"Immediately Dangerous to Life and Health" (IDLH):

Facility managers should also know that the National Institute for Occupational Safety and Health (NIOSH) uses "Immediately Dangerous to Life and Health" (IDLH) values for respirator selection.

The Occupational Safety and Health Administration (OSHA) defines an IDLH as an atmospheric concentration of any toxic, corrosive or asphyxiant substance that poses an immediate threat to life or that would cause irreversible or delayed adverse health effects or would interfere with an individual's ability to escape from a dangerous atmosphere.