Immediately dangerous to life or health (IDLH) is a term used by U.S. Occupational Safety and Health (OSHA) regulations to designate a situation in which the atmosphere poses an imminent threat of serious physical harm or death. The standard also encompasses exposures that might hinder an individual as they attempted to escape a contaminated area. For example, firefighters battling a fire where toxic fumes are present may be exposed not only to airborne toxins but also have their view obscured by smoke, preventing them from spotting an escape route.
Work conditions are Immediately dangerous to life or health (IDLH) if the exposure to a chemical will cause serious harm and a very short time period. The term describes dangerous or emergency situations. Unsafe atmospheric conditions, in comparison, may result in temporary illness or harm but are not life-threatening if the exposure duration is short.
The National Institute for Occupational Safety and Health (NIOSH) maintains a table of IDLH values for various chemicals such as chlorine, methylamine, and coal tar pitch volatiles. The tables define the acute exposure concentration of each chemical that may cause death or severe physical harm.
The duration of the exposure required to cause harm defines whether it is an immediate threat. Even if the harm caused by an exposure does not manifest immediately, the chemical concentration may still be designated as IDLH.
The purpose of identifying IDLH levels is to ensure that proper procedures are in place to allow workers to not only avoid dangerous exposures but also safety escape a dangerous environment in the event their safety equipment fails.
Before an employer initiates a respiratory protection program, it must identify whether an atmosphere is or has the potential to be immediately dangerous to life or health (IDLH). This information is used as part of the employer's respirator selection criteria. For high-risk environments, the most reliable, self-contained personal protective equipment is required.
OSHA guidelines may also require employers to implement protocols such as the provision of standby rescue personnel, safety lines or additional safety equipment for IDLH work environments.