Occupational exposure limits

Chemical and biological health hazards and control

Details of a COSHH assessment

Occupational exposure limits

One of the main purposes of a COSHH assessment is to control adequately the exposure of employees and others to hazardous substances. This means that such substances should be reduced to levels which do not pose a health threat to those exposed to them day after day at work. The HSC has assigned occupational exposure limits (OEL) to a large number of hazardous substances and the HSE publishes an annual update in a publication called Occupational Exposure Limits EH4O. Two types of exposure limit are published:

1    Maximum Exposure Limit (MEL) – this limit is set for substances which may cause serious health problems and must not be exceeded and should be reduced to as low as is reasonably practicable


Details of a COSHH assessment

         Table 12.1 Examples of Occupational Exposure Limits (OEL)

2    Occupational Exposure Standard (OES) – is the level at or below which little health risk is likely to occur to persons exposed to the hazardous substance. If the level is exceeded, then steps should be taken to reduce the exposure. Work need not cease when the OES is exceeded provided that reasons are given and that the exposure levels will be reduced as soon as is reasonably practicable.

The two exposure limits are subject to time-weighted averaging. There are two such time-weighted averages (TWA) – the long-term exposure limit (LTEL) or 8-hour reference period and the short-term exposure limit (STEL) or 15-minute reference period. The 8-hour TWA is the maximum exposure allowed over an 8-hour period, so that if the exposure period was less than 8 hours the maximum exposure limit is increased accord-ingly with the proviso that exposure above the LTEL value continues for no longer than 1 hour.

For example, if a person was exposed to a hazardous substance with a OES of 100 mg/m3 (8-hour TWA) for 4 hours, no action would be required until an exposure level of 200 mg/m3 was reached. (Exposure at levels between 100 and 200 mg/m3 should be restricted to 1 hour.)

If, however, the substance has an STEL of 150 mg/m3, then action would be required when the exposure level rose above 150 mg/m3 for more than 15 minutes. 

The STEL always takes precedence over the LTEL. When a STEL is not given, it should be assumed that it is three times the LTEL value.

The publication EH4O is a valuable document for the Health and Safety Professional since it contains much additional advice on hazardous substances for use during the assessment of health risks, particularly where new medical information has been made public.

It is important to stress that if an MEL is exceeded, the process and use of the substance should cease immediately. In the longer term, the process, the control and monitoring measures should be reviewed and health surveillance considered. The overriding requirement for any hazardous substance, which has an MEL, is to reduce exposure to as low as is reasonably practicable.

Finally, there are certain limitations on the use of the published exposure limits:

•    they are specifically quoted for an 8-hour period (with an additional STEL for many hazardous substances). Adjustments must be made when exposure occurs over a continuous period longer than 8 hours;

•    they can only be used for exposure in a workplace and not to evaluate or control non-occupational exposure (e.g. to evaluate exposure levels in a neighbourhood close to the workplace, such as a playground);
•    OESs are only approved where the atmospheric pressure varies from 900 to 1100 millibars. This could exclude their use in mining and tunnelling operations;
•    they should also not be used when there is a rapid build-up of a hazardous substance due to a serious accident or other emergency. Emergency arrangements should cover these eventualities.


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