IDLH

From Freepedia

An IDLH, a situation Immediately Dangerous to Life and Health, defined by the NIOSH as follows: "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 such an environment." Examples include smoke or poisonous gases.

IDLH values are often used for the selection of breathing apparatus that must be available to workers or firefighters in a given situation. However, they do not technically include areas with oxygen deficiency (below 19.5 percent) where breathing apparatus is also required.