What is an IDLH value?

An immediately dangerous to life or health value is meant to protect first responders and other workers when they are exposed to toxic chemicals


By Shelbie Watts, FR1 Editorial Assistant

In the 1970s, the National Institute for Occupational Safety and Health developed IDLH values to protect first responders and other workers from toxic workplace chemicals. IDLHs are supposed to help save your life, but what exactly do they mean?

Here’s a breakdown of what an IDLH value is, as well as how to use it to ensure safety in dangerous conditions.  

In the 1970s, the National Institute for Occupational Safety and Health developed IDLH values to protect first responders and other workers from toxic workplace chemicals. (Photo/Wikimedia Commons)
In the 1970s, the National Institute for Occupational Safety and Health developed IDLH values to protect first responders and other workers from toxic workplace chemicals. (Photo/Wikimedia Commons)

 What does IDLH mean?

IDLH stands for immediately dangerous to life or health, and NIOSH defines the condition as an incident "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."

An IDLH value determines the concentration at which exposure to a chemical without a respirator would cause permanent or escape-impairing health effects.

According to NIOSH, the values:

  • Ensure that the worker can escape from a given contaminated environment in the event of failure of the respiratory protection equipment
  • Indicate a maximum level above which only a highly reliable breathing apparatus is permitted

Basically, if the concentration of a chemical is equal to or higher than its IDLH value, you’d better be well equipped. 

How are IDLH values chosen?

As of their most recent revision in 1994, more than 380 substances currently have IDLH values.

NIOSH calculates the values using the LD50 or LC50, which are the measures for the doses or concentrations that it would take to kill half the members of a tested population.

When establishing the limits, NIOSH also considers two factors:

  • Can the worker escape from the environment without suffering permanent damage?
  • Can the worker escape from the environment without suffering severe eye or respiratory tract irritation?

According to NIOSH, the organization “has continued to review available scientific data to improve the methodology used to derive acute exposure guidelines, in addition to the chemical-specific IDLH values.”

Why do I need to know an IDLH value?

The primary goal of an IDLH value is to help determine what kind of respirator to use in situations involving airborne chemicals.

If the condition is above the IDLH, only self-contained breathing apparatuses should be used. However, if it is below the value, an air purifying respirator is adequate.

Before the 1994 revisions, the IDLH values were associated with exposure duration of 30 minutes, but according to the current definition, workers should not be exposed to an IDLH environment without the proper respirator for any duration of time.

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