OSHA PEL explained with TWA, STEL, ceiling and action level
An OSHA PEL is the legal limit on a worker's airborne exposure to a chemical, usually an 8-hour TWA. See TWA, ceiling, STEL, action level and lookup.
OSHA PEL (permissible exposure limit) explained
An OSHA PEL, or permissible exposure limit, is the legal maximum on a US worker's airborne exposure to a substance. Most are 8-hour time-weighted averages (TWAs) in ppm or mg/m3, listed in the Z-tables of 29 CFR 1910.1000.
These permissible exposure limits are mandated by OSHA. OSHA says approximately 500 PELs have been established. Some substances have a separate standard, such as benzene, with its own TWA, short-term exposure limit (STEL) and action level.
PEL definition and where the limits live
Under 1910.1000, an employee's exposure to any substance in Tables Z-1, Z-2 or Z-3 "shall be limited in accordance with the requirements" of that section. Table Z-1 is "Limits for Air Contaminants", Z-2 covers substances with a TWA, an acceptable ceiling and an acceptable maximum peak, and Z-3 covers mineral dusts.
Table Z-1 has columns for substance, CAS number, ppm, mg/m3 and skin designation (OSHA Table Z-1). Its footnote says the PELs "are 8-hour TWAs unless otherwise noted; a (C) designation denotes a ceiling limit", measured from breathing-zone air samples. For gases and vapors, ppm means parts per million by volume at 25 °C and 760 torr. When a value appears only in the mg/m3 column it is exact, and when it sits next to a ppm value it is approximate.
General industry uses 1910.1000. Construction has its own table in 29 CFR 1926.55, and shipyards use Table Z in 29 CFR 1915.1000.
TWA, ceiling, STEL and action level
A PEL can be a TWA, a ceiling, a peak or a STEL. An action level is not itself a PEL. The air contaminant rule and substance standards set these terms:
| Limit | Averaging time | What it means |
|---|---|---|
| 8-hour TWA | 8-hour shift | Average exposure may not exceed the limit in any 8-hour shift of a 40-hour week |
| Z-1 ceiling (C) | Instantaneous | Never exceeded. If instantaneous monitoring isn't feasible, assess it as a 15-minute TWA |
| Z-2 peak | Set per substance | Exposure may go above the ceiling only up to the peak, for the time allowed |
| STEL | 15 minutes | A short-term average limit, set in standards such as benzene |
| Action level | Usually 8 hours | A trigger for duties such as monitoring and medical surveillance, not an exposure cap |
The OSHA TWA in 1910.1000(a)(2) says exposure "shall not exceed the 8-hour Time Weighted Average given for that substance in any 8-hour work shift of a 40-hour work week."
A Z-1 limit marked "C" "shall at no time exceed the exposure limit." Only when instantaneous monitoring is not feasible is that ceiling assessed as a 15-minute TWA.
Table Z-2 adds a third number, the peak. Toluene shows how it works: 200 ppm 8-hour TWA, 300 ppm acceptable ceiling and 500 ppm acceptable maximum peak for 10 minutes (1910.1000).
Table Z-1 has no STEL column. STELs come from substance-specific standards. Benzene workers may not be exposed above 5 ppm "as averaged over any 15 minute period" (1910.1028), and formaldehyde has a 2 ppm STEL (1910.1048).
Action levels are defined in substance standards, not the generic Z-tables. For benzene, the action level is 0.5 ppm as an 8-hour TWA, against a 1 ppm PEL. Exposure at or above that action level, but at or below the TWA, requires repeat monitoring at least every year. Medical surveillance applies if benzene exposure is at or above the action level on 30 or more days a year. You'll often hear that the action level is half the PEL. That holds for benzene and for respirable crystalline silica (25 µg/m3 against a 50 µg/m3 PEL). But it does not hold for lead (30 µg/m3 against a 50 µg/m3 PEL) or formaldehyde (0.5 ppm against a 0.75 ppm TWA).
For mixtures, 1910.1000(d)(2)(i) adds each concentration divided by its limit (C1 ÷ L1 + C2 ÷ L2 and so on), and the sum "shall not exceed unity (1)". Skin designations warn against skin contact. They are not an airborne limit.
How OSHA enforces PELs
Administrative or engineering controls must first be determined and implemented whenever feasible. If those controls cannot reach full compliance, the employer uses respirators or other protective equipment. Respirator use must follow 1910.134.
To check an 8-hour TWA, OSHA inspectors take representative full-shift samples, defined as "a minimum of the total time of the work shift less one hour" (OSHA Technical Manual).
A chemical with no PEL is not a free pass. The General Duty Clause covers recognized hazards causing or likely to cause death or serious physical harm. The Field Operations Manual says exposure above an ACGIH TLV or NIOSH REL, where no OSHA PEL exists, "can be considered under Section 5(a)(1)". Below the PEL but above a TLV or REL, citations "will not normally be issued". Maximum penalties are $16,550 per serious violation and $165,514 per willful or repeated violation, and 2025 penalty amounts remain in effect for 2026.
Why many PELs are out of date
OSHA says many of its PELs are "outdated and inadequate". Most Z-table values were adopted from the Walsh-Healy Public Contracts Act, which in turn had adopted the 1968 ACGIH TLVs. A 2014 Federal Register notice says OSHA adopted the current PELs in 1971, then tried a 1989 update that would have lowered limits for 212 chemicals and added 164 more. The Eleventh Circuit vacated that rule in 1992. The Z-table PELs remain in effect.
The annotated PEL tables show those PELs beside Cal/OSHA PELs, NIOSH RELs, and ACGIH TLVs. OSHA recommends the alternatives because exposure above some of them may still be hazardous even when the employer meets the PEL. Cal/OSHA and NIOSH columns are dated November 2021, and the tables omit skin-absorption and sensitization notes. NIOSH RELs and ACGIH TLVs aren't federal law. Cal/OSHA PELs are binding in California: toluene is 10 ppm under Title 8 §5155 against a 200 ppm federal TWA, and carbon monoxide is 25 ppm against 50 ppm.
Where to find the PEL for a chemical
- Search the OSHA Occupational Chemical Database by name or CAS number. PELs appear in the "Exposure Limits" table for each chemical.
- Check the binding text in 1910.1000. Some Z-1 rows give no number and send you elsewhere: benzene points to 1910.1028, and toluene points to Table Z-2.
- Use the right table for the work: 1910 for general industry, 1926.55 for construction, 1915.1000 for shipyards.
- Check applicable state-plan limits, such as California's Table AC-1.
- Read Section 8 of the safety data sheet. HazCom Appendix D requires the OSHA PEL, the ACGIH TLV, and any other exposure limit the preparer uses or recommends, where available.
A missing Z-table entry alone doesn't establish that there is no federal PEL. Check substance-specific standards too. Where no PEL exists, the General Duty Clause can still apply.
The NIOSH Pocket Guide lists OSHA PELs beside NIOSH RELs, but its REL TWA runs up to a 10-hour workday in a 40-hour week. The OSHA TWA is 8 hours.
RegAffairs AI answers what an OSHA PEL is and where to find the limit for a chemical.