Calculating the COSHH 8-Hour TWA
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)?
COSHH 8-Hour TWA Formula
TWA = (C1 × T1 + C2 × T2 + …) ÷ 8
Example:
“Exposure of X ppm for 2 hours and Y ppm for 6 hours = …”
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.
If you are looking for support with your risk assessments, please get in touch, or contact our colleagues at 'C&C Consulting' who would be happy to assist you with your Health and Safety Requirements.
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.
Frequently Asked Questions
What is a COSHH 8-hour TWA?
A COSHH 8-hour Time Weighted Average (TWA) is the average level of exposure to a hazardous substance over a standard 8-hour working day. It helps determine whether exposure remains within safe limits set by workplace regulations.
How do you calculate an 8-hour TWA?
To calculate an 8-hour TWA, multiply each exposure level by the time spent at that level, add those values together, and divide the total by 8 hours. This gives you the average exposure across the full working day.
Why is TWA important under COSHH?
TWA is used to assess whether workers are being exposed to harmful substances above safe limits. Staying within these limits is a legal requirement under COSHH and helps protect long-term health.
What happens if exposure exceeds the TWA limit?
If exposure goes above the recommended TWA limit, employers must take action to reduce it. This could include improving ventilation, changing processes, or providing additional protective equipment.
Is the 8-hour TWA the only exposure limit to consider?
No, some substances also have short-term exposure limits (STELs), which measure exposure over shorter periods, typically 15 minutes. Both TWA and STEL should be considered when assessing risk.
How can exposure be monitored effectively?
Exposure can be monitored using air sampling, sensors, and regular assessments. Many organisations also use digital systems to record and manage this data, making it easier to stay compliant and identify risks early.
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