What Are Workplace Exposure Limits? A Plain-English Guide for UK Tradespeople
If you work with dust, fumes, solvents, or any hazardous substance, you've probably come across the term workplace exposure limit — or WEL. But what does it actually mean, and what do you have to do about it? Here's a clear, practical explanation grounded in the rules that actually apply in the UK.
The Legal Basis: COSHH 2002
Workplace exposure limits sit within the Control of Substances Hazardous to Health Regulations 2002 (COSHH). Under COSHH, employers must assess the risk from hazardous substances and ensure that exposure is either prevented or, where that isn't reasonably practicable, adequately controlled. WELs are central to that control standard.
They are published by the HSE in a document called EH40/2005 Workplace Exposure Limits, which is updated periodically. The current edition (fourth edition, 2020) lists hundreds of substances alongside their legal limit values. EH40 is the definitive UK reference — if a substance has a WEL, it appears there.
