Short-term exposure limit

A short-term exposure limit (STEL) is the acceptable average exposure over a short period of time, usually 15 minutes as long as the time-weighted average is not exceeded. 
STEL is a term used in occupational health, industrial hygiene and toxicology. The STEL may be a legal limit in the United States for exposure of an employee to a chemical substance. The Occupational Safety and Health Administration (U.S. OSHA) has set OSHA-STELs for 1,3-butadiene, benzene and ethylene oxide. For chemicals, STEL assessments are usually done for 15 minutes and expressed in parts per million (ppm), or sometimes in milligrams per cubic meter (mg/m3).
The American Conference of Governmental Industrial Hygienists publishes a more extensive list of STELs as threshold limit values (TLV-STEL).