What is an occupational exposure limit (OEL)?

An occupational exposure limit (OEL) is the maximum average airborne concentration of an active pharmaceutical ingredient (API) or a hazardous chemical to which nearly all workers can be exposed to for 8-hours per day, 5-days per week, for an entire working lifetime (which is estimated to be 40 years). Unless otherwise specified, it is a time-weighted average (TWA) over the 8-hour work shift.

For some APIs or hazardous chemicals that have irritating or effects that occur rapidly, there are also STELs (short-term exposure limits), which is the maximum an employee can be exposed to for either a 15-minute or 30-minute period, or in some cases, there may be ceiling limits established. A ceiling limit the maximum permissible airborne concentration of a substance that must never be exceeded at any time during a work period, even for a short duration.

In the United States, the Occupational Safety and Health Administration's has established regulatory limits called Permissible Exposure Limits (PEL),  OSHA has established PELs for 470 forms of approximately 300 substances. Only a handful of these  listed hazardous substances are APIs or excipients.