[DEPARTMENT FOR INDUSTRIAL AFFAIRS
ASSESSMENT
of
HAZARD SUBSTANCES MANAGEMENT
in
DENTAL LABORATORIES
Jane Avvvvad
Joe Crea
Industry Services Office
Department for Industrial Affairs
July 1997
1
Acknowledgements
We wish to acknowledge the cooperation of the dental laboratories who participated in this
study.
We wish also to thank the following individuals and organisations for their support and
advise:
Graham Wingate, Chairman, Australian Commercial Dental Laboratories Association,
Allan Joslin, President, Clinical Dental Technicians Association,
John Hoist, Lecturer, Dental Studies, Torrens Valley Institute of TAFE
Rosalie Dutson, Executive Officer, Australian Dental Association
Geoffrey Staples, Laboratory Representative, Halas Dental Limited.
2
CONTENTS
Page
Acknowledgements
1
SUMMARY
3
BACKGROUND
4
Introduction
Methodology
HEALTH HAZARDS
4
4
6
Hazardous Substances
Exposure Standards
Common Procedures in Dental Laboratories
Dust Generating Procedures
Liquids, Vapours and Gases
Methyl Methacrylate
Other Liquids and Gases
6
6
8
8
9
9
10
SURVEY OF DENTAL LABORATORIES
11
RESULTS
12
COMMENTS AND RECOMMENDATIONS
15
APPENDICES
19
Appendix 1: Local Exhaust Ventilation Suggestions
for the Dental Laboratory
Appendix 2: Examples of Local Exhaust Ventilation
in Dental Laboratories
Appendix 3: Hazardous Substances Survey
BIBLIOGRAPHY
19
21
24
31
3
SUMMARY
Dental technicians work with a variety of toxic substances. These include; precious and nonprecious metal alloys used in the manufacture of crowns, bridges and dentures; crystalline
silica as constituent of porcelain and investment material; and methyl methacrylate used in
preparing dentures.
Potential exposure to toxic dusts, which may include, crystalline silica, chromium, cobalt,
nickel, platinum, palladium, copper, zinc, molybdenum, gallium, aluminium, methyl
methacrylate or gypsum, is possible during sandblasting, grinding and polishing operations,
which are regularly performed in the dental laboratory. Exposure to methyl methacrylate
vapour and liquid or polymethylmethacrylate dust, during denture making procedures, also
presents a potential hazard.
There are numerous published reports of pneumoconiosis and occupational asthma in dental
laboratory technicians. Hand dermatitis in dental technicians, due to the handling of methyl
methacrylate monomer, is also well documented.
A survey of 17 dental laboratories in South Australia metropolitan area was carried out to
qualitatively evaluate the potential health hazards associated with exposure to dusts and
methyl methacrylate, and to determine how the dental laboratories are managing the
hazardous substances regulations which is to be adopted from the 1st of July 1997.
It has been found that the laboratories visited had difficulty in managing the complete
compliance with hazardous substances regulations. However, many of the laboratories
surveyed, had adopted some control measures for dust and chemicals, in an effort to provide
safe and clean working conditions for the employees.
Improvements or modifications to the local exhaust ventilation systems, personal protective
equipment together with some changes in work procedures were recommended.
Assistance with legislative requirements was also offered to participants in the survey to
ensure that they fulfil their duties under the hazardous substances regulations.
4
BACKGROUND
Introduction
The Hazardous Substances requirements under the South Australian OHS&W Regulations
1995, were initially approved and gazetted to take effect from the 1st April 1997. In order to
give industry more time to implement the demanding requirements of the hazardous
substances regulations, the Hon. Minister of Labour, Mr D Brown, extended the complete
implementation date to the 1st July 1997.
The Department for Industrial Affairs, as part of a new approach in providing workplace
relations and health and safety services for employers and employees, has initiated several
'industry based' projects. One of the projects, conducted by the Health & Community Health
Services Team has been, the 'Assessment Of Hazardous Substances Management In Dental
Laboratories'.
The project was designed to provide the industry with information and support in regards to
the implementation of the Hazardous Substances Regulations in dental laboratories in South
Australia.
The principal aim of the project was to:
•
To assess dental laboratories management of the Occupational Health Safety and Welfare
Legislation and in particular their compliance to the Hazardous Substances Regulations.
The objectives of the project were to:
• Carry out inspections of a cross section of the dental laboratories to ensure compliance
with the OHS & W Legislation in particular the Hazardous Substances Regulations;
• Observe work practices, and evaluate control measures associated with the use of
chemicals in dental laboratories; and
• Make specific recommendations on improved work practices, exposure control and assist
organisations to meet the legislative requirements regarding Hazardous Substances.
Methodology
A letter was sent to all the dental laboratories listed in the Yellow pages (about 106),
notifying them of the project details and outcomes. They were also given the opportunity to
volunteer in this study. Dental Associations, namely, the Australian Dental Association,
Australian Commercial Dental Laboratories Association, Clinical Dental Technicians
Association, were also contacted to ensure that their members could participate in the study.
The Torrens Valley Institute of TAFE, which provide the training for dental technicians, was
also notified.
Unfortunately, only five laboratories volunteered. Another twelve laboratories were randomly
selected from the list in the Yellow pages, in order to give a reasonable cross section of the
industry in South Australia.
5
A questionnaire was prepared, which included the requirements of the Hazardous Substances
Regulations, such as, an inventory of hazardous chemicals, Material Safety Data Sheets
(MSDS), labelling, storage of chemicals, emergency procedures, risk assessments and control
procedures in dental laboratories. ( a copy of the questionnaire is attached, see appendix 3)
Inspections of laboratories commenced on the 6th May 1997 and were completed on 2nd June
1997. Recommendations on the findings were given verbally during the visit and followed up
with a written report.
This report, includes issues/problems found in the dental industry together with relevant
recommendations for the implementation of policies and procedures to minimise exposure to
hazardous substances. This will be circulated across dental laboratories in South Australia.
