Permissible exposure limit

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The Permissible Exposure Limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a substance or physical agent. For substances it is usually expressed in parts per million (ppm), or sometimes in milligrams per cubic metre (mg/m³). Units of measure for physical agents such as noise are specific to the agent. Permissible Exposure Limits are established by the Occupational Safety and Health Administration (OSHA).

A PEL is usually given as a time-weighted average (TWA), although some are Short Term Exposure Limits (STEL) or Ceiling Limits. A TWA is the average exposure over a specified period of time, usually a nominal eight hours. This means that, for limited periods, a worker may be exposed to concentrations higher than the PEL, so long as the average concentration over eight hours remains lower.

A Short Term Exposure Limit is one that addresses the average exposure over the a 15-30 minute period of maximum exposure during a single work shift.

A Ceiling Limit is one that may not be exceeded, and is applied to irritants and other materials that have immediate effects.