IDLH is an initialism for Immediately Dangerous to Life and Health, and is defined by the NIOSH as exposure to airborne contaminants that is "likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment." Examples include smoke or other poisonous gases at sufficiently high concentrations.
The 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."
IDLH values are often used to guide the selection of breathing apparatus that made available to workers or firefighters in specific situations.
The NIOSH definition does not include oxygen deficiency (below 19.5 percent) although atmosphere-supplying breathing apparatus is also required. Examples include high altitudes and unventilated, confined spaces.
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.). It also uses the broader term "impair", rather than "prevent", with respect to the ability to escape. 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).
The OSHA definiition is part of a legal standard, which is the minimum legal requirement. 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.