6
HEALTH HAZARDS
Hazardous substances
Dental laboratory technicians may be exposed to a variety of hazardous substances.
The most common hazardous materials are summarised below:
The non-precious metal alloys may contain chromium, cobalt, nickel, molybdenum,
beryllium, and small quantities of iron, manganese, gallium, ruthenium and aluminium. The
composition varies depending on the specific use. For example, non precious alloys used for
crowns and bridges typically contain about 65-70% nickel and 20-30% chromium, while nonprecious alloys for partial dentures typically do not contain nickel and have about 30%
chromium and 60-65% cobalt. Beryllium was added to improve hardness, strength, fatigue,
corrosion resistance and elasticity. ( beryllium is no longer used in the composition of some
non-precious alloys).
Semi-precious alloys are palladium alloys with gold (2%), and smaller quantities of copper,
tin and gallium. These alloys are the most widely used for crown and bridge and partial
denture work (about 60-70%).
Precious alloys contain a high gold content (50-70%) and smaller quantities of silver,
palladium, tin and zinc. Their use is limited due to the high cost.
Crystalline silica has been used as a blasting abrasive, is present in porcelain, in pumice and
refractory investment materials used for casting procedures.
Aluminium oxide used as a blasting abrasive ( replaced crystalline quartz)
Methyl methacglate is used in the manufacturing of dentures.
Gypsum powders used in castings
Acids, such as hydrofluoric acid ( used for etching porcelain or veneers), and hydrochloric
acid ( cleaning metal alloys).
Solvents such as isopropyl alcohol, methyl ethyl ketone, limonene (orange oil), acetone,
chloroform and methylated spirits.
Decomposition products from burning waxes in a furnace.
Disinfecting solution may contain glutaraldehyde.
Exposure Standards
The criteria used to assess occupational exposure to airborne contaminants are given in the
National Occupational Health and Safety Commission's (NOHSC) publication,' Exposure
Standards for Atmospheric Contaminants in the Occupational Environment, 1995". The
exposure standard represents airborne concentration of individual chemical substances which,
according to current knowledge, should neither impair the health of, nor cause undue
discomfort to, nearly all workers. Exposure Standard -TWA (time weighted average) of a
substance is defined as the airborne concentration to which nearly all workers may be
repeatedly exposed 8 hours per day, 40 hours per week, without adverse effect.
7
Due to wide variations in individual susceptibility, however, a small percentage of workers
may experience discomfort from some substances at concentrations below the exposure
standard; a smaller percentage may be affected more seriously by aggravation of a preexisting condition or by development of an occupational illness.
The basis on which the exposure standards are established may differ from substance to
substance; protection against impairment of health may be a guiding factor for some, whereas,
reasonable freedom from irritation, narcosis, nuisance or other form of stress may form the
basis for other substances.
The exposure standards should be used as guides in the control of health hazards and should
not be used as a fine lines between safe and dangerous concentrations.
When assessing the hazard from a mixture of airborne contaminants, the combined effect of
this multiple exposure may be considerably greater then the sum of the effects from individual
components (synergistic effect). At present the understanding of interaction effects is
incomplete.
Therefore, the best practice is to maintain concentrations of all atmospheric contaminants as
low as practicable since complete details about toxicities and interaction of substances are
seldom known.
The exposure standards (TWA) of some of the commonly used material in the dental
laboratory are:
Crystalline silica
Chromium metal dust
Cobalt metal dust
Nickel metal dust
Beryllium dust
Silver metal dust
Copper metal dust
gypsum dust ( calcium sulphate)Methyl methacrylate
0.2 mg/m3 (under review)
0.5 mg/m3
0.05 mg/m3 (Sen)*
1.0 mg/m3 (Sen)*
0.002 mg/m3 (Carc. 2)**
0.1 mg/m3
1.0 mg/m3
10.0 mg/m3
100 ppm (Sk, Sen)*
Acrylic dust does not have an assigned exposure standard. This does not mean that it is
non-hazardous. There may be insufficient information on the health effects of this dust
to allow the National Occupational Health and Safety Commissioni to assign an
exposure standard. It is therefore a good policy to keep the exposure to acrylic dust as
low as practicable.
* Skin (Sk) notation indicates that the substance are readily absorbed through the skin. The
adopted exposure standards only consider absorption via inhalation, and are valid only on
the condition that significant skin absorption cannot occur. Therefore special measures
should be taken to prevent absorption through the skin.
*Sensitiser (Sen) notation indicates that some substances can cause a specific immune
response in some people. Such substances are called sensitisers and the development of a
specific immune response is termed 'sensitisation.'
**Beryllium is defined as a 'carcinogen category 2' by the National Occupational Health and
Safety Commission.' This means that it is a probable human carcinogen and there is
'sufficient evidence to provide a strong presumption that human exposure might result in
the development of cancer.'
8
Common Procedures in Dental Laboratories
The following processes carried out in dental laboratories have the potential to produce
airborne concentrations of hazardous substances or skin problems :
•
•
•
•
•
Bench grinding, hand grinding, polishing and sand blasting of metal castings and denture
material ( containing methyl methacrylate)
Mixing of methyl methacrylate monomer with a powder containing the polymer
Kneading the partially cured acrylic, moulding it over the plaster cast impression and then
manipulating the moulded acrylic to achieve the desired dimensions. The whole process is
carried out without wearing gloves.
Cleaning denture with methyl methacrylate monomer using bare finger.
Burning off waxes in a furnace at high temperatures.
Dust Generating Procedures
A significant amount of dust formation is possible during the grinding, abrasive blasting and
polishing procedures which are regularly carried out in dental laboratories. Owing to the
variety of materials used, the composition of the dust formed varies greatly.
From a toxicological viewpoint, there are five factors which determine the risk associated with
specific airborne particulate, namely:
•
•
•
•
•
The type of dust inhaled;
The concentration of the dust inhaled;
The size of dust particles
The duration of exposure ( possibly years); and
The individual's biological defence mechanism and personal state of health.
