STELs vs. Ceiling Limits

In the OHS Code (Schedule 1, Table 1), ceiling or 15-minute occupational exposure limits are provided for some airborne contaminants. What does this value represent? Is it the same this as a Short-Term Exposure Limit (STEL)?  

What is a Ceiling?

A ceiling (15-minute occupational exposure) limit represent airborne concentrations of contaminants that should never be exceeded in the workplace. These limits often supplement other limits, such as the 8-hour OELs, but they can also stand alone.  

What is a STEL?

A STEL, on the other hand, represents the airborne concentration of contaminants deemed acceptable for workers to be exposed to for 15-minute periods of time, assuming a standard 8-hour workday and 40-hour workweek. Up to four exposures per day of concentrations as high as the STEL are acceptable, if these exposure events have at least 60 minutes between them and the overall 8-hour Time Weighted Average (TWA) is not exceeded.

The TWA represents the level of airborne exposure, averaged over an 8-hour workday and 40-hour workweek. The average worker can be safely exposed to the TWA concentration of an airborne contaminant day after day without adverse health effects.  

What to do Without a STEL or Ceiling

STELs, like ceiling limits, supplement other limits (such as the OEL/TLV or TWA). If no ceiling or STEL is provided, ACGIH recommends that five times the TWA be used instead, and that only three short-term exposures to that level for up to 30 min per day be permitted. The STEL is meant to account for the acute effects of substances that have primarily chronic affects. 

Here's an Example:

As you can see, understanding what each value means can be critical when determining whether a worker is over-exposed. In the case of benzene, the ceiling value and the STEL are the same – meaning that worker exposure can never exceed 2.5 ppm under any circumstances. 

Workers can be safely exposed to levels between the OEL/TWA and the STEL/Ceiling limit in the frequencies stated above for the STEL (up to 4x per shift with 60 minutes between) as long as the average exposure over the shift (8-hours) is still below the OEL/TWA value.  

Reach out to an Expert!

As you can imagine, many complex exposure scenarios can arise in a workplace. The best way to protect your workers is to ensure the air quality within your environment is regularly monitored and evaluated by a qualified industrial hygienist. Questions about this? Call JADA Solutions (HSE) Inc. to talk to a professional!  


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