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IDLH means “immediately dangerous to life or health.” Under OSHA’s respiratory protection standard it is an atmosphere that poses an immediate threat to life, would cause irreversible adverse health effects, or would impair a person’s ability to escape. NIOSH publishes numeric IDLH values for individual chemicals, and OSHA’s rule decides which respirators may be used when an atmosphere is, or must be treated as, IDLH. This page is general safety information, not a hazard assessment for any specific workplace.
Definition and scope: two uses of one term
“IDLH” appears in two places that do different jobs, and mixing them up is the most common source of confusion.
| Where it appears | What it is | What it is used for |
|---|---|---|
| OSHA 29 CFR 1910.134(b) | A regulatory definition: an atmosphere that poses an immediate threat to life, would cause irreversible adverse health effects, or would impair an individual’s ability to escape from a dangerous atmosphere. | Deciding which respirators an employer may provide (paragraph (d)). |
| NIOSH IDLH values | Chemical-specific concentrations published by NIOSH. The NIOSH Pocket Guide describes them as meant to let a worker escape if respiratory protection fails. | Technical reference for hazard evaluation and emergency planning; not an enforceable exposure limit. |
Sources: OSHA 29 CFR 1910.134 and NIOSH Pocket Guide introduction, both read 2026-11-07.
How IDLH works in practice
What a NIOSH IDLH value is based on
The original IDLH values date from the 1970s and were revised in 1994. In 2014 NIOSH issued Current Intelligence Bulletin 66, “Derivation of Immediately Dangerous to Life or Health (IDLH) Values” (NIOSH 2014-100), which sets the newer derivation criteria. NIOSH’s IDLH index states that it aims to re-evaluate all IDLH values with the CIB 66 criteria, and that until that work is finished the list mixes values from the 1994 and CIB 66 approaches. The Pocket Guide notes that IDLH values are based on effects that might occur during a 30-minute exposure, but stresses that workers should leave immediately rather than stay for that period.
What is the IDLH value for chlorine?
NIOSH’s chlorine documentation lists a current IDLH of 10 ppm, based on acute inhalation toxicity data in humans (Freitag 1941; ILO 1971; NPIRI 1983). The original value was 30 ppm, based on an ILO (1971) statement that exposure to 30 ppm causes intense coughing fits. The NIOSH documentation is dated May 1994 and the page was last reviewed 4 December 2014.
This is the “two values” trap. A table, safety data sheet or training slide that still shows 30 ppm is quoting the superseded figure. NIOSH’s index says that where an entry lists two values, the one based on CIB 66 should be treated as current NIOSH policy; check the basis of any value before relying on it.
What respirator is safe in an IDLH atmosphere?
OSHA 1910.134(d)(2)(i) requires employers to provide, for IDLH atmospheres, either a full-facepiece pressure-demand self-contained breathing apparatus (SCBA) certified by NIOSH for a minimum service life of 30 minutes, or a combination full-facepiece pressure-demand supplied-air respirator with an auxiliary self-contained air supply. Respirators provided only for escape from IDLH atmospheres must be NIOSH-certified for escape from the atmosphere in which they will be used ((d)(2)(ii)).
Two further rules matter for planning:
- All oxygen-deficient atmospheres (oxygen content below 19.5% by volume) shall be considered IDLH, (d)(2)(iii).
- Where the employer cannot identify or reasonably estimate employee exposure, the employer shall consider the atmosphere to be IDLH, (d)(1)(iii).
The standard also states that employers must not apply maximum use concentrations (MUCs) to IDLH conditions and must instead use the respirators listed in (d)(2). For protection factors and filter classes in non-IDLH use, see respirator levels; for the written program, fit testing and medical evaluation, see the laboratory respiratory protection program guide.
IDLH, PEL, REL and STEL compared
What are PEL and STEL?
