4.1 Workplace exposure limits (WELs) Flashcards
Workplace exposure limits (WELs) are
occupational exposure limits (OELs) set
under the CoSHH regulations in order to help protect the health of workers
In the UK WELs are set at levels which are either:
The level at which no adverse effects on human health (NOAEL) would be
expected to occur based on the known and/or predicted effects of the substance,
or if this is not reasonably achievable.
A level corresponding to what is considered to represent good control, taking
into account the severity of the likely health hazards and the costs and efficacy of
control solutions.
NOAEL is
the highest point on the exposure response curve at which there are no
adverse health effects observed in humans
Control of exposure to substances hazardous to health is adequate if:
The principles of good practice set out in schedule 2A of CoSHH (page 87) have
been applied
WELs have not been exceeded
Exposure is reduced to as low a level as is reasonably practicable for:
- Carcinogens
- Mutagens
- Respiratory sensitisers or any other substance which the risk assessment has
shown to be a potential cause of occupational asthma.
WELs apply only to people at work and to conditions where the atmospheric
pressure is normal, i.e. between 900 and 1100 millibars, and WELs are expressed in
the following way:
Parts per million (PPM) for gases and vapours
Milligrams per cubic metre (mg/m
-3
) for dust, fume, airborne particles.
Short-term
exposure limits
(STELs)
15 minute reference period
Protect against acute adverse health effects arising from brief
exposures
Example: eye irritation, narcosis from an alcohol based
substance, irreversible tissue damage such as asbestos
Long-term
exposure limits
(LTELs)
8 hour reference periods
Control health effects arising from prolonged or accumulated
exposure
Restricting the total intake by inhalation over one or more work
shifts
Example: ammonia, styrene, trichloroethylene
If the working exposure is less than 8 hours, the LTEL may be exceeded in
proportion to the hours worked, as long as
the STEL is never exceeded
Some dusts have a specific LTEL, for example: silica dust. However, where there is
no WEL assigned, dust may still be harmful, especially if it is respirable i.e. enters
the alveoli. In this case CoSHH sets the WEL for ‘nuisance dust’ LTEL as:
Respirable dust = 4 mg/m3
Total inhalable dust = 10 mg/m3
Limitations to the application of exposure limits
There are a number of problems associated with standard setting.
Inhalation may not be the sole route of entry into the body. Some substances
have the ability to penetrate intact skin and become absorbed into the body, thus
contributing to systemic toxicity.
The exposure limits relate to personal monitoring.
The doses received by workers may vary depending upon, for example, their
work rate. A higher work rate will cause workers to breathe more deeply and
inhale more air. This is not taken into consideration during standard setting … but
must be taken into account during CoSHH assessment.
Working conditions may impose additional stress on the body, for example: high
temperatures, pressures and humidity may increase the toxic response to a
substance.
Errors in estimating workplace exposure could have significant effects on the
controls decided upon.
Toxicological knowledge may change with new knowledge.
More prescriptive risk controls lead to the introduction of the eight ‘principles of good
practice’, which apply to all agents regardless of whether a substance has a WEL:
- Design and operate processes and activities to minimise emission, release and
spread of substances hazardous to health. - Take into account all relevant routes of exposure.
- Ensure control measures are proportionate to the health risk.
- Choose the most effective and reliable control options to minimise the escape
and spread of hazardous substances. - Where adequate control cannot be achieved by other means, provide, in
combination with other control measures, suitable personal protective equipment
(PPE). - Check and regularly review control measures to ensure their continuing
effectiveness. - Inform and train all employees on the hazards and risks and the control
measures developed to minimise the risks. - Ensure that the introduction of control measures does not increase the overall
risk to health and safety.
The occupational exposure limits for lead are:
Lead (other than lead alkyls): 0.15 mg/m3
Lead alkyls: 0.10 mg/m3.
The term ‘significant’ in relation to exposure to lead is defined in Regulation 2 of
CLAW as:
1) Where any employee is, or is likely to be, exposed to a concentration of lead in
the atmosphere exceeding half the occupational exposure limit.
2) Where there is substantial risk of any employee ingesting lead.
3) Where there is the risk of contact between the skin and lead alkyls or other
substances containing lead that can be absorbed through the skin.
Blood Lead Action level Suspension level
General employee
Young person under 18
Woman of child bearing age
Blood Lead Action level Suspension level
50 μg/100ml 60 μg/100ml
40 μg/100ml 50 μg/100ml
25 μg/100ml 30 μg/100ml
LTEL
long term exposure
limit (asbestos)
0.1f/cm3
(equivalent to 0.1 f/ml) over 4 hours
