Some types of atmospheres contain concentrations of hazardous substances that places the worker in immediate danger because these concentrations would impair the ability to leave the work area (self rescue) or potentially cause irreversible health effects, including serious injury or death in a matter of minutes.
There are particular conditions that are considered "Immediately Dangerous to Life or Health (IDLH)". These include:
· A known contaminant at a concentration known to be IDLH
· A known contaminant at an unknown concentration with the potential to be IDLH
· An unknown contaminant at an unknown concentration
· An untested confined space
· An oxygen-deficient atmosphere
· Contaminants at or above 20% of their lower explosive limit (LEL—the concentration at which the gas or vapour could ignite)