IDLH

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.
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Immediately Dangerous to Life and Health (IDLH or NIOSH IDLH) is a limit for personal exposure to a substance defined by the United States National Institute for Occupational Safety and Health (NIOSH), normally expressed in parts per million (ppm). This concentration is considered to be the limit beyond which an individual will not be capable of escaping death or permanent injury without help in less than thirty minutes.
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The maximum level to which a healthy individual can be exposed to a chemical for 30 minutes and escape without suffering irreversible health effects or impairing symptoms. Used as a "level of concern." (See: level of concern.)