Permissible exposure limit
A permissible exposure limit (PEL) is a legal limit in the United States for a worker's exposure to a chemical substance or physical agent, such as noise. PELs are established and enforced by the Occupational Safety and Health Administration (OSHA). Most OSHA PELs were adopted shortly after passage of the Occupational Safety and Health Act of 1970.
For air contaminants, exposure limits are commonly expressed in parts per million (ppm) or milligrams per cubic meter (mg/m3). Units for physical agents such as noise are specific to the hazard being measured. A PEL is usually expressed as an 8-hour-time-weighted average, although some standards also use short-term exposure limits or ceiling limits.
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, usually a nominal eight hours. This means that for limited periods, a worker may be exposed to concentration excursions higher than the PEL as long as the TWA is not exceeded and any applicable excursion limit is not exceeded. An excursion limit typically means that "...worker exposure levels may exceed 3 times the PEL-TWA for no more than a total of 30 minutes during a workday, and under no circumstances should they exceed 5 times the PEL-TWA, provided that the PEL-TWA is not exceeded." Excursion limits are enforced in some states (for example Oregon) and on the federal level for certain contaminants such as asbestos.
A short-term exposure limit is one that addresses the average exposure over a 15-30 minute period of maximum exposure during a single work shift. A ceiling limit is one that may not be exceeded even for a moment, and is applied to irritants and other materials that have immediate effects.