Calculating The COSHH Regulations 8-Hour Time-Weighted Average
Workplace exposure limits (WELs) under the COSHH regulations are designed to manage exposure to certain substances. The most prevalent type of WEL is the 8-hour long-term exposure limit. But how do you determine the 8-hour time-weighted average (TWA)?
Understanding the COSHH Regulations' 8-Hour Time-Weighted Average
Almost every business will encounter hazardous substances in some capacity. Therefore, it’s crucial for businesses to understand workplace exposure limits (WELs), as some hazardous substances are legally regulated.
The Health and Safety Executive (HSE) sets these exposure limits through the Control of Substances Hazardous to Health (COSHH) Regulations. While not all hazardous substances have WELs, hundreds do.
Ways hazardous substances can enter the body:
Calculating Variations in Exposure
What if exposure levels fluctuate?
Different stages of a process might expose workers to varying concentrations of a substance, and exposure might even pause during breaks.
In this case, you’ll need to account for these different exposure levels in your calculations.
For instance, if an employee works an 8-hour shift with 2 hours of exposure at 0.25 mg/m³ and 4 hours of exposure at 0.30 mg/m³, the time-weighted average can be calculated as follows:
((2x0.25)+(4x0.3))/8 =
The time-weighted average exposure is 0.2125mg/m3.
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