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Permissible exposure limit: what it means for the CHST

Toe Board editorial teamUpdated 3 min read

A permissible exposure limit, or PEL, is the legal maximum amount of a chemical, dust or physical agent such as noise that OSHA allows a worker to be exposed to. Many PELs are written as an 8-hour time-weighted average, so short peaks are allowed as long as the average over the shift stays at or under the limit.

A line graph of exposure across an 8-hour shift with peaks and dips, a dashed horizontal line marked PEL and a flat line marked 8-hour average sitting below it
Peaks can cross the line. The shift average is what a TWA limit is measured against.
Use this diagram

Where the numbers live

For construction, the general table of airborne limits is in 29 CFR 1926.55 and its appendix. Substances with their own full standards carry their own PEL inside that standard: respirable crystalline silica in 1926.1153, lead in 1926.62 and asbestos in 1926.1101, among others. Noise is in 1926.52, as a table of allowed hours at each sound level starting at 90 dBA for 8 hours.

Our health hazards guide gives the silica and lead figures: both have a PEL of 50 micrograms per cubic meter as an 8-hour average.

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Three kinds of limit

  • Time-weighted average. The average over a full shift, the most common form. See time-weighted average for the arithmetic.
  • Short-term exposure limit. An average over a short window, usually 15 minutes, for substances where brief high exposure matters.
  • Ceiling. A level that must not be exceeded at any moment.

A substance can carry more than one. Read which kind a limit is before comparing a sample result with it.

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What a PEL is not

A PEL is a legal floor for compliance, not a line between safe and harmful. Many of OSHA's general air limits were adopted around 1970 and have not been updated, and OSHA itself points employers to newer recommended limits from NIOSH and other bodies. Many substance standards also set an action level, a lower number that switches on monitoring and medical duties before the PEL is reached.

Common mix-ups

  • Comparing a 15-minute sample with an 8-hour limit. Match the sample period to the type of limit.
  • Treating a respirator as meeting the PEL. The PEL is about the air in the worker's breathing zone. Controls come first under the hierarchy, and respirators are used when those are not enough.
  • Using the general industry table for construction. The rules are in different parts. Cite Part 1926 on a construction site.

On the CHST exam

Exposure limits sit in the hazard and risk identification and control domain, 36.6% of the CHST, with silica, lead, noise and chemicals all named in its knowledge list.

Practice it

Noise is the easiest PEL to work by hand. Try the noise dose calculator, which turns hours at each sound level into a dose and an 8-hour TWA. All twenty terms are in the CHST terms glossary.

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