Guide
Reading Exposure Limits: PEL, REL, TLV, IDLH, STEL and Ceiling
An occupational exposure limit is the airborne concentration of a chemical that a worker may breathe under defined conditions. A PEL is a legally enforceable OSHA limit, a REL is a NIOSH recommendation, a TLV is an ACGIH guideline, a STEL is a short-term (15-minute) limit, a Ceiling is a value that should not be exceeded at any time, and IDLH is a level that is immediately dangerous to life or health.
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The terms at a glance
| Term | Full name | Who sets it | Legal status |
|---|---|---|---|
| PEL | Permissible exposure limit | OSHA | Legally enforceable in US workplaces |
| REL | Recommended exposure limit | NIOSH | Recommendation; not enforceable on its own |
| TLV | Threshold limit value | ACGIH | Guideline; not enforceable unless adopted by a regulator |
| TWA | Time-weighted average | (a form of limit) | Average over a typical 8-hour shift (10-hour for NIOSH RELs) |
| STEL | Short-term exposure limit | OSHA, NIOSH, ACGIH | Average over a short period, typically 15 minutes, not to be exceeded |
| Ceiling (C) | Ceiling limit | OSHA, NIOSH, ACGIH | Concentration that should not be exceeded at any time |
| IDLH | Immediately dangerous to life or health | NIOSH | Reference level for respirator selection and emergency planning |
The glossary has entries for each, including PEL. The numeric values for many substances appear in the NIOSH Pocket Guide and in OSHA's tables, and our chemical pages show them with sources where available.
PELs: the legal limit, but not always the safest
OSHA's PELs are in its air contaminants standard (29 CFR 1910.1000 and related tables) and in substance-specific standards. Many of the general PELs were adopted decades ago and have not been revised, so for some substances newer science suggests lower levels. For that reason, safety professionals often compare the PEL with the NIOSH REL and ACGIH TLV and use the lowest value as a working target, while still meeting the legal minimum. SDS authors list the limits they know about in Section 8.
Units: ppm, mg/m3 and what they mean
Limits are given as parts per million (ppm) by volume in air, or as milligrams per cubic metre (mg/m3). Gases and vapors are commonly in ppm; dusts, mists and fumes are in mg/m3. The two can be converted for gases using the molar mass at standard conditions, but the converted value is only approximate. Make sure that you compare limits in the same unit. A "skin" notation means skin absorption may contribute significantly; air monitoring then does not tell the whole story. See choosing gloves.
Using the numbers
- A limit is not a line between safe and unsafe. Individual susceptibility varies, and the aim is to keep exposures as low as practical.
- Compare measured exposure over the same averaging time as the limit: 8-hour average against TWA, short sample against STEL.
- Exposure monitoring by a qualified person is needed to know whether you are above or below a limit. Smell is not a reliable warning for many chemicals.
- For mixtures with several ingredients acting on the same organ, combine their effects as an occupational hygienist would; see SDSs for mixtures.
- For respirator selection, use the limits, the IDLH and the protection factor of the respirator type, within a respiratory protection program.
IDLH in plain terms
IDLH values are NIOSH's estimate of a concentration that could cause death, irreversible health effects or prevent escape within a short period (30 minutes in the original definition). They are used for emergency planning and in choosing respirators, and are not working limits. Work in an IDLH atmosphere requires specialized equipment and procedures.
Limits in other countries
Other countries set their own values: for example, the EU has indicative and binding occupational exposure limit values at EU level and national limits, the UK publishes workplace exposure limits, and Germany, Canada and Australia have their own tables. A foreign sheet may list a value that does not apply at your site. Check which authority governs your workplace. See also the toxicology data in Section 11 that underlie many limits.
Frequently asked questions
What is the difference between a PEL and a TLV?
A PEL is an OSHA limit that is legally enforceable in US workplaces. A TLV is a guideline published by ACGIH. Neither is automatically stricter than the other.
What does TWA mean?
Time-weighted average: the average airborne concentration over a work shift, usually 8 hours. Short peaks are covered by STEL or Ceiling values.
What is a STEL?
A short-term exposure limit, normally a 15-minute average that should not be exceeded at any time during the workday.
Is being below the limit safe?
Not necessarily. Limits do not guarantee protection for every person, do not account for skin absorption unless noted, and may be out of date. Keep exposures as low as practical.