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19th June, 2019

Calculating The COSHH Regulations 8 Hour Time Weighted Average

Harmful substances are used in nearly every business. Some hazardous substances have legal exposure limits. These exposure limits are set by the HSE under the Control of Substances Hazardous to Health (COSHH) Regulations. While thousands of substances come under the COSHH regulations, around 500 have workplace exposure limits (WELs).

Exposure to a hazardous substance can happen in a variety of ways:

  • Through breathing it in
  • Direct contact
  • Absorbed through the skin
  • By swallowing

The main route of entry for hazardous substances (and the hardest to control) is inhalation. This is where exposure limits come into play, putting a measurable limit on the amount of exposure that can occur, by law. It is important to be aware of the exposure limits that apply to the substances in use, so you can assess the exposure within your workplace, and take action to avoid exposure limits being exceeded.

Workplace exposure limits that apply under the COSHH regulations are subject to a time-weighted average (TWA).

The 8-hour TWA

The most common type of workplace exposure limit (WEL) is the 8-hour limit, or long term exposure limit. If a substance has an 8-hour exposure limit, this actually means that the exposure in any 24 hour period is measured over 8 hours.

The term ‘8-hour reference period’ relates to the procedure whereby the occupational exposures in any 24-hour period are treated as equivalent to a single uniform exposure for 8 hours (the 8-hour time-weighted average (TWA) exposure).

Find out more about workplace exposure limits (WELs) in COSHH workplace exposure limits (WELs) explained.

The 8-hour time-weighted average is a legal limit that should not be exceeded. But how do you know if you are exceeding the exposure limit in your workplace?

In order to calculate if the level is being exceeded or when action needs to be taken, you need to work out the time-weighted average of the employees exposed and compare this to the exposure limit set by the HSE.

Where an operative works an 8-hour shift and is exposed to a substance during that period at a level of 50mg.m-3 the time-weighted average would be calculated as:

(8x50)/8 = 50mg.m-3

Broken down, this calculation is 8 hours exposure of 50mg.m-3, divided by 8 for the time-weighted average we are measuring. Pretty straightforward. But of course, in the real world, exposure doesn't always happen like this. Let's look at some more realistic examples.

Example Calculations

Changes in exposure

What if the exposure isn't always the same level? Some parts of a process may expose employees to a higher concentration of a substance than other parts of your work. Breaks during their shift may stop exposure altogether.

In this situation, working sessions may be broken down to different exposure levels in your calculation. For example, where an employee works an 8-hour shift and is exposed to a substance for 3 hours at 0.15mg.m-3 and for 2 hours as 0.20mg.m-3 the time-weighted average can be calculated by:

((3x0.15)+(2x0.2))/8 = 0.11mg.m-3

Longer shifts

Not everyone works 8-hour shifts. But you can't just reset the exposure clock every 8 hours. The 8 hour reference period is a reference period in every 24 hours in which exposure occurs. Therefore if an employee is working a 10-hour shift and exposed to 6mg.m-3 while at work, the calculation of the exposure level would be:

(10x6)/8 = 7.5mg.m-3

We still divide by 8 to get our 8-hour average. And we still must not exceed the 8-hour WEL that is given.

Shorter shifts

Similarly, if an employee is working a 4-hour shift, they can be exposed to higher levels within those 4 hours, than they would be if they were working the full 8 hours.

The difference with shorter exposure is you can adjust the workplace exposure limit, providing this does not exceed any other exposure limit such as a short term (15-minute) exposure limit. However, if you are going to expose an employee to an exposure level above the workplace exposure limit at any point during the shift, this exposure should be limited to one hour.

An example is where a person is exposed to a hazardous substance with a WEL of 18mg.m-³ (8 hour TWA) for 4 hours, an adjusted exposure level of 36mg.m-³ would apply over the 4 hours. However, exposure levels above 18mg.m-3 should be restricted to 1 hour.

((3x18 mg.m-³)+(1x55 mg.m-³))/4 = 27.25 mg.m-³*

*above the 8-hour exposure limit but within the revised time-weighted average for shorter exposure period.


Once you have calculated the 8-hour time-weighted average for the employee, you can compare this with the workplace exposure limit, to determine if the level of exposure is acceptable. Where the exposure limit is exceeded, action must be taken to bring exposure within the set limits for the particular substance(s).

Find out more in workplace exposure limits explained, and get help with COSHH with our COSHH assessment templates.

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This article was written by Emma at HASpod. Emma has over 10 years experience in health and safety and BSc (Hons) Construction Management. She is NEBOSH qualified and Tech IOSH.

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