All the above factors are important in the way an inhaled dust affects the worker, because they
govern how much of a material enters the body, where it finally lodges and what sort of toxic
effect it can exert.
Dust particles must usually be smaller than 5 microns ( 5 millionth of a meter) in diameter in
order to reach the alveoli or inner recesses of the lungs. This type of dust is referred to as
respirable dust. Larger particles, between 7 and 100 microns, referred to as inhalable dust,
are trapped in the upper respiratory system, namely, in the nasal passages, throat, larynx,
trachea and bronchi, from which they are removed by a swallowing action or by blowing the
nose. It must be noted that some dusts are chemically active and if swallowed they may be
dissolved in gastric acid or juices in the digestive system, and thus enter the blood stream.
Airborne dusts of health significance are generally 0.1 to 5 microns. Particles less than 0.1
microns in size are usually exhaled and do not deposit in the lungs.
The dust particles produced in the dental laboratory during grinding, abrasive blasting and
polishing operations will produce dust in both the repirable and inhalable dimensions.
The most common particle sizes found during grinding operations of porcelain, metals, and
gypsum were between 1 and 5 microns, with 0.2 to 1 microns the next most frequent. All
these particle sizes are considered to be respirable and therefore potentially hazardous!'
9
Several overseas studies2-14 reported pneumoconiosis (lung disease due to dust exposure) in
dental technicians from exposure to crystalline silica, acrylic resin dust and metals
(chromium, cobalt, tungsten, molybdenum, nickel, aluminium, titanium).
The studies concluded that a higher incidence of pneumoconiosis was found among dental
technicians with longer exposure patterns. However, a single cause for the dental technician's
pneumoconiosis could not be identified; but the above materials, which are commonly used
in the dental laboratories, are the best recognised causes of4pneumoconiosis, and all have been
implicated as possible causal agents in dental technicians.1
Unacceptable dust measurements in the breathing zone of the technicians were only measured
during grinding operations with inadequate local ventilation.3'15'16 The authors concluded that
the major portion of dust from processing dental materials can be removed by local ventilation
with a suction capacity of about 30 litres per second, and mounted as close as possible to the
workpiece (less than 10cm from workpiece).
Liquids, Vapours and Gases
Methyl methacrylate
The major exposure routes to methyl methacrylate in the dental laboratory are by the skin
contact and by inhalation. Methyl methacrylate is a sensitizing, irritating and neurotoxic
(poisonous to nerve tissue) chemical which must be handled with care. Methyl methacrylate
has a distinctive pungent odour which can be detected at concentrations well below that
required to induce harmful effects (0.3 ppm),I8'19 It therefore provides good "warning
properties".
Most denture base acrylics are marketed as a powder and a liquid. The powder commonly
contains small prepolymerised acrylic particles, the most common component being
polymethyl methacrylate. It also contains a catalyst ( eg benzoyl peroxide), a plasticizer,
usually dibutyl phthalate, and pigments which may be organic pigments or metal salts, such
as, iron, cadmium, titanium, calcium and zinc salts. The main component of the liquid is
methyl methacrylate together with additives such as cross-linking agents ( ethylene glycol
dimethylacrylate), a catalyst ( N, N-dimethyl para-toluidine), inhibitors (hydroquinone, pmethoxyphenol) and UV absorbers ( benzophenone).
The mixture of powder and liquid is polymerisable at room temperature ( self-curing) or at
high temperatures ( heat-cured).
The highest potential exposure via inhalation to the methyl methacrylate monomer occurs
during the mixing process. Vapour of methyl methacrylate monomer is also released during
grinding and polishing of the polymethyl methacrylate. This vapour arises due to the presence
of free monomer in the denture material or from the de-polymerisation during grinding and
polishing.
Skin contact occurs during kneading of the partially cured polymer and the finishing
procedure to achieve the desired dimensions. The process is carried out with unprotected
hands. The monomer is also applied with the finger to produce a smooth finish of the denture.
Several studies18-22'3°'31 have reported a high incidence of allergic dermatitis among dental
technicians. The mean duration of self-reported dermatitis was 7 years ( from 2 weeks to 40
years). Abatement during holiday periods and reappearance after the resumption of work was
reported by 67% and 42% respectively.18 In a questionnaire study of Finnish dental
technicians,2° 34% of the technicians reported dermatitis and other hand symptoms, such as,
finger numbing, feeling coldness and whitening in areas of frequent contact to methyl
methacrylate monomer. The authors conclude that other factors, such as, frequent hand
10
washing contributes to hand dermatitis. They also found that people with a skin disease in
childhood or with allergic conjunctivitis or rhinitis seemed to have a higher rate of acrylateassociated dermal problems than others.
In dental technology, the dental technicians are obliged to handle monomer-polymer dough
with their bare hands, partly because of the demands for dexterity and partly because of bad
habits.18,20 According to the studies,20'23 only a few try to protect their hands in some way.
The protection is not easy, it was claimed, as "all" commercial surgeon's gloves with
sufficient flexibility and durability are permeable to methyl methacrylate monomer.
Furthermore, a permeable glove only creates an occlusion effect and thus strengthens the
sensitizing and irritating potential of the monomer. Several studies20'2' suggested that the use
of a disposable polyethylene glove with a separate rubber finger-hood would be effective
enough to prevent methyl methacrylate penetration during quick procedures, of up to 10
minutes. However, further development of non-touch techniques was strongly recommended.
In one study, 24 different types of gloves were tested for penetration by immersing in methyl
methacrylate liquid. The penetration of the monomer occurred during the first minute of
immersion in the conventional surgical gloves ( 0.13 to 0.16 mm thickness), and
disintegration of the rubber was visible after 3 minutes. Penetration occurred after 20 minutes
for the viton butyl glove ( 0.27 mm thickness) with slight change of the material observed at
that time. The best performer was a PVP glove, which consisted of an outer layer of
polyethylene, a middle layer of ethylene vinyl alcohol copolymer, and an inner layer of
polyethylene to a thickness of 0.07 mm. Penetration occurred after 30 minutes with no visible
material degradation evident after 6 hours.
