IDLH
An Immediately dangerous to life or health value, developed by the National Institute for Occupational Safety and Health (NIOSH), is the airborne concentration from which a worker could escape without injury or irreversible health effects in the event of a failure of respiratory protection equipment. An IDLH condition is a situation 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.
Selected Key Regulation
Source
NIOSH