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NIOSH Recommended Exposure Limit (REL)

Last updated: January 15, 2019

The NIOSH Recommended Exposure Limit (REL) is an occupational exposure limit (OEL) put forth by the National Institute for Occupational Safety and Health, a U.S. federal agency responsible for conducting and publishing research on the prevention of work-related injury and illness.

The NIOSH RELs are one of the major OELs used in the United States. Other major OELs include OSHA’s Permissible Exposure Limit (PEL) and the American Conference of Governmental Industrial Hygienists' (ACGIH) Threshold Limit Values (TLVs).

The NIOSH RELs are not legally binding OELs. However, because they are readily available and easy to look up, they provide employers and workers with an important comprehensive reference for understanding safe exposure limits. OSHA’s list of legally binding PELs only covers a limited number of chemicals; therefore, employers using chemical substances that do not have PELs may need to use RELs or another recognized OEL (such as the ACGIH’s TLVs) in order to ensure they are meeting their general duty to keep their employees safe.

Although the NIOSH RELs are published for use in workplaces, they also constitute recommendations to OSHA to update a specific PEL to reflect the NIOSH REL or to create a new REL for a given chemical. In situations where the OSHA PEL and the NIOSH REL differ, which is common, the exposure limit provided by the REL is generally significantly lower than the PEL. This is because the NIOSH RELs are a strictly science-based measurement that attempts to reflect the limit below which no worker harm can occur. OSHA PELs also take into account the economic interests of employers and only aim to act as a limit below which serious harm should not occur to most workers. Like other OELs, RELs are given as either a time-weighted average (TWA) limit that is indexed to eight-hour exposures, a short-term exposure limit indexed to 15-minute exposures, or a ceiling (maximum) exposure limit.

The NIOSH is part of the U.S. Center for Disease Control. RELs are listed in NIOSH’s Pocket Guide to Chemical Hazards, which is available on the NIOSH website.


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