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30th July, 2018

COSHH Workplace Exposure Limits (WELs) Explained

Under the COSHH regulations, workplace exposure limits (WELs) are assigned to a large number of hazardous substances, which must not be exceeded.

Around 500 substances have WELs assigned to them, and these hazardous substances could be chemicals, fumes, dusts or fibres.

WELs are legal limits of exposure, averaged over a specified period of time. So within a period of time (often 8 hours, but sometimes as short as 15 minutes), you are only allowed to be exposed to a certain amount of a hazardous substance.

This list is legally binding, as it reproduces the list of workplace exposure limits (WELs) which have been approved by the Health and Safety Executive.

WELs apply to lots of substances you will have heard of like arsenic, asphalt, carbon dioxide, carbon monoxide, cement, flour dust, gypsum, hardwood dust, limestone and petroleum fumes. But also some you might not be so familiar with, like piperazine dihydrochloride.

Substances which have been assigned a WEL include those that are carcinogenic or mutagenic could cause asthma and other substances that are of significant risk through high levels of inhalation.

Sometimes it's not possible to eliminate exposure to a hazardous substance completely, even with controls like extraction, ventilation and PPE in place.

Exposure limits are in place to control the effects of substances, depending on the nature of the substance and the effects of exposure. Some effects require prolonged exposure, while others may be seen after brief exposures.

Time-weighted averages

Workplace exposure limits are subject to time-weighted averages (TWA). The long-term exposure limit (LTEL) or 8 hour reference period and the short-term exposure limit (STEL) or 15 minute reference period.

These are the two time periods used for WELs:

  • Long-term (8 hours)
  • Short-term (15 minutes)

There are certain rules that apply to these time-weighted exposures such as that the short-term exposure level takes priority over the long-term, and that the LTEL can be increased if the exposure period is less.


Calculating WELs

To know if you are under the legal workplace exposure limits, you need to calculate exposure.

A simple example of someone being exposed to 0.2mg.m-3 of a hazardous substance for 8 hours would give a time-weighted average of 0.2mg.m-3.

(0.2 x 8) / 8 = 0.2 mg.m-3

But it's not that often your calculations will be that simple. Its more likely exposure will be for a shorter period while a specific task is carried out.

For example, someone is exposed to 0.25mg.m-3 for 3 hours. You would calculate those 3 hours and then the remaining 5 at 0 to get the 8-hour time-weighted average.

((0.36 x 3) + (0 x 5)) / 8 = 0.135mg.m-3

WELs calculation

The 8-hour LTEL is the maximum exposure allowed over an 8 hour period. When calculating the exposure level, if the exposure period is less than 8 hours then the exposure limit can be increased providing that exposure above the LTEL value continues for no longer than an hour and that the LTEL is not exceeded over the 8 hours.

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.

However, the STEL will always take priority over the LTEL. So if using the example above, the substance had a STEL of 15mg.m-³, then action would be required if the exposure level rose above this for more than 15 minutes.

For those substances with no specified STEL, it is recommended that a figure of three times the LTEL is used as a guideline for controlling short-term peaks in exposure.

Checking exposure levels

How do you know if you are within the workplace exposure limit?

Usually through monitoring exposure.

Remember, WELs are legal limits, so you must make sure that these levels are not being exceeded.

Regulation 10 of the COSHH regulations imposes a duty to monitor the exposure of employees to substances hazardous to health in certain specified situations. Sampling strategies may involve measurement of the hazardous substance in the breathing zone of the worker (personal sampling) or in the workplace air.

To find out current workplace exposure limits (WELs) check out the HSE publication EH40 Workplace Exposure Limits, which can be downloaded from the HSE website.

Beyond WELs

If a substance is not assigned a WEL, that doesn't make it safe. Under COSHH, you should still make sure exposure is controlled to a level to which is safe for any hazardous substance used or created at work.

WELs apply to exposure by inhalation, as this is the main route of entry into the body, for most substances.

However, the COSHH regulations apply to all routes of entry as some substances have the ability to penetrate intact skin and become absorbed into the body, and hazardous substances can all harm through skin contact, contact with eyes, and injection and ingestion.

In addition to complying with WELs, it is important to also carry out a COSHH assessment and control all potential exposure routes.

This article was written by Emma at HASpod. Emma has over 10 years experience in health and safety and a degree in construction management. She is NEBOSH qualified and a member of IOSH.

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