Air quality is an issue in many types of work, especially manufacturing, where substances are either produced in the plant for sale and distribution to other companies or used in the different processes being performed in the plant. Depending on the competence of the ventilation structures of the plant, these released substances will either be blown out from the working environment or gradually accumulate in the air inside the plant. Substances that accumulate over time, thus becoming more concentrated can have a negative effect on worker health. Exposure limits, or the concentration levels of a substance to which individuals can be safely exposed, are designed to help monitor air quality and keep workers safe. This article will explain the meaning of five essential exposure limits that are used to determine what safe actually means.

Threshold Limit Value

Threshold Limit Value (TLV) is a registered and reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH). It refers to the maximum average concentration of a hazardous material floating in the workplace to which workers can be exposed during an eight-hour work day and 40-hour workweek, over a working lifetime, without experiencing significant adverse health concerns. It 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.

When the concentration of an airborne substance in the working environment is below the TLV, workers are presumed to be safe. In effect, the TLV is the prevailing concentration of a substance in the working environment between safe and dangerous levels. (Read more in Indoor Air Quality: The Invisible Hazard in Every Workplace.)

Time-Weighted Average Exposure Value

Time-weighted average exposure value (TWAEV) refers to the average airborne concentration of a biological or chemical agent to which a worker may be exposed over an eight-hour work day or a 40-hour work week. It is also referred to as time weighted average (TWA).

Short-Term Exposure Value

Short-term exposure value (STEV) is defined as a 15-minute TWAEV exposure value that should not be exceeded at any time during the work day. STEV is the maximum airborne concentration of a chemical or biological agent to which a worker may be exposed in any 15-minute period, even if the TWAEV is not exceeded. This is applied to irritants and other substances that have fast and immediate ill effects. (For related reading, see Proper Ventilation to Improve Indoor Air Quality.)

Ceiling Exposure Value

The ceiling exposure value (CEV) refers to the maximum concentration of an airborne substance - either biological or chemical - that should never be exceeded under any circumstance. It is the maximum concentration to which a worker may be exposed at any time. This means that a worker who is exposed to a concentration higher than the CEV may suffer health effects, regardless of the duration of exposure. This exposure limit is strictly observed for chemicals and biological agents that could cause irreversible health effects. It should never be exceeded.

Permissible Exposure Limit

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. It serves as a regulatory limit.

Bottom Line

It is the responsibility of the employer and the safety supervisors to ensure that air quality is being monitored and that all the limits from TLV to the CEV are respected. Exposing workers to conditions beyond these limits puts them at immediate risk as well as long term risks in the form of respiratory issues and other long term health problems. Simply put, know your air quality and protect your workers accordingly. (Also check out Employer Liability for Asbestos Exposure.)