency depends on
circumstances
When the mean level of exposure exceeds 0.5 times the OEL it is likely that a significant proportion
of individual results will exceed the limit. Such a situation should be regarded as ‘out of control’
and action should be taken to reduce the levels. Respiratory protective equipment should be
provided in the short term. Frequent monitoring is desirable if the exposure can be controlled by
operating practices, as it will help to reinforce the need for improvement and provide confirmation
when improvement has been achieved. If, however, improvement is only possible through
engineering change, monitoring is of limited value once the situation has been characterised.
5.4
Review of the Programme
The monitoring programme should be reviewed whenever:
 a chemical risk assessment indicates the need to do so;
 the data obtained from the programme suggests the need to do so (e.g. if levels increase, more
frequent monitoring may be advisable);
 there is a significant change to a process, plant or operating practices, whether this is expected
to increase or to decrease exposure;
 at least every 5 years.
Air Sampling Strategies
Page 9