Understanding Workplace Exposure Limits (WELs)

What is a WEL?

Workplace exposure limits (WELs) are occupational exposure limits in Great Britain approved by HSE. They are set to help protect workers’ health. WELs are concentrations of hazardous substances in the air, averaged over a set period of time. These can include: Dust, fumes, chemicals, vapours, mists, nanotechnology, gases, biological agents and germs that cause disease.

Many thousands of substances are used at work but only around 500 substances have WELs.


Why use WELs?

Controlling exposure limits is vital in ensuring safety within a workplace. Exposure to a substance can include:

  • Breathing in fumes, dust, gas or mist
  • Skin contact
  • Injection into the skin
  • Swallowing.

Some illnesses caused by exposure to hazardous substances in the workplace (occupational diseases) may not appear until a long time after the first exposure. Therefore, it is important to know in advance how to protect the health of people working with hazardous substances and of others who may be affected in the surrounding environment.


Types of WELs

There are several different types of limits used, some are only specific for certain governing bodies. The two most common workplace exposure limits are TWA and STEL.

  • Time Weighted Average (TWA) is based on an 8-hour workday and a 40-hour work week.
    For example, the TWA for carbon monoxide is 20ppm. This means that an average of 20ppm is considered to be the safe Threshold Limit Value (TLV®) for an 8-hour workday.
  • Short Term Exposure Limit (STEL) is based on a 15 minute average.



Substances that have been assigned a WEL are subject to the requirements of the Control of Substances Hazardous to Heath (COSHH) regulations. These regulations require employers to prevent or control exposure to hazardous substances. Under COSHH, control measures are defined as adequate only if:

  1. The principles of good control practice are applied
  2. Any WEL is not exceeded
  3. Exposure to asthmagens, carcinogens and mutagens are reduced to as low as is reasonably practicable.

The absence of a WEL does not indicate that a substance is safe. For these substances, exposure should be controlled to a level to which nearly all the working population could be exposed, day after day at work, without any adverse effects on health.


HSE EH40 Workplace Exposure Limits
The following information relates to the most common gases Shawcity customers encounter and has been extracted from Table 1 of the HSE’s EH40/2005 Workplace Exposure limits, published 2020. The full document can be found at https://www.hse.gov.uk/coshh/basics/exposurelimits.htm

The Control of Substances Hazardous to Health (COSHH) Regulations 2002 impose requirements by reference to Table 1 or the Notices of Approval which are therefore legally binding.


WELs table


Monitoring WELs

Frequent monitoring is a useful indicator to check whether the control measures in place are effective. This means measuring the substance in the air that a worker breathes while they are doing a task.

Instrumentation worn by the worker or placed in specific locations can help determine personal or environmental levels of exposure. Advice should be obtained from a competent Occupational Hygiene professional and measurements should be carried out during low, moderate and peak working hours to provide a greater overview of WELs during these periods for analysis.


Monitoring solutions

Shawcity offers an extensive range of personal and area monitoring instrumentation suitable for a wide range of applications including particulate, liquids, gas, vapours and mist. For more information or to discuss your monitoring requirements, contact Shawcity:

01367 899419


Contains public sector information published by the Health and Safety Executive 

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