In most of the dental laboratories workers are only exposed to methyl methacrylate vapour for
relatively short periods. Therefore, it would be difficult to have exposures exceeding 100 ppm
averaged over an eight-hour work day. However, moderately high concentrations (>100ppm)
of methyl methacrylate may be experienced briefly (eg mixing operations) and this may result
in nausea, drowsiness and other effects.18'I9 A study involving personal exposure monitoring
and a self response questionnaire of 35 dental technicians26 indicated that symptoms
suggestive of possible neurotoxic action by methyl methacrylate may have occurred at low
airborne concentrations of the monomer.
Pneumoconiosis resulting from high exposures to acrylic resin dust, produced during grinding
and polishing operations, has been reported."
Cases of occupational asthma from exposure to methyl methacrylate have also been
reported.25
Other liquids and gases
Other substances which are found in dental laboratories, such as, acids, disinfecting solutions
containing glutaraldehyde, cleaning liquids which may contain solvents (eg. isopropyl
alcohol, limonene (orange oil), chloroform, acetone, methylated spirits), polishing liquids (
ethylene glycol, sulphuric acid), liquid component of the investment mixture (ethylene
glycol), separators (hexane), do not present a major hazard because the quantities used are
small and the frequency is low. However, skin contact with these materials should be avoided
by using appropriate protective gloves ( refer to MSDS for the particular substance).
The various waxes used in the dental laboratory may not be hazardous in their normal state,
but during the burning off the wax in the furnace, at high temperatures, some toxic
decomposition gases will be released. Therefore, appropriate engineering controls should be
adopted.
11
SURVEY OF DENTAL LABORATORIES
A survey of 17 dental laboratories, in the metropolitan area of South Australia, was carried out
between May and June 1997. Eleven laboratories had staff of less than three, and eight of
these were self employed, which is very common in this industry. Six of the laboratories had
between three and ten technicians.
The survey was based on a simple questionnaire dealing with the management of the
hazardous substances requirements of the OHS&W Regulations, 1995. (a copy of the
questionnaire is attached). Discussion was encouraged with both the laboratory owner and
technicians, in regards to the questionnaire and particular tasks.
During the inspection we looked at work procedures and the type of control measures adopted
to control the hazard. Smoke tubes were used to provide a qualitative assessment of the local
exhaust facilities.
The main work performed in the laboratories surveyed can be divided into three principle
activities:
Prosthetics- making acrylic/cast partial dentures ( 4 laboratories)
1.
Crown and Bridge work with precious and non-precious metals( 3
2.
laboratories)
The above, plus making orthodontic appliances, mouthguards etc. ( 10
3.
laboratories)
All the dental activities noted above were mainly performed by the larger laboratories.
12
RESULTS
The results of the hazardous substances survey is presented in table 1.
From table 1 the following observations can be made:
1. Specific issues regarding the management of the hazardous substances requirements under
the OHS&W Regulations, 1995, were not well addressed. The main reasons given by the
laboratory owners were: they were not aware of the requirements of this particular
legislation; they could not understand it, and for self employed, they were under the
impression that the hazardous substances regulations were not applicable to them.
2. The basic requirements of the Hazardous Substances Regulation, namely, the development
of a hazardous substances register of all chemicals used in the laboratory and the
accessibility of Material Safety Data Sheets (MSDS), were not completed by any of the
laboratories visited. However, some of them had made a start. In order to assist them in
gathering this information, we provided them with information sheets on the relevant
regulations and a form to compile the chemicals used, supplier's name and availability of
MSDS's.
Risk assessments, which follow these basic regulatory requirements, were not carried out
by any laboratory.
Safe working procedures and procedures for handling chemical spillages had not been
developed by any of the laboratories visited.
Staff training was performed by only one laboratory.
3. Approximately half of the laboratories surveyed had an emergency evacuation procedure
developed and displayed. While most laboratories had fire extinguishers, not all of these
were maintained or correctly labelled.
Seventy percent of the laboratories had adequate first aid facilities which included an
eyewash and a burns module.
4. Storage of chemicals was satisfactory in most laboratories. However, several laboratories
(70%) decanted some substances to smaller containers which were not adequately
labelled.
5. Fifleen of the laboratories (88%), made an attempt to control the potential hazards in the
laboratory by installing local exhaust ventilation. However, the efficiency of these
systems, in most cases was not satisfactory. For dust control, during bench and hand
grinding operations, the extraction system was, in a few cases, a vacuum cleaner which
was kept in the laboratory. Although it produced sufficient suction ( when the operation is
carried out close to the suction inlet), the filter is inadequate because it cannot trap the
very fine dust. As a consequence of this, the fine dust is regenerated back in the working
area and thus, in the breathing zone of the operator. Only four laboratories (24%), had an
extraction system which had good suction and a high efficiency filter, which could capture
particles less than 5 microns in diameter.
Most laboratories had a hood type extraction system over the furnace to capture emissions
release from the burning of waxes.
Extraction for methyl methacrylate usage was not provided by the four laboratories that
constantly used the product.
13
Smoke tube tests also demonstrated that, in all cases the extraction efficiency could be
improved by, working closer to the suction inlet and providing additional enclosure
around the suction inlet or hood.
6. As can be seen from the table, most of the laboratories surveyed had some form of
personal protective equipment. The disposable gloves, which most laboratories
possessed, were mainly used for infection control. Protective gloves were not used during
mixing procedures of monomeric methyl methacrylate with the powder or the handling
of the partially polymerised dough, because they "affected the manual dexterity" and
furthermore, most glove materials were permeable to the methyl methacrylate monomer.
Most technicians wore glasses (76%) during grinding procedures, however, some wore
prescription glasses with hardened lenses but no side guards, which increases the risk of
eye injury. Glasses worn should meet the Australian Standard (AS 1337-1984)
requirements.