A PEL (permissible exposure limit) is an OSHA limit from 29 CFR 1910.1000 that OSHA enforces; TWA PELs must not be exceeded during any 8-hour shift of a 40-hour week. A STEL (short-term exposure limit), marked “ST” in the Pocket Guide, is a 15-minute TWA exposure that should not be exceeded at any time during a workday. A ceiling value, marked “C,” should not be exceeded at any time. An REL (recommended exposure limit) is a NIOSH recommendation whose TWA covers up to a 10-hour workday in a 40-hour week.
| Value | Issued by | Time basis (per the Pocket Guide) | Question it answers |
|---|---|---|---|
| PEL | OSHA | 8-hour TWA for TWA-type limits; ST and C also used | What exposure is legally permitted over a shift? |
| REL | NIOSH | TWA for up to 10 hours; ST and C also used | What does NIOSH recommend over a shift? |
| STEL (“ST”) | Marked on PEL or REL entries | 15-minute TWA, not to be exceeded at any time in a workday | What short peak is tolerable? |
| Ceiling (“C”) | Marked on PEL or REL entries | Not to be exceeded at any time; not averaged | What instantaneous level is the cap? |
| IDLH | NIOSH value; OSHA definition | Based on effects of a 30-minute exposure; workers should leave at once | At what point could a person fail to escape or suffer irreversible harm? |
PEL, REL, STEL and ceiling describe exposures to be kept under control during work. IDLH describes an escape threshold, not a level at which working is acceptable with a respirator that is not IDLH-rated. Where PEL and REL differ, the different averaging periods (8 hours versus up to 10 hours) are part of the reason a bare comparison can mislead. See the NIOSH Pocket Guide explainer for reading an entry, and ethylene oxide exposure for a worked example of OSHA limits, action levels and monitoring triggers.
Where it matters for labs
- Spill and release planning. Because an unknown concentration must be treated as IDLH, a response plan that assumes an air-purifying respirator will do when no measurement exists conflicts with 1910.134(d)(1)(iii).
- Confined or gas-filled spaces. Any oxygen-deficient atmosphere (below 19.5%) is IDLH by rule, whatever the contaminant, which is relevant to cryogen and inert-gas storage areas.
- Reading reference tables. Confirm which derivation (1994 or CIB 66) a listed value uses, as the chlorine entry shows.
Common misunderstandings
- “IDLH is an exposure limit.” It is an escape and respirator-selection concept, not a limit under which work may be done.
- “The 30-minute basis means a worker can stay 30 minutes.” NIOSH states workers should leave immediately.
- “A chemical’s IDLH value is fixed.” NIOSH values are being re-evaluated under CIB 66 and some entries carry two values.
- “A cartridge respirator is acceptable if the concentration is below the MUC.” MUCs are not applied to IDLH conditions.
Frequently asked questions
What is the definition of IDLH according to OSHA?
In 29 CFR 1910.134(b), an IDLH atmosphere is one that poses an immediate threat to life, would cause irreversible adverse health effects, or would impair an individual’s ability to escape from a dangerous atmosphere.
What is the IDLH value for chlorine?
NIOSH lists 10 ppm as the current value; the original value was 30 ppm. See the chlorine section above.
What is PEL and STEL?
PEL is OSHA’s enforceable permissible exposure limit (8-hour TWA basis for TWA-type limits); STEL is a 15-minute TWA that should not be exceeded at any time during a workday.
What type of respirator is safe to use in an IDLH atmosphere?
Under 1910.134(d)(2), a NIOSH-certified full-facepiece pressure-demand SCBA (minimum 30-minute service life) or a combination full-facepiece pressure-demand supplied-air respirator with auxiliary self-contained air. Escape-only respirators must be NIOSH-certified for the atmosphere.
Sources
- OSHA, 29 CFR 1910.134, Respiratory protection, read 2026-11-07.
- NIOSH, Immediately Dangerous to Life or Health (IDLH) Values, index, read 2026-11-07.
- NIOSH, Chlorine IDLH documentation, read 2026-11-07.
- NIOSH, Pocket Guide to Chemical Hazards, introduction, read 2026-11-07.
- NIOSH, Current Intelligence Bulletin 66 (NIOSH 2014-100).
