Industrial Hygiene (IH) Basics: What is a Permissible Exposure Limit (PEL)?

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If you’re new to industrial hygiene, you may have heard of permissible exposure limits (PELs) but may perhaps not know what they are. Don’t worry–there are good reasons to be confused.

In this article, which is just one in our ongoing Industrial Hygiene (IH) Basics series–we’ll explain what an OSHA PEL is, tell you where you can find them, give you some tips on how to use them, and give you some reasons and nudges for using alternative occupational exposure limits that are more protective than PELs.

What Is a Permissible Exposure Limit (PEL)?

A permissible exposure limit, also known as a PEL, is a limit for an employee’s exposure to (most commonly) hazardous chemicals but also to other harmful agents such as noise. OSHA sets PELs as the legal requirement in the United States.

OSHA has established PELs for hundreds of hazardous chemicals. Most of these PELs are found in the OSHA 1910.1000 Z Tables–Z1Z2, and Z3.

You’ll find OSHA PELs for noise in 1910.95, Occupational Noise Exposure.

PELs are typically given as a time-weighted average, or TWA. However, they are sometimes given as short-term exposure limits (STELs) or ceilings.

What Is a Time-Weighted Average (TWA)?

The time-weighted average, or TWA, is an average exposure level over a specific time.

Here’s how OSHA explains the TWA:

The average exposure to a contaminant over a given period of time, typically 8-hours.

What Is a Short-Term Exposure Limit (STEL)?

A short-term exposure limit, or STEL, is an average exposure limit over a short time.

Here’s how OSHA explains the STEL:

The average exposure to a contaminant to which a worker may be exposed during a short time period (typically 15 – 30 minutes).

What is a Ceiling Limit?

A ceiling is a maximum allowable exposure limit. It’s not an average exposure limit over time, like the TWA and STEL, but a maximum exposure at any one time.

Here’s how OSHA explains a ceiling limit:

The exposure limit a worker’s exposure may never exceed.

OSHA goes into more detail on ceiling limits here:

Substances with limits preceded by “C”—Ceiling Values. An employee’s exposure to any substance in Table Z-1, the exposure limit of which is preceded by a “C”, shall at no time exceed the exposure limit given for that substance. If instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time during the working day.

Does An Employer Face Any Responsibilities for Exposures Below the PEL?

Yes, the employer does have specific responsibilities even when exposures are below the PEL. These requirements from OSHA kick in when exposure limits reach what’s known as an action level.

Here’s how OSHA explains an action level:

Action level means a concentration designated in 29 CFR part 1910 for a specific substance, calculated as an eight (8)-hour time-weighted average, which initiates certain required activities such as exposure monitoring and medical surveillance.

Watch for a more detailed article on OSHA action levels soon.


Are OSHA’s PELs Outdated?

OSHA created most of its PELs shortly after OSHA came into effect in 1970, and even OSHA admits many of them are outdated, meaning the OSHA PELs often don’t reflect current knowledge of hazardous exposure levels and that the OSHA PELs are often set much higher than they would be in an ideal circumstance.

Here’s how OSHA puts it:

OSHA recognizes that many of its permissible exposure limits (PELs) are outdated and inadequate for ensuring protection of worker health. Most of OSHA’s PELs were issued shortly after adoption of the Occupational Safety and Health (OSH) Act in 1970, and have not been updated since that time. Section 6(a) of the OSH Act granted the Agency the authority to adopt existing Federal standards or national consensus standards as enforceable OSHA standards.

Nonetheless, the OSHA PELs found in OSHA regulations (and mainly within the 1910.1000 Z Tables) are still legally binding, and they essentially set a minimum compliance standard for employers.

If OSHA PELs Are Out of Date, Should Employers Comply With Other Occupational Exposure Limits?

OSHA not only acknowledges that many of its own PELs are out of date and not adequately protective, they also suggest employers consider using more modern, updated occupational exposure limits set by other organizations. Again, here’s how OSHA puts it:

Industrial experience, new developments in technology, and scientific data clearly indicate that in many instances these adopted limits are not sufficiently protective of worker health. This has been demonstrated by the reduction in allowable exposure limits recommended by many technical, professional, industrial, and government organizations, both inside and outside the United States. Many large industrial organizations have felt obligated to supplement the existing OSHA PELs with their own internal corporate guidelines. OSHA’s Hazard Communication standard (1910. 1200 Appendix D) requires that safety data sheets list not only the relevant OSHA PEL but also the ACGIH® TLV® and any other exposure limit used or recommended by the chemical manufacturer, importer, or employer preparing the safety data sheet.

To provide employers, workers, and other interested parties with a list of alternate occupational exposure limits that may serve to better protect workers, OSHA has annotated the existing Z-Tables with other selected occupational exposure limits. OSHA has chosen to present a side-by-side table with the Cal/OSHA PELs, the NIOSH Recommended Exposure Limits (RELs) and the ACGIH® TLVs®s. The tables list air concentration limits, but do not include notations for skin absorption or sensitization.

Watch for future articles by us explaining RELs, TLVs, and similar exposure limits.


Conclusion: Industrial Hygiene (IH) Basics: Permissible Exposure Limits (PELs)

We hope you found this Industrial Hygiene Basics article on the permissible exposure limits (PELs) by OSHA helpful.

This IH Basics article, and all our IH Basics articles, has been created in collaboration with Morgan Bliss, a Certified Industrial Hygienist and an Assistant Professor of Safety Management at Central Washington University. We’d like to thank Morgan for her expertise and 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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Jeffrey Dalto

Jeffrey Dalto

Jeffrey Dalto is an Instructional Designer and the Senior Learning & Development Specialist at Convergence Training. He's worked in training/learning & development for 20 years, in safety and safety training for more than 10, is an OSHA Authorized Outreach Trainer for General Industry OSHA 10 and 30, has completed a General Industry Safety and Health Specialist Certificate from the University of Washington/Pacific Northwest OSHA Education Center, and is a member of the committee creating the upcoming ANSI Z490.2 national standard on online environmental, health, and safety training.

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