The correct respiratory equipment for dust control was available in three( 18%) of the
laboratories visited, however, nine other laboratories ( 53%) provided disposable masks
which were approved only for infection control and not for dust control.
14
TABLE 1: RESULTS OF HAZARDOUS SUBSTANCES SURVEY
Yes
13
No
4
Hazardous substances register- developed
0
17
Material Safety Data Sheets -availability & accessibility
1
16
Labelling requirements
5
12
Storage of chemicals
17
0
4(11)*
2
17
0
1
8
16*
1
3(9)*
5
13*
4
Risk assessment- conducted and documented
0
17
Safe Work procedures- developed and documented
0
17
Emergency evacuation procedures
9
8
Fire extinguishers - type, maintained, labelled,
convenient
15
2
Procedures for handling spillages
0
17
First aid facilities ( including eyewash & burns module)
12
4
House-keeping in acceptable state
Ventilation - local extraction
- general ventilation
Staff training
Personal Protective Equipment - gloves
- respirators
- glasses
Comment
Some had food facilities in
the laboratory.
Many chemicals kept in
small quantities.
*1 1 other labs had some local
ventilation but needs
improvement.
General ventilation was
either A/c, exhaust fan on
ceiling/window or natural.
Not applicable for eight labs,
as they were self employed.
*surgical gloves used mainly
for infection control.
heat resistant gloves
*many used infection control
mask for dust control
*many had prescription
glasses with a hard coated
surface, but no side guards
for full protection.
Many had extinguishers but
the were not maintained
15
COMMENTS AND RECOMMENDATIONS
The survey indicates that dental laboratories would not meet the legislative requirements by
the 1st July 1997, the implementation date. The main reason given for this delay was
ignorance of the hazardous substances regulation and many self-employed technicians did not
think that the regulation applied to them.
Nevertheless, the majority of the laboratories did make an effort, regardless of the legislative
requirements, to provide some control measures to protect the technicians in their work
environment. However, as discussed above, most of these control measures require some
improvements or modifications if they are to be effective in controlling the dusts, vapours and
gases produced in the work environment.
The regulations state the prevention or control of hazardous substances "shall be secured by
measures other than the provision of personal protective equipment. Control measures should
be implemented in accordance with the hierarchy of controls." The hierarchy of controls is a
list of control measures, in priority order, that can be used to eliminate or minimise exposure
to hazardous substances. Application of the hierarchy of controls measures involves firstly
assessing whether a hazardous substance can be eliminated. If this is not practicable,
substitution should be considered. Following this, consideration should be then given to each
of the following control measures; isolation, engineering controls, safe work practices and
personal protective equipment.
Upon consideration of the hierarchy of control measures, the following recommendations
should control exposure to hazardous substances in the dental laboratory:
• Substitution
If practicable, substitute hazardous products with non or less hazardous ones.
For example:
light cured acrylic "special trays" could be substituted for methyl methacrylate based
resin in the form of a polymer( powder) and monomer (liquid). This would
eliminate exposure to methyl methacrylate vapour during mixing procedures and
minimise skin contact to the monomer during doughing and other handling
techniques.
Beryllium, a constituent of non-precious metal alloys, is a carcinogen, and therefore,
should be replaced with a substance which is not carcinogenic and less hazardous.
Polishing and abrasive blasting compounds containing crystalline silica, could be
replaced, where possible, with compounds which contain less hazardous substances.
• Ventilation Control of Dust
Dry abrasive machining of acrylics and metals would be expected to liberate
particulates of various size distributions, depending on the material being ground and
the coarseness of the abrasive surface. In the case of bench grinding, large particles
would tend to fly off tangentially from the contact point whereas the physiologically
more important fine particles would remain close to the wheel periphery. Coarse
dust particles ( >30 microns) generally have fixed directions of "flow" so that local
exhaust collection trays may be correctly positioned to account for this dust.
Intermediate and small sized dust particles are more difficult to capture.
Nevertheless, analysis of particle motion and breathing zone dust concentrations
have been carried out27 for grinding, buffing and polishing operations and criteria
16
for ventilation of such operations published.28 (see, for example Figure 1- Note that
the criteria given in figure 1 are meant to cover high density dusts and high
peripheral wheel speeds so that the recommended exhaust volume are probably
excessive for work carried out in the dental laboratory. Therefore, use this example
as a guide only).
The followilf principles of ventilation taken from the US National Safety Council
publication,2 "Fundamental of Industrial Hygiene," apply:
(i) Enclose the operation as much as possible to reduce the rate of airflow
needed to control the contaminant, and to prevent cross drafts from
blowing the contaminant away from the field of influence of the
exhaust hood.
(ii) Always locate a hood so that the contaminant is moved away from the
breathing zone of the operator.
(iii) Locate and shape the exhaust hood so the initial velocity of the
contaminant will throw it into the hood opening (eg see figure 1,
appendix 1).
(iv) Locate the hood as close as possible to the source of the contaminant
(the required extraction volume varies with the square of the distance
from the source).
(v) Design the hood so it will not interfere with the operator.
The grinder or polisher should have an adjustable guard placed as close as possible
to the top part of the wheel. This is an important addition because it tends to trap the
finer dust as it is carried around the wheel in the air stream set up by the wheel's
rotation.
During hand grinding operations, similar principles about ventilation outlined above
apply.
( see appendix 2 for examples of extraction systems in dental laboratories)
Once captured in the exhaust hood the dust needs to be filtered out of the air exhaust,
and in this regard, ordinary vacuum cleaners are possibly inefficient, that is, they
retain only intermediate and coarse dust but allow some of the fine dust to pass
through. Hence an outdoors location for the dust filter is recommended.
If the filter has to be located in the laboratory ( eg in multi-storey buildings) an
appropriate extraction system with a high efficiency filter should be used.
