Immediately dangerous to life or health

[1] The Occupational Safety and Health Administration (OSHA) regulation (1910.134(b)) defines the term as "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.

"[2] IDLH values are often used to guide the selection of breathing apparatus that are made available to workers or firefighters in specific situations.

The OSHA definition is arguably broad enough to include oxygen-deficient circumstances in the absence of "airborne contaminants", as well as many other chemical, thermal, or pneumatic hazards to life or health (e.g., pure helium, super-cooled or super-heated air, hyperbaric or hypo-baric or submerged chambers, etc.).

For example, blinding but non-toxic smoke could be considered IDLH under the OSHA definition if it would impair the ability to escape a "dangerous" but not life-threatening atmosphere (such as tear gas).

Users or employers are encouraged to apply proper judgment to avoid taking unnecessary risks, even if the only immediate hazard is "reversible", such as temporary pain, disorientation, nausea, or non-toxic contamination.