Immediately dangerous to life or health

The term "immediately dangerous to life or health" (IDLH) is defined by the US National Institute for Occupational Safety and Health (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. It is calculated using the LD50 or LC50.

Source: Wikipedia — Immediately dangerous to life or health (CC BY-SA 4.0)