In order to minimise friction losses, the ductwork, ideally, should be circular,
smooth, wide, as short in length and as few bends as practicable.28
• Ventilation Control of Gases and Vapours
Small scale methyl methacrylate and other solvent work, such as mixing and
moulding, shaping should to be carried out in the presence of local extraction
ventilation. The ventilation system may be in the shape of a hood, a cabinet (eg
fume cupboard) or a table with lateral extraction (see figure 2, appendix 1). The
design in the extraction system is left to the employers discretion. However, the
general ventilation principles outlined above must be adhered to. Whichever
ventilation system is adopted, it must be remembered that the vapours generated
from methyl methacrylate or other solvent vapours, are drawn away from the
breathing zone of the worker and exhausted, through the ductwork, to the outside
atmosphere or trapped on an activated charcoal filter ( if unable to exhaust to the
outside). If you are using lateral extraction , vertical baffles at the edges of the table
17
are desirable because it increases the efficiency of the extraction system (see figure
2, appendix 1).
Gases released by burning waxes in a furnace should be exhausted out of the
laboratory, to the outside atmosphere. Most of the laboratories visited had an exhaust
hood placed over the furnace, which was satisfactory. Once again, in order to
maximise the efficiency of the extraction system, the hood should be place as close
as practicable to the furnace, or if this is not possible, extend the chimney outlet
towards the hood inlet.
Ensure that any extraction system used in the laboratory is regularly cleaned and
maintained in order to provide maximum efficiency.
• Training
Ensure that dental technicians receive adequate information and training on the
health effects of the materials they work with, and on control measures used to
minimise exposure to hazardous substances ( including engineering controls and
protective equipment).
• Legislative
Ensure that everyone working with hazardous substances is familiar with the
legislative requirements, which includes; hazardous substances register, material
safety data sheets (MSDS), labelling, risk assessment, risk control, record
keeping, training and employer and employee responsibilities.
• Personal Hygiene
Ensure that personal hygiene is maintained at a high standard because chemicals
which are handled in everyday procedures can easily be ingested by contaminating
food or cigarettes. Food preparation and coffee and tea facilities should not be in the
work area. Avoid bad habits such as, licking the tip of a porcelain brush. Dipping in
water and then rolling in a tissue, for example, will achieve the same result. Washing
of hands after contact with chemicals (eg. methyl methacrylate) is encouraged. If
suitable gloves are not available a suitable barrier cream could be tried.
•
Personal Protective Equipment
If Personal protective equipment is supplied, ensure that it is approved for the
intended application. Eye protection during grinding procedures should meet
Australian Standard requirements (AS 1337-1984) and if technicians wear
prescription glasses, ensure that the lenses are impact resistant and appropriate side
guards are fitted.
Attempts must be made by the technicians to protect their hands from contact with
methyl methacrylate. PVA (polyvinyl alcohol) gloves are classified as having
excellent permeation rate and breakthrough properties,32 however, they may not
give the technician sufficient flexibility and dexterity when handling the
monomer-polymer dough. Nevertheless, they are worth considering. They would
certainly be suitable during methyl methacrylate mixing and decanting procedures.
Other suitable gloves which were discussed earlier are, viton butyl gloves with a
18
breakthrough time of 20 minutes and PVP gloves with a breakthrough time of greater
than 30 minutes.
In an overseas report,21 the authors suggested that a disposable polyethylene
glove with a separate rubber fingerhood would be effective enough to prevent
methyl methacrylate penetration during quick procedures ( up to 10 minutes). It is
also advisable for the technicians to look at developing non-touch techniques in
order to minimise contact with methyl methacrylate. For example, avoid using the
finger dipping in methyl methacrylate liquid for smoothing and shaping the dentures,
instead, a satisfactory result may be achieved using a brush or another tool.
Respiratory protection is used as last resort only when other control measures ( eg
engineering controls) are not possible. If the worker needs to take additional control
measures from dust exposure, then, an approved dust respirator may be used which
complies with Australian Standards AS 1715- 1991 and AS 1716-1991. A half-face
cartridge respirator or a disposable respirator with class P1 filter is approved for use
against mechanically generated particulates from grinding and polishing operations.
The disposable respirators used for infection control are not recommended for dust.
If a respirator is used ensure that the worker is instructed on how to wear the
respirator correctly and on how to test for a good facial seal. At the end of the day,
if a cartridge type respirator is used, it should be inspected, cleaned, placed in a
plastic bag and stored away from any possible contamination. The cartridge should
be replaced on a regular basis. Depending on the degree of contamination, If a
disposable dust respirator is used, it may be stored away from any possible
contamination for re-use.
For all respirators which are stored, ensure that there is some way of identifying the
respirator with the user to avoid hygiene problems.
Other maintenance details are provided in the Australian Standard AS 1715-1991.
• Emergency Procedures
Ensure that emergency procedures for evacuation are developed and displayed and
procedures for chemical spillages are documented. Staff should also be trained in
these procedures. An appropriate fire extinguisher, located in a convenient place,
should also be available in the laboratory. This should be maintained regularly and a
suitable sign fixed above the extinguisher.
• First Aid
Ensure that first aid facilities are available in the laboratory which includes, an
eyewash and a burns module, as per the Approved Code of Practice, under the OHS
&W Act, 1986, "Occupational Health and First Aid in the Workplace. 1991".
• Additional Information
For additional information or clarification, please call the Industry Services Office,
Department for Industrial Affairs.
19
LOCAL EXHAUST VENTILATION SUGGESTIONS
IN THE DENTAL LABORATORY
APPENDIX 1:
Adjustable ibnigue - keep
as close to wheel os
1
1
1
L
25 D of least.
---t
-
r
Direct/on of rotation.
—1 I/1" for hard wheel.
3 for soft wheel.
_ ____
\
H-
-
\
0
..---""
.
