Time-weighted average (WES-TWA)

The average airborne concentration of a substance calculated over an eight-hour working day.

Short-term exposure limit (WES-STEL)

The 15-minute time weighted average exposure standard. Applies to any 15-minute period in the working day and is designed to protect the worker against adverse effects of irritation, chronic or irreversible tissue change, or narcosis that may increase the likelihood of accidents.

The WES-STEL is not an alternative to the WES-TWA; both the short-term and time-weighted average exposures apply. Exposures at concentrations between the WES-TWA and the WES-STEL should be less than 15 minutes, should occur no more than four times per day, and there should be at least 60 minutes between successive exposures in this range.

Ceiling (WES-Ceiling)

A concentration that should not be exceeded at any time during any part of the working day.