What is mean by IDLH?

IDLH, means for “Immediately dangerous to life or health” this term is defined by the US National Institute for Occupational Safety and Health (NIOSH) defines the condition as an incident “that poses a threat of exposure to airborne contaminants when that exposure is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from a dangerous an environment or atmosphere.” as exposure to airborne contaminants, including smoke or the other poisonous gases at sufficiently high concentrations.

According to the Respiratory Protection Standard, all oxygen-deficient atmospheres are considered IDLH atmospheres and so supplied-air respirators are required.
If there is at least 19.5% oxygen in the atmosphere, then before selecting an APR you must identify the chemical and its concentration.
APRs only clean the air, they don’t supply oxygen. If there is less than 19.5%, the oxygen you must use an air-supplied respirator, such as a Self Contained Breathing Apparatus (SCBA).

Since the development of the original IDLH values in the 1970s and their subsequent revision in 1994, since the development of the original IDLH values in the 1970s and their subsequent revision in 1994.
According to NIOSH, the organization “has continued to review available scientific data to improve the methodology used to derive acute exposure guidelines, in addition to the chemical-specific IDLH values.”

The OSHA definition is arguably broad enough to include oxygen-deficient circumstances in the absence of “airborne contaminants”, as well as many other chemicals, thermal, or pneumatic hazards to life or health. It also uses the broader term “impair”, rather than “prevent”, concerning 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 definition 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.

The primary goal of an IDLH value is to help determine what kind of respirator to use in situations involving airborne chemicals.
If the condition is above the IDLH, only self-contained breathing apparatuses should be used. However, if it is below the value, an air-purifying respirator is adequate.