The National Institute of Occupational Safety and Health (NIOSH) defines an immediately dangerous to life or health condition as a situation "that poses a threat of exposure to airborne contaminants when that exposure is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment" (NIOSH IDLH Historical Documentation).  The IDLH limit represents the concentration of a chemical in the air to which healthy adult workers could be exposed (if their respirators fail) without suffering permanent or escape-impairing health effects.

IDLH limits are derived by NIOSH based on animal and human data. Two factors are considered when establishing IDLH limits. Workers must be able to escape from the environment where they are exposed to hazardous chemicals without suffering (a) permanent health damage or (b) severe eye or respiratory tract irritation (or other conditions) that might impair their ability to escape. To find out more about the IDLH development process, go to the IDLH Program website.

There are more than 380 substances with IDLH limits. See the current list of substances that have IDLHs. You can also find IDLH values in CAMEO Chemicals or in the online version of the NIOSH Pocket Guide.

IDLH limits were created mainly to assist in making decisions regarding respirator use. Above the IDLH, only supplied air respirators should be used; below the IDLH, air purifying respirators may be used, if appropriate. Until 1994, an exposure duration of 30 minutes was associated with the IDLH. However, the current definition has no exposure duration associated with it; workers should not be in an IDLH environment for any length of time unless they are equipped and protected to be in that environment.

When modeling a toxic chemical release, ALOHA will provide you with default toxic Level of Concern (LOC) options. The default option is designed to be the best available LOC option for emergency exposure situations where the public is (or may be) exposed to a chemical: AEGLs are used preferentially, then ERPGs, then PACs, and then the IDLH limit.

Today, most common toxic chemicals are defined under at least one of the public exposure guidelines, so ALOHA will rarely default to using the IDLH limit as the toxic LOC. If public exposure guidelines are available, we recommend that you use those preferentially as your toxic LOCs, because they are much better suited to emergency response situations typically modeled in ALOHA than limits designed for workers.

Although the IDLH value is a workplace exposure limit, there are historic examples of the IDLH being used in public exposure situations (before separate public exposure guidelines were commonly available). For example, Technical Guidance for Hazards Analysis—which was developed in 1987 by the Environmental Protection Agency and other federal agencies to provide guidance for hazard planning—primarily used 1/10th of the IDLH limit as an LOC.

If you use the IDLH value as your toxic LOC, note that the resulting threat zone estimate will only include a single threat zone—because there is only one IDLH value per chemical. This is different than toxic threat zone estimates based on the three-tiered public exposure guidelines, which will typically result in a threat zone picture with red, orange, and yellow threat zones. A threat zone indicates the area where the LOC is predicted to be exceeded at some time after the chemical release began.