Short-Term Exposure Limit (STEL) is the maximum airborne concentration of a substance or agent to which workers can be exposed for a short duration, usually 15 minutes, without experiencing adverse health effects such as irritation, dizziness, or organ damage. The STEL is higher than that allowed by an 8 hour exposure limit. Exposures at the STEL should not occur more than four times per day, and there should be at least 60 minutes between successive STEL exposures. In the United States, STELs are published and enforced by OSHA.
Brief Explanation of the Term “Short-Term Exposure Limits”
STELs are established to protect workers from acute health effects that may occur from brief, high-level exposures. STELs are intended to supplement Time-Weighted Average (TWA) exposure limits. While TWAs address average exposures over an 8-hour workday, STELs address short-term spikes in exposure that could cause immediate harm. STELs are commonly expressed in the same units as TWAs (e.g., ppm, mg/m³). Regulatory agencies and organizations such as OSHA, NIOSH, and ACGIH define STEL values for various hazardous chemicals.
Example: Workers at a fertilizer plant are exposed to ammonia gas during maintenance procedures on storage tanks.
Ammonia is an irritant, and brief, high-level exposures can cause immediate respiratory distress, eye irritation, and other acute effects. The STEL for ammonia (CAS No. 7664-41-7) is 35 ppm (parts per million) over 15 minutes, as set by OSHA. During tank maintenance, when valves are opened or connections are broken, there’s a risk of short-term ammonia releases. During these maintenance tasks, industrial hygienists would use real-time air monitoring equipment to measure ammonia levels in the workers’ breathing zones.
If workers in a fertilizer plant are exposed to 50 ppm of ammonia vapors for 15 minutes, the exposure exceeds the STEL. To ensure compliance with safety standards, the employer must stop the maintenance work, increase ventilation in the area, limit the exposure duration, and provide respirators, such as air-supplied respirators.
The goal is to prevent workers from experiencing acute health effects from short-term ammonia exposures, even if their 8-hour TWA exposure remains below the permissible limit. It is also important to note that not only the peak exposure but also the frequency of those peak exposures are important. STEL limits also require at least 1 hour between peak exposures.
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