Permissible Exposure Limit

By Tabitha Mishra
Last updated: September 5, 2021

What Does Permissible Exposure Limit Mean?

The permissible exposure limit (PEL) is a legal limit that describes the maximum amount or concentration of a toxic airborne substance that an employee is allowed to be exposed to during a given period of time. PELs are a type of benchmark used to protect (or limit) workers from harm caused by exposure to toxic substances.

According to a survey by IHSN, a prominent OHS magazine, OSHA PELs are the primary method through which U.S. industrial hygienists monitor workplaces for potential respiratory hazards.

Safeopedia Explains Permissible Exposure Limit

PELs are published and enforced by the United States Occupational Safety and Health Administration (OSHA). They are primarily based on an eight-hour time-weighted average (TWA), which means that a PEL’s exposure limit is concerned with the average amount of exposure time over an eight hour per-day, 40 hour per-week work schedule.

The TWA rule means that a worker can be exposed to an airborne toxic substance for two hours per day at a concentration above what the PEL allows, so long as the average amount of exposure over an eight-hour period remains below the PEL.

PELs, STELs, and Ceiling Limits

Some PELs also have short-term exposure limits (STELs) and ceiling limits. STELs define the maximum concentration of a toxic substance that an employee may be exposed to during a fifteen-minute period. STELs also use a time-weighted average to calculate exposure levels.

Ceiling limits define the maximum concentration of a toxic substance that an employee may be exposed to during any given period of time. In other words, an employee’s legal right to a safe workplace is breached the moment they are exposed to any concentration of a substance that exceeds that substance’s ceiling limit.

Limitations of Relying on PELs

Although OSHA’s PELs are the United States’ legal limits to which an employee may be exposed to a given airborne toxin, they do not provide an adequate means of protecting employee health. Numerous toxins do not have any PELs, and numerous PELs that do exist are very out of date and allow workers to be exposed to levels of hazardous substances that scientific research has proven can cause severe harm to human health.

OSHA has this to say about its PELs: "OSHA recognizes that many of its permissible exposure limits (PELs) are outdated and inadequate for ensuring protection of worker health.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."

OSHA’s PELs are out of date due to a combination of industry pressure to keep PELs low as well as legal and political barriers. In 1989, OSHA released an update to its PELs that increased employee protection for 212 toxic substances and introduced new PELs for an additional 164 substances. OSHA was sued over these changes, and the resulting U.S. court ruling forced it to eliminate all of them, as well as to remove all its newly introduced PELs.

Although OSHA still has some authority to introduce changes, it must now do so very slowly. One of its more recent PEL changes was to respirable silica dust exposure; in 2016, OSHA reduced the PEL for silica from 250 to 50 micrograms per eight-hour time-weighted average.

State-Level PELs

Due to the poor effectiveness of OSHA’s PEL system, numerous U.S. states – such as Washington and California – have implemented their own system of PELs. These PELs impose stricter exposure limits and set limits for a wider array of toxins. OSHA specifically recommends that employers consult California’s list of PELs, which are not legally enforceable outside California, in order to understand what limits are necessary to keep employees safe.

Other Exposure Limits

PELs are not the only governmental occupational exposure limit (OEL) published in the United States. The National Institute of Occupational Safety and Health (NIOSH), a governmental agency that’s part of the Centers for Disease Control, publishes “Recommended Exposure Limits” (REL). Unlike OSHA’s PELs, NIOSH’s RELs are based solely on scientific research on how much toxin a human can be exposed to before they are harmed.

Outside of governmental OELs, the American Conference of Governmental Industrial Hygienists (ACGIH) provides an internationally recognized list of OELs that are referred to as Threshold Limit Values (TLVs). Many countries base their OELs on ACGIH TLVs; for example, every Canadian jurisdiction bases its OELs either fully or partially on them.


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