_
,
z
,
..
i
v_
i
_
1
Trop with cleonout
when desirable.
r-0.75 D if
! possible
i\
\
\f‘
Minimum duct veloci.• 3500 fpm
Entry loss .. 0.65 VP for straight take-off
0.40 VP for tapered take-off
Wheel cham.
inches
Wheel width*
inches
Exhaust volume
cfm
Exhaust volume
cfrn
Good enclosure
Poor enclosure
/o9
2
300
400
over 9 to /6
3
500
6/0
over 16 t 19
4
610
740
over 19 to 24
5
740
/200
over 24 to 30
6
/040
/500
over 30 to 36
6
/200
/990
* /n cases of extra wide wheels, use
wheel width to determine exhaust
volume.
AMERICAN CONFERENCE OF
GOVERNMENTAL INDUSTRIAL HYGIENISTS
BUFFING AND POLISHING
DATE
1-82
I
VS -406
Figure 1: Conventional Exhaust Hood Design for a Buffing and Polishing Machine.
Courtesy American Conference of Govermnental Industrial Hygienists
20
APPENDIX 1:
LOCAL EXHAUST VENTILATION SUGGESTIONS
IN THE DENTAL LABORATORY
450 slope min
Slots -size for 1000 in
Baffles ore
desirable
A
o.10
PI
II
Maximum plenum velocity
1/2 slo/ velocity
= 350 ofm//inea/ ft of hood
Hood length =required working space
Bench width= 24"maximum
Duct velocity = 1000 - 3000 fpm
Entry loss=178 slot VP #025 duct VP
Figure 2: Swzgested Ventilation Arrangement for a Solvent Work Table.
Courtesy American Conference of Governmental Industrial Hygienists
- 21 -
APPENDIX 2:
EXAMPLES OF LOCAL EXHAUST VENTILATION
IN DENTAL LABORATORIES
BENCH GRINDING OPERATIONS
Note: The perspex guard should be adjustable and kept
as close to the wheel as possible to trap the line dust
Note: The exhaust inlet is as close as possible the grinding wheel. The front
perspex guard is missing.
- 22 -
APPENDIX 2:
EXAMPLES OF LOCAL EXHAUST VENTILATION
IN DENTAL LABORATORIES
HAND GRINDING OPERATIONS
Note: The glass Guard provides a Barrier Against the fine Dust.
Note: The hand grinding operation is performed close to the suction inlet and the PVC
pipe cut at an angle increases the dust trapping efficiency. The front guard(as
shown above) is missing.
- 23 -
APPENDIX 2:
EXAMPLES OF LOCAL EXHAUST VENTILATION
IN DENTAL LABORATORIES
"HOME MADE" EXHAUST VENTILATION CABINET
FOR HANDLING VOLATILE LIQUIDS
Note: the cabinet should be made of materials which do not react with the chemicals used.
The cabinet should have a bottom in the form of a tray to trap any spillages.
24
APPENDIX 3:
HAZARDOUS SUBSTANCES SURVEY
Ref No:
DOD
Organisation:
Contact Person:
Address:
Phone Number
No: of Workers-
Fax Number
25
Materials Used
Material
Supplier
Do you have
an MSDS
Where is the material
stored
26
Tasks
Worker
Main Tasks
How Long
(hours/min) is the
worker exposed to
the substance per day
27
Housekeeping
YN
• Is the workplace and amenities kept in a clean and hygienic state?
00
• Is the floor in good condition?
I=1
• Do disposal containers have lids?
DO
• Are all passages/spaces provided for normal movement and
emergency egress kept clear?
DO
Hazardous Substances Inventory
• Has a register been developed/maintained containing list
00
of all hazardous substances?
• Is this register readily accessible to all employees who could
be exposed to a hazardous substance?
1:1
• Are MSDS accessible for all hazardous substances?
DO
Storage/labelling
• Are containers holding hazardous substances labelled with the
identity of substance and the basic health and safety information?
00
• Do storage conditions confirm with MSDS recommendations?
DO
• Are potentially reactive chemicals stored together?
00
• Is any mixing or other handling of chemicals done in the storage area? DO
• Is ventilation available?
Local exhaust ventilation
general ventilation
00
00
• Are labels altered in any way
00
• Are decanted substances that are not used immediately labelled
with product name, risk phrase(s) and safety phrase(s)
00
28
YN
• Do containers containing hazardous substance remain correctly
labelled until they are cleaned
DO
• Are staff provided with appropriate instruction and training in
regards to hazardous substances
Do
• Are records of training made and kept for at least five years
from the date of the last entry
OD
Risk Assessment
• Has a risk assessments been conducted on the hazardous substances used?
DO
• If Yes, on which hazard substances?
• Was this recorded?
DO
• Has monitoring been conducted in the workplace?
Do
If Yes, Who by9
Which hazard substances'?
Routes of exposure?
• Has the worker been monitored for exposure?
whole work shift (no. of hours)
half work shift (no. of hours)
less than half work shift (no. of hours)
What type of sample was collected? Personal or Static
If personal monitoring was conducted, was the result less then the Exposure Standard?
• Was the monitoring for a:
• Were the recommendations from monitoring program carried out
Do
• Is there a documented safe work procedure for the process?
OD
• Have workers been trained in all safe work procedures?
Do
DO
• Is personal protective equipment available for use?
Type supplied.
29
YN
• Have staff been provided with training on the correct use and
maintenance of personal protective equipment?
00
• Are all the controls identified in the MSDS used?
( both personal and equipment/process)
00
• Is there appropriate signage
00
Emergency Procedure
• Are emergency facilities available and adequate? ( showers, eyewash)
00
• Are fire extinguishers of the correct type?
OD
• Are they regularly checked and maintained?
00
• Are they correctly labelled and located conveniently?
OD
• Are fire exits clear and accessible?
DO
• Has an evacuation procedure been developed and implemented?
DO
• Are evacuation procedures displayed?
OD
• Have procedures for handling spillages of hazardous substances been
developed and implemented?
CIO
• Is equipment available for the clean-up of spillage of hazardous substances?
DO
• Have staff received training in cleaning up hazardous substance spills?
00
• Are records of Hazardous substances readily available to emergency services?
DO
First Aid Facilities
• Do facilities comply with the Code of Practice?
00
• Do any staff members have first aid training?
00
30
Control Measures
Process
Hazardous substance
Control Measures
1\1° workers
exposed
31
BIBLIOGRAPHY
1.
National Occupational Health and Safety Commission: Exposure Standards For
Atmospheric Contaminants in the Occupational Environment. (NOHSC:1003, 1995),
Australian Government Publishing Service, Canberra.
2.
Sherson D, Maltbaek N, Olsen 0: Small Opacities among Dental Laboratory
Technicians in Copenhagen. Br J of Ind. Med. 1988; 45: 320-324
3.
Rom WN, Lockey JE, Lee JS, Kimball AC, Bang KM, et al: Pneumoconiosis and
Exposure of Dental Laboratory Technicians. AJPH 1984; 74: 1252-1257
4.
Kronenberger H, Morgenroth K, Tuengerthal S, at al: Pneumoconiosis in Dental
Technicians: Clinical, Physiological, Radiological, and Histological Findings. Am
Rev Respir Dis 1981; 123:127
5.
Morgenroth K, Kronenberger H: Lung Changes from Dental Materials. Quintessence
of Dental Technology 1984; 45-48
6.
Selden Al, Persson B, Bornberger-Dankvardt SI, Winstrom LE, Bondin LS: Exposure
to Cobalt Chromium Dust and lung Disorders in Dental Technicians. Thorax 1995;
50: 769-772
7.
Choudat D: Occupational Lung Diseases among Dental Technicians. Tubercle and
Lung Disease 1994; 75: 99-104
8.
Hansen HM: Silicosis In a Dental Technician. Ugeskrift for Laeger 1983; 145: 23782379
9.
Rispal P, Wen L, Precigout V, Aparicio M: Nephropathy with Silica in a Dental
Technician. Presse Medicale 1991; 20: 176
10.
Sherson D, Maltbaek N, Heydorn K: A Dental Technician with Pulmonary Fibrosis: A
Case of Chromium-Cobalt Alloy Pneumoconiosis? European Respiratory Journal
1990; 3: 1227-1229
11.
Barrett TB, Pietra GG, Maycock RL, Rossman MD, Minda JM, Johns LW: Acrylic
Resin Pneumoconiosis: Report of a Case in a Dental Student. Am Rev Respir Dis
1989; 139: 841-843
12.
Loewen GM, Weiner D, McMahan J: Pneumoconiosis in an Elderly Dentist. Chest
1988; 93: 1312-1313
13.
Morgenroth K, Kronenbrger H, Michalke G, Schnabel R: Morphology and
Pathogenesis of Pneumoconiosis in Dental Technicians. Pathology Research abd
Practice 1985; 179: 528-536
14.
Anonymous: Lung Disease in Dental Laboratory Technicians (Editorial). Lancet
1985; 1: 12001
32
15.
Brune D, Beltesbrekke H: Dust in Dental Laboratories. Part III: Efficiency of
Ventilation Systems and Face Masks. The Journal of Prosthetic Dentistry 1980;
44:211-215
16.
Brune D, Beltesbrekke H: Dust in Dental Laboratories. Part I. Types and Levels in
Specific Operations. The Journal of Prosthetic Dentistry 1980; 43: 687-692
17.
Brune D, Beltesbrekke H, Strand G: Dust in Dental Laboratories. Part II:
Measurement of Particle Distributions. The Journal of Prosthetic Dentistry 1980; 44:
82-87
18.
Rajaniemi R, Tola S: Subjective Symptoms Among Dental Technicians Exposed to the
Monomer Methyl Methactylate. Scand J Work Environ Health 1985; 11: 281
19.
National Occupational Health and Safety Commission: Documentation of the Exposure
Standards. (NOHSC:1003, 1995), Australian Government Publishing Service, Canberra.
20.
Rajaniemi R: Clinical Evaluation of Occupational Toxicity of Methyl Methactylate
inonomer to Dental Technicians. J Soc Occup Med 1986; 36: 56-59
21.
Estlander T, Rajaniemi R, Jolanki R: Hand Dermatitis in Dental Technicians. Contact
Dermatitis 1984; 10: 201-205
22.
Fisher AA: Allergic Sensitization of the Skin and Oral Mucosa to Acrylic Denture
Materials. Journal of the American Medical Association 1954; 156: 238
23.
Pegum JS, Medhurst FA: Contact Dermatitis from Penetration of Rubber Gloves by
Acrylic Monomer. British Medical Journal 1971; 2: 141
24.
Dane E, Vedel P, Jensen JS: Skin Protection Against Methyl methacrylate. Acta
Orthopaedica Scandinavica 1987; 58: 236-238
25.
Losewicz S, Davison AG, Hopkirk A, et al: Occupational Asthma Due to Methyl
Methactylate and Cyanoctylate. Thorax 1984; 39: 712-13
26.
Money C, Moss S, Fortin CM: An Assessment of the Health Status of Dental
Technicians Exposed to Methyl Methactylate. British Health and Safety Society
Newsletter 1987; 15: 11-15
27.
US National Institute for Occupational Safety and health, Research Report: Ventilation
Requirements for Grinding, Buffing and Polishing Operations. HEW Publication No.
75-107: 1974
28.
American Conference of Governmental Industrial Hygienists Inc.: Industrial
Ventilation -A Manual of Recommended Practice. 1987, 19th Edition, Ohio, USA
29.
Olishifski J(ed): Fundamentals Of Industrial Hygiene. US National Safety Council,
Chicago, USA, 2nd Edition,1979: 642
30.
Farli M, Gasparini M, Franalanci S, Gola M, Sertoli A: Occupational Contact
Dermatitis in 2 Dental Technicians. Contact Dermatitis 1990; 22: 282-287
33
31.
Rycroft RIG: Contact Dermatitis for Acrylic Compounds. British Journal of
Dermatology 1977; 96: 685-687
32.
Ansell Edmont: Ansell Edmont Chemical Resistance Guide. Ansell Edmont Industrial
Inc. 1990