As used by US NIOSH, unless noted otherwise, the 15 minute time weighted average exposure that should not be exceeded at any time during a work day.
A 15 minute TWA exposure that should not be exceeded at any time during a working day even if the eight-hour TWA average is within the TWA exposure standard. Exposures at the STEL should not be longer than 15 minutes and should not be repeated more than four times per day. There should be at least 60 minutes between successive exposures at the STEL.
Represented as STEL or TLVSTEL, this is the maximum concentration to which workers can be exposed for a short period of time (15 minutes) for only four times throughout the day with at least one hour between exposures. Also the daily TLVTWA must not be exceeded.