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Immediately Dangerous to Life and Health Limits (IDLHs)
IDLHs are workplace exposure limits that are meant to protect workers when they are exposed to a toxic chemical in the course of their work.
This page discusses the following topics:
What are IDLHs?
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."
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.
How are IDLHs chosen?
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
- Permanent health damage, or
- 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, select the link titled IDLH Program at the bottom of this page.
What substances have IDLHs?
There are more than 380 substances with IDLH limits (as of the early 1990s when they were last updated). To see the current list of substances that have IDLHs, select the link titled IDLH Chemicals at the bottom of this page.
You can also search for a specific substance on the CAMEO Chemicals site to find response recommendations and chemical data (including IDLHs) or in the online version of the NIOSH Pocket Guide by selecting the appropriate link at the bottom of this page.
How should IDLHs be used?
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. This is no longer the case. 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.
How does ALOHA use IDLHs?
In ALOHA, you can choose an IDLH limit as a Level of Concern (LOC) when modeling a toxic chemical release--if an IDLH has been defined for that chemical. (A toxic LOC is the value above which the toxic gas concentration might be high enough to harm people.)
ALOHA allows you to specify up to three toxic LOCs, but there is only one IDLH value per chemical. If you select the IDLH limit as your only LOC, ALOHA will use it in combination with other scenario information to generate a threat zone plot where a single zone indicates the area where the LOC was exceeded at some time after the chemical release began. To determine how long the LOC was exceeded at a particular location, read the Ask Dr. ALOHA article on working with the concentration graph.
Note: On the other hand, threat zone plots generated with public exposure guidelines (such as AEGLs, ERPGs, and TEELs) typically have red, orange, and yellow zones that each indicates a different hazard level.
However, most common toxic chemicals today are defined under at least one of the public exposure guidelines, which are much better suited to emergency response situations (where the public is exposed to a toxic chemical) than limits designed for workers. If public guidelines are available, we recommend that you use those as your toxic LOCs in ALOHA--rather than the IDLH limit.
When modeling a toxic chemical release, ALOHA will provide you with default toxic LOC options. The default option is designed to be the best available LOC option for public exposure situations: AEGLs are used preferentially, then ERPGs, then TEELs, and then the IDLH limit (if no public guidelines are defined for the chosen chemical).
Before public exposure guidelines were commonly available, the IDLH limit was used in public exposure situations. 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.
Where else can I find information on IDLHs?
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- IDLH Program Read all about IDLHs on this National Institute of Occupational Safety and Health site, which contains a list of current IDLHs and details about IDLH development. [leaves OR&R site]
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- IDLH Chemicals See the current list of chemicals with IDLHs (and an explanation of how the value for each chemical was derived) on this National Institute of Occupational Safety and Health site. [leaves OR&R site]
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- CAMEO Chemicals Search the CAMEO chemical database online and find IDLHs for specific chemicals. At this separate Office of Response and Restoration site, you can also print customized reports with response recommendations and find out how chemicals would react if they mixed. [leaves OR&R site]
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- NIOSH Pocket Guide Search the online version of the National Institute of Occupational Safety and Health pocket guide to find IDLH values and other information for nearly 700 substances and substance groups. [leaves OR&R site]
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