What is a short-term exposure limit (STEL) and when is it applied?

• A. The permissible average exposure over 8 hours.
• B. The permissible average exposure over 15 minutes.
• C. The maximum single exposure.
• D. The maximum allowable weekly exposure.

Answer: (B)

The main idea is that a short-term exposure limit sets a cap on how high the concentration can be during a brief, recent period to prevent acute health effects. It is applied as a 15-minute average, meaning the exposure should not exceed this limit in any 15-minute window. This targets peak exposures that could cause immediate symptoms, even if the longer shift-average exposure would be acceptable. It’s different from an 8-hour average (which looks at long-term exposure) and from a ceiling limit (which caps exposure at any instant). So the correct understanding is that the STEL is the permissible average exposure over a 15-minute period.