In this article, we’re going to continue our discussion of exposure limits within the industrial hygiene world by explaining short-term exposure limits (STELs) and ceiling (C) limits.
For more articles like this one on IH topics, check the links at the bottom of this article.
And read on to learn more about STELs and ceilings.
A short-term exposure limit, or STEL, is the highest amount that a worker can be exposed to over a 15-minute period. It should never be exceeded during the work day.
One important note about STELs – they are usually 15-minute exposures, but can also be 5-minutes or 10-minutes, depending on the chemical.
Here’s how the American Conference of Governmental Industrial Hygienists (ACGIH) defines a short-term exposure limit:
The concentration to which workers can be exposed continuously for a short period of time without suffering from (1) irritation, (2) chronic or irreversible tissue damage, (3) certain toxic effects, and (4) narcosis of sufficient degree to increase the likelihood of accidental injury, impair self-rescue, or materially reduce work efficiency.
A STEL can be a stand-alone exposure limit, or it may supplement the time-weighted average (TWA) time limit. The STEL usually addresses acute (short-term) effects, while the TWA addresses chronic (long-term) effects.
A worker can be exposed to 4 different STEL exposures during an 8-hour shift, as long as at least 1-hour (60 minutes) separates each STEL exposure.
A ceiling is also a type of short term exposure limit that can never be exceeded at any time–not even for a brief moment (although you should see the OSHA definition below for further context on that). Ceiling limits are sometimes used in place of STELs based on the type of air contaminant and its toxicological properties.
Exposures are monitored to ensure they don’t exceed the ceiling by using instantaneous monitoring that provides real-time information. If instantaneous monitoring is not possible, the exposure must be measured as a 15-minute STEL.
As is the case with STELs, a ceiling can be a stand-alone exposure limit. Ceilings often act to supplement full-shift TWAs.
Here’s how OSHA explains ceiling limits in 1910.1000(a)(1):
Substances with limits preceded by “C”—Ceiling Values. An employee’s exposure to any substance in Table Z-1, the exposure limit of which is preceded by a “C”, shall at no time exceed the exposure limit given for that substance. If instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time during the working day.
OSHA provides some additional guidance about ceiling limits in 1910.1000(b)(2):
Acceptable ceiling concentrations. An employee’s exposure to a substance listed in Table Z-2 shall not exceed at any time during an 8-hour shift the acceptable ceiling concentration limit given for the substance in the table, except for a time period, and up to a concentration not exceeding the maximum duration and concentration allowed in the column under “acceptable maximum peak above the acceptable ceiling concentration for an 8-hour shift.
Many chemicals with an established 8-hour TWA do not have an established STEL or ceiling limit. But, you still need to control those higher short-term exposures to workers. There is something called the “3/5 Rule” in industrial hygiene that can be used in these instances. The 3/5 Rule basically states that a worker exposure can get to 3 times the TWA for up to 15-minutes, but should never exceed 5 times the TWA at any time, as long as the 8-hour TWA is not exceeded.
This 3/5 Rule also requires no more than 4 higher short-term exposures per work shift, with at least 1-hour between each exposure.
We hope you found this article on short-term exposure limits and ceiling limits helpful. For more on this IH topic and other IH topics, check out our Industrial Hygiene Basics elearning course–we’ve provided a short sample below or you can click the link you just passed for more information.
A quick note that this article, and all our IH Basics articles, has been created in consultation with Morgan Bliss, a Certified Industrial Hygienist and an Assistant Professor of Safety and Health Management at Central Washington University. Many thanks to Morgan for knowledge and time; we encourage you to visit Morgan Bliss on LinkedIn.
Be sure to check out these other articles related to industrial hygiene and/or in our IH Basics series:
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