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Exploring Exposure Limits: What do those numbers mean?

If you’ve ever attended safety training, you surely have heard the terms PEL, TLV, REL and STEL.  Do these letters really have a meaning or are they just a bunch of alphabet soup?  These acronyms all represent different occupational exposure limits (OELs) that are derived by different organizations. An OEL is representative of the highest concentration a healthy worker can be exposed to for a full work week over the duration of their working life without experiencing an adverse effect.  Although similar, they each have a different goal and meaning.

Permissible Exposure Limit (PEL)

The Permissible Exposure Limit, or PEL, is the most widely known exposure limit.  This is the OSHA 8-hour time weighted average (TWA) exposure limit and is the only limit directly enforceable by regulation.  OSHA limits must be approved by Congress and take into account both health benefits and industry costs.  PELs are difficult to change because of the congressional approval required.  These limits apply to the general working population and are not meant to be applied to other, possibly more sensitive, populations.

Threshold Limit Value (TLV)

The Threshold Limit Value, or TLV, is set by ACGIH.  These 8-hour TWA limits are more representative of recent information and are outlined in the TLV guidebook, which is published annually.  Since they are consensus standards, they are not directly enforceable.  In addition, they are often stricter than the published PEL for the same chemical.   ACGIH does not consider financial impact when deriving TLVs, only health impact.

Recommended Exposure Limit (REL)

A Recommended Exposure Limit, or REL, is based on research done at NIOSH.  A REL is most commonly a 10-hour TWA (although some do exist as 8-hour limits).  NIOSH is the CDC funded research complement to OSHA, however the REL is also not directly enforceable and is considered a consensus standard as well.  The main goal of an REL is to recommend what an updated PEL should be to OSHA.

Short Term Exposure Limit (STEL)

The STEL is very similar to the other limits, however it is based on a 15-minute exposure, as opposed to a full working shift.  This is done to prevent employers from exposing employees to very high levels of an agent for short periods of time.  All agencies adopt STELs for some chemicals.


Now that you have all the OELs to consider in evaluating workplace exposure, how do you use that information?  Especially if they are all different, how do you move forward?  The best answer is to follow the most strict limit.  This always ensures the highest level of employee protection.  In addition, OSHA published the annotated Z table (Table-Z1) as a means of recognizing the advancement of OEL development since the 1970’s, which is when most PELs were published.  To circumvent the need to gain legislative approval, the annotated Z table which compares the OSHA exposure limits to that of NIOSH, ACGIH and Cal OSHA, was added to the OSHA website.  This means that OSHA can now use the general duty clause to enforce these exposure limits as well.

Occupational exposure limits can be confusing, especially when there are so many OELs established for the same chemical.  However, with a little insight into the origin of each limit, you can choose the best protective measures for your employee’s health.


This is the third blog in a ten-part Industrial Hygiene series that we are featuring monthly. We welcome your input on the series! To provide feedback, or for questions about industrial hygiene and for information on how input from a Certified Industrial Hygienist can benefit your safety program, please email [email protected].

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