7.3 Introduction to Occupational Exposure Limits

The purpose of occupational exposure limits

Occupational exposure limits (OELs) are set to prevent occupational disease and other harm to workers exposed to hazardous chemicals. OELs assume that the people exposed are healthy adult workers. They are tools that help employers protect the health of workers who may be exposed to chemicals in the working environment.

In general, an OEL indicates the level of exposure, for a given length of time (usually 8 hours), to a chemical hazard that is not likely to affect a worker's health.

Many professional organisations around the world set such limits, for example the American Conference of Governmental Industrial Hygienists (ACGIH) and the National Institute for Occupational Safety and Health (NIOSH) in the United States.

Workplace exposure limits (WELs) are the occupational exposure limits of Great Britain, approved by the HSE. Many thousands of substances are used at work, but only about 500 have WELs. The publication EH40, Workplace exposure limits, lists all the current GB WELs.

Limits are set on the basis of the chemical properties of the substance, experimental studies on animals and humans, and toxicological and epidemiological data.

Long-term and short-term limits

The effects of exposure to hazardous substances vary a great deal with the nature of the substance and the pattern of exposure, and some effects need prolonged or accumulated exposure.

WELs are concentrations of hazardous substances in the air, averaged over a stated period, which is called a time-weighted average (TWA). Two periods are used.

• Long-term exposure limit (LTEL). Designed to protect the workforce from concentrations that, over a long time, could cause chronic ill health. It uses 8 hours, to control effects by limiting the total intake over one or more work shifts.
• Short-term exposure limit (STEL). Relates to peak exposure and is designed to protect against immediate acute effects. It is usually 15 minutes, to control effects that may be seen after a brief exposure.

A STEL limits short-term exposure during work. It is not an emergency-entry allowance or a safe time to spend in a leak. Emergency response needs its own competent assessment, suitable equipment and rescue arrangements; do not use a workplace limit to authorise entry into an unknown or immediately dangerous atmosphere.

Both the LTEL and the STEL must be monitored to give a correct picture of exposure. A substance with no WEL is not guaranteed to be safe, and monitoring and control of exposure may still be needed.

A legal limit or guideline, such as an OEL, should never be seen as a line between safe and unsafe. The best approach is always to keep exposure, or the risk from a hazard, as low as possible.

For example, many carcinogens have no exposure limit, because it is difficult to say with certainty that exposure below a set point is unlikely to cause harm. For carcinogens and other specific agents such as allergens, the principle of as low as reasonably practicable applies. In practice this means exposure should be eliminated or reduced as much as possible.

Summary notes for Element 7 · Cheat sheet · Glossary. A study aid, not published or endorsed by NEBOSH.