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Recommended Exposure Limit (REL)

Last updated: June 30, 2018

The recommended exposure limit (REL) is a numeric value defining the maximum concentration or level of exposure to a hazardous substance or condition that is permitted in a workplace. RELs are set for hazards such as excessive noise, chemical fumes, airborne particulates, and biological agents. The recommended exposure limit for each hazard is used to ensure a safe working environment.

The U.S. National Institute for Occupational Safety and Health (NIOSH) establishes recommended exposure limits for hazards in the workplace. NIOSH defines limits for not only hazardous substances but also for physical or environmental hazards such as excessive noise. To determine the REL for a particular hazard, NIOSH policy makers combine information from several sources, including engineering and trade data as well as research studies. Other private and regulatory organizations also define exposure limits for workplace hazards. Other exposure guidelines include permissible exposure levels (PELs), occupational exposure limits (OELs), workplace environmental exposure limits (WEELs), and threshold limit values (TLVs).

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In most instances, a recommended exposure limit's definition is based on a time-weighted average (TWA) for the exposure. Time-weighted averages are measure the average exposure a worker would experience during a work day of up to 10 hours or a 40-hour work week. An REL based on a time-weighted average indicates that an employee may be exposed to the agent at a level that on average does not exceed the stated value.

In some instances, there is a maximum value at which an employee could suffer harm even with a single or short-term exposure. If a hazard could cause harm with a single short-term or high concentration exposure, a ceiling (C) or short-term exposure value (STEV) will be defined.

When an REL is based on the average level of exposure a worker is expected to experience in a particular environment, the concentration or level of exposure to a hazardous agent in the workplace may exceed the REL. If a ceiling level has been set, than an exposure at that level of any duration is considered harmful. Workplace exposures may not exceed this maximum ceiling level at any point--even if the average level of exposure remains below the REL. Employers are expected to monitor and take preventive measures to ensure that the average for a TWA value or acute exposure for a C value does not exceed the stated RELs.

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