Exposure limits play a critical role in keeping your workers safe. They mark the cut-off point at which exposure to a given substance is no longer safe. When that cut-off point is reached, appropriate engineering controls or PPE must be used to keep workers protected (for related reading, see SCBA 101 – Meet the Respirator that Will Save Your Life).

Ventilation systems, respirators, and other forms of hazard control, however, are not always enough. Unless you understand the exposure limits for the materials they are handling or working with, you cannot be sure that they are adequately protected.

In this article, we'll go over five exposure limits you need to know to ensure the safety of your workers.

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 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 concerns.

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.

When the concentration of an airborne substance in the working environment is below the TLV, workers are presumed to be safe.

Time-Weighted Average

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 eight-hour work day or a 40-hour work week.

Unlike the threshold limit value, it does not span an entire working lifetime.

Short-Term Exposure Value

Short-term exposure value (STEV) is defined as 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.

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 TWA (which measures exposure over the course of an entire work day) is not 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.

Ceiling Exposure Value

The ceiling exposure value (CEV) is the maximum concentration of an airborne substance (either biological or chemical) 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.

Bottom Line

It is the responsibility of the employer and the safety supervisors to ensure that air quality is being monitored and that every applicable exposure limit – from the TLV to the CEV – are respected. Exposing workers to conditions that exceed those indicated by these measures puts them at risk of immediate or long-term, chronic ill effects.