Short Term Exposure Limit
Short-Term Exposure Limit (STEL) is the 15-minute time-weighted average airborne concentration of a substance that must not be exceeded, with the usual WorkSafeBC/ACGIH rule allowing no more than 4 such 15-minute periods in an 8-hour shift and at least 1 hour between successive excursions.
On a manufacturing or shop floor, STEL controls brief high-exposure tasks such as solvent wiping, spray application, drum transfer, welding fume peaks, degreasing, or maintenance work where concentration spikes can occur even if the full-shift average looks acceptable. Air monitoring is evaluated as a 15-minute sample or equivalent short-duration assessment for the specific task. If a task can create repeated peaks, employers must plan controls so workers do not exceed the STEL and do not exceed the allowed number of excursions in a shift. Controls typically include local exhaust ventilation, enclosure, process substitution, work scheduling, and respiratory protection when engineering controls are not sufficient.
- Relying only on the 8-hour TWA and ignoring short peaks that exceed the STEL.
- Allowing too many 15-minute excursions or failing to maintain the required 1-hour separation between them.
- Using the wrong limit for the substance because WorkSafeBC may adopt or modify ACGIH values, so the current B.C. exposure-limit table must be checked for the specific chemical.
Is STEL the same as a ceiling limit?
No. A ceiling limit is not to be exceeded at any time, while a STEL is a 15-minute average with limited permitted excursions under the ACGIH/WorkSafeBC framework.
Why use STEL when the 8-hour TWA is compliant?
Because some substances cause acute effects from short peaks even when daily average exposure remains acceptable.
Which rule governs in B.C.?
WorkSafeBC’s OHS Regulation and exposure-limit tables govern, with many limits derived from ACGIH TLVs unless the Board has set a different value.