A workplace exposure limit (WEL) is the maximum concentration of an airborne substance, averaged over a reference period, to which workers may be exposed. WELs are published by the HSE in guidance note EH40/2005 (as amended).
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What a WEL is
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TWA and STEL
Most substances have an 8-hour time-weighted average (TWA) limit reflecting a normal working day, and many also have a 15-minute short-term exposure limit (STEL) to control acute peaks. Some substances carry a 'Sen' (sensitiser) or 'Sk' (skin) notation that adds further duties.
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How compliance is assessed
A single measurement below the limit is not enough on its own. BS EN 689 sets out a statistical approach that accounts for day-to-day variability, so that a confident judgement of compliance can be made for a whole exposure group.
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Frequently asked questions
Are WELs safe levels?
WELs are legal maximums, not targets — COSHH still requires exposure to be reduced to as low as reasonably practicable, especially for carcinogens and sensitisers.
Do all substances have a WEL?
No — where there is no UK WEL, exposure is benchmarked against recognised international or in-house standards and minimised.