Industrial Hygiene (IH) Basics: IDLH Limits

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In recent IH Basics articles, we discussed permissible exposure limits (PELs, action levels (ALs), and STELS and Ceiling Limits.

In this article, we’re going to continue looking at exposure limits in the fascinating worlds of IH and occupational safety and health with an investigation into IDLH (immediately dangerous to life or health) limits.

So read on to learn about IDLH atmosphere and don’t be shy about downloading the free Guide to Performing a Job Hazard Analysis from the bottom of this article.

What Does IDLH Stand for and Mean?

IDLH stands for immediate danger to life or health. It’s generally used to describe an atmosphere, such as in saying “an IDLH atmosphere,” and so it typically means an atmosphere that’s immediately dangerous to life or health.

The purpose of an IDLH value is two-fold: 1) to make sure a worker can escape from a contaminated environment if their respiratory protection equipment fails, and 2) to set a maximum exposure level, because any exposure above the IDLH requires a supplied air respirator like an SCBA.

Unlike other occupational exposure limits, an IDLH value does not have a time limit or exposure duration associated with it. Workers should not stay in an IDLH environment for any period of time without suitable personal protective equipment.

The Origins of IDLH Values

According to the National Institute for Occupational Safety and Health (NIOSH),IDLH values were originally developed in the 1970s by both OSHA and NIOSH for high-risk exposure concentrations as part of an effort to develop respirator selection criteria.

The 1974 IDLH values were revised in 1994; you can find a table of IDLH values here.

IDLH Definitions by NIOSH and OSHA

NIOSH defines an IDLH atmosphere this way:

…one 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.

OSHA’s respiratory protection standard (29 CFR 1910.134) defines this definition of an IDLH atmosphere:

… an atmosphere that poses an immediate threat to life, would cause irreversible adverse health effects, or would impair an individual’s ability to escape from a dangerous atmosphere.

OSHA offers this definition of IDLH in their HAZWOPER regulation (29 CFR 1910.120):

An atmospheric concentration of any toxic, corrosive or asphyxiant substance that poses an immediate threat to life or would cause irreversible or delayed adverse health effects or would interfere with an individual’s ability to escape from a dangerous atmosphere.

In addition, OSHA’s permit-required confined spaces regulation (29 CFR 1910.146) defines IDLH this way:

Any condition that poses an immediate or delayed threat to life or that would cause irreversible adverse health effects or that would interfere with an individual’s ability to escape unaided from a permit space. Note: Some materials–hydrogen fluoride gas and cadmium vapor, for example–may produce immediate transient effects that, even if severe, may pass without medical attention, but are followed by sudden, possibly fatal collapse 12-72 hours after exposure. The victim “feels normal” from recovery from transient effects until collapse. Such materials in hazardous quantities are considered to be “immediately dangerous to life or health.

Selecting Respirators for IDLH Atmospheres

OSHA lists the following requirements for respirators used in IDLH atmospheres:

  • Full facepiece pressure-demand self-contained breathing apparatus (SCBA) that’s certified by NIOSH and has a minimum service life of thirty minutes
  • Combination full facepiece pressure-demand supplied-air respirator (SAR) with auxiliary self-contained air supply

In addition, OSHA notes that:

  • All oxygen-deficient atmospheres (meaning they contain less than 19.5% O2 by volume) are considered IDLH atmospheres
  • If it’s impossible to identify or reasonably estimate exposure, then the atmosphere must be considered IDLH

Conclusion: IDLH = Immediately Dangerous to Life or Health

We hope you found this IH Basics article helpful.If you are curious for even more information about how IDLH values are developed, NIOSH’s Current Intelligence Bulletin 66 provides a thorough explanation. Let us know if you have additional question related to IDLH.

To learn more about IH, check out our Industrial Hygiene Basics elearning course–here’s a quick sample video for you.

You might also find the following online safety training courses on related topics:

Also, we’d like to acknowledge that this article, like all our IH Basics articles, has been created in consultation with Morgan Bliss, a Certified Industrial Hygienist and an Assistant Professor of Safety and Health Management at Central Washington University. Many thanks to Morgan for knowledge and time; we encourage you to visit Morgan Bliss on LinkedIn.

Be sure to check out these other articles related to industrial hygiene and/or in our IH Basics series:

Let us know if you have an IH question and we’ll write an article about it!

And don’t leave without downloading our free guide to conducting a JHA.

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Job Hazard Analysis Guide

Learn how to perform a job hazard analysis on the job with our free step-by-step guide.

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Jeff Dalto, Senior Learning & Performance Improvement Manager
Jeff is a learning designer and performance improvement specialist with more than 20 years in learning and development, 15+ of which have been spent working in manufacturing, industrial, and architecture, engineering & construction training. Jeff has worked side-by-side with more than 50 companies as they implemented online training. Jeff is an advocate for using evidence-based training practices and is currently completing a Masters degree in Organizational Performance and Workplace Learning from Boise State University. He writes the Vector Solutions | Convergence Training blog and invites you to connect with him on LinkedIn.

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