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5 Essential Exposure Limit Terms Worth Knowing

By Safeopedia Staff
Published: August 22, 2018 | Last updated: August 7, 2022 10:35:56
Key Takeaways

Understanding exposure limits is an essential step in keeping workers safe from dangerous materials.

Exposure limits play a critical role in keeping your workers safe. They mark the cut-off point at which the concentration of a given airborne contaminant is no longer at a safe level. When that cut-off point is reached, appropriate engineering controls or PPE must be used protect workers.

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In the United States, occupational exposure limits (OELs) are set by three organizations:

These limits are typically expressed in milligrams of contaminant per cubic meter of air (mg/m3).

In this article, we'll go over five exposure limits you need to know to keep your workers safe.

Threshold Limit Value (TLV)

Threshold limit value (TLV) is set by the ACGIH. It refers to the maximum average concentration of an airborne hazardous material to which workers can be exposed during an eight-hour work day and a 40-hour work week, over a working lifetime, without experiencing significant adverse health outcomes.

TLV is expressed in parts per million (PPM) of surrounding air for gases and vapors. For fumes, mists, and dusts, it is expressed in terms of milligrams per cubic meter (mg/m3) of surrounding air.

Time-Weighted Average (TWA)

Time weighted average (TWA) refers to the average airborne concentration of a biological or chemical agent to which a worker may be exposed over an 8-hour work day or a 40-hour work week.

Unlike the TLV, the TWA does not span an entire working lifetime. TWA values are calculated as the sum of exposure during a workday to a particular hazardous substance in ppm-hours and dividing it by an eight-hour period.

The TWA for exposure to any substance is used when the concentration and time of exposure vary. It is also applicable to short-term samples (such as a 15-minute TWA) and considers particular variables, dose rates, and duration.

Permissible Exposure Limit (PEL)

Permissible exposure limit (PEL) is the time-weighted average threshold limit of exposure to a chemical, physical agent, or any hazardous substance within an eight-hour period.


Because the PEL is based on the time-weighted average exposure limit, a worker may be exposed to a concentration higher than the PEL on the condition that the average concentration over an eight-hour period remains lower than the PEL.

PELs are set by OSHA and expressed as a time-weighted average (TWA) of 8 to 10 hours. It is sometimes also expressed as workplace environmental exposure limits (WEELs). OSHA's annotated PEL tables can be found on their website.

Ceiling Exposure Value (CEV)

The ceiling exposure value (CEV) is the maximum concentration of an airborne substance to which a worker may be exposed at any time.

Unlike TLV, TWA, and STEV, workers exposed to a concentration higher than the CEV may suffer adverse health effects regardless of the duration of exposure. It should never be exceeded, even for a moment.

Short-Term Exposure Value (STEV)

Short-term exposure value (STEV) is a 15-minute TWA exposure value that should not be exceeded at any time during the work day. It applies to irritants and other substances that have fast-acting or immediate adverse effects.

The concentration level for a STEV is measured using a single air sample taken during the applicable exposure period, or a TWA of several samples taken during the 15-minute period.

OSHA uses the term "ceiling limit" to define short-term exposure values, which is the maximum limit beyond which the permissible exposure limit (PEL) should not exceed for a particular contaminant.

Bottom Line

Employers and safety supervisors are responsible for ensuring that workers are not exposed to harmful concentrations of hazardous materials. Achieving this may require several measures, such as continuous air monitoring, the use of ventillation systems, or providing respirators for any employee working with these substances.

Failing to do so could result in workers developing severe, chronic health conditions with no known cure. So, take the time to familiarize yourself with exposure limits - it's the only way to make sure your workplace is safe for everyone.


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Written by Safeopedia Staff

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At Safeopedia, we think safety professionals are unsung superheroes in many workplaces. We aim to support and celebrate these professionals and the work they do by providing easy access to occupational health and safety information, and by reinforcing safe work practices.

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