Short-term exposure limit (STEL)

Some chemicals cause irritation, narcosis or other acute effects at peak levels that an 8-hour average can hide. A worker could, for example, be exposed to a high level for a short task and then very little for the rest of the shift, keeping the Time-weighted average (TWA) low while still getting a harmful peak. The STEL sets a ceiling on those peaks.

The STEL is normally measured as a 15-minute average, and ACGIH guidance says exposures at the STEL level should be spaced well apart and should not occur more than a limited number of times per day. OSHA, NIOSH and ACGIH all publish short-term limits for some substances, and the values for the same chemical can differ between them.

A STEL is different from a Ceiling limit, which must never be exceeded at any instant. For tasks like opening reactors, cleaning with solvents, charging tanks or spraying, STEL compliance usually requires sampling for the short task period, not only a full-shift sample.

If an SDS lists a STEL, use the same units as the TWA, usually Parts per million (ppm) or milligrams per cubic meter. Where no STEL is listed, ACGIH publishes general excursion guidance that limits how far and how long peaks may rise above the TWA; apply it with the help of a qualified hygienist.