Occupational Exposure Limits (OEL) or Permissible Exposure Limits (PEL) are defined as the maximum concentration of a substance to which a worker may be exposed to over a given period of time. Chemicals may have more than one OEL for different “averaging” or exposure times. Common types of exposure limits are explained below:
Time Weighted Average (TWA) – A TWA exposure limit is the limit for the average exposure over a specified period, typically 8 hours, which represents a standard work shift.
Short Term Exposure Limit (STEL) – A STEL exposure limit is the limit for the average exposure over a shorter period of time, typically 15 minutes. These limits often apply to substances that produce acute (or fast acting) effects on the human body. Many organic solvents have both STEL and TWA exposure limits.
Ceiling or Maximum Allowable Concentration (MAC) – A Ceiling or MAC exposure limit is concentration of the chemical that should not be exceeded for any period of time.
Where can I find applicable OELs?
Country-specific OELs are often defined within local regulations and should be referenced for legal compliance. Examples of some country-specific TWA and STEL for some common substances are provided below:
Note: The examples provided below were accurate at time of publication and may be updated from time to time. The most current regulations regarding country-specific OELs should always be referenced.
What if there is no OEL in my country’s regulations?
Some countries may not have established OELs for all substances in use at your facility. In this case, it is recommended that accepted OELs from other jurisdictions or governmental bodies be adopted.
American Conference of Governmental Industrial Hygienist – ACGIH The ACGIH is an Internationally recognized organization that publishes (for purchase) Threshold Limit Values (TLVs®) and Biological Exposure Indices (BEIs®).
Extended Work Shifts
As noted above, TWA exposure limits are based on an averaging time of 8 hours. If employees work longer than 8 hours, TWA exposure limits must be adjusted to account for extended exposure time and reduced recovery time.
Exposure to Multiple Chemicals
Workers are often exposed to multiple chemicals at the same time (i.e. chemicals mixtures). This can sometimes result in more severe impacts to a worker’s health.