Calculating the COSHH regulations 8 hour time weighted average header image

17th April, 2012

Calculating the COSHH regulations 8 hour time weighted average

Certain hazardous substances used within your organisation may be restricted by exposure limits. These exposure limits are set by the HSE under the Control of Substances Hazardous to Health (COSHH) Regulations.

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.

The most common type of workplace exposure limit (WEL) is the 8 hour limit, or long term exposure limit.

The 8 hour time weighted average 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

Working sessions may be broken down as different processes may expose the employee to the substance at different levels, and breaks may stop exposure. 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

The 8 hour reference period is a reference period in every 24 hours in which an 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

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 8 hour time weighted average exposure limit but within 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).

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