What is the difference between an OEL vs. a TWA

An occupational exposure limit (OEL) is an upper limit or the acceptable concentration of a hazardous substance in workplace air for a particular material. It is typically set by competent national authorities and enforced by legislation to protect worker health. There are many hazardous substances for which there are no legally-mandated occupational exposure limits.

In the pharmaceutical industry, the majority of large pharmaceutical companies have established their own internal OELs.

A time-weighted average (TWA) refers to the airborne concentration of a hazardous substance that is weighted for a certain time duration, usually 8 hours. When you perform industrial hygiene monitoring, the result you obtain is a TWA for the time duration monitored.

The majority of OELs that have been established are for 8-hour TWA, however; some companies develop 10-hour and 12-hour OELs. If a company varies from the standard 8-hour period, this should be clearly stated on the safety data sheet. Unless otherwise stated, if an OEL is on a safety data sheet, it can be assumed to an 8-hour TWA.

The most correct way to state an OEL is to indicate OEL-TWA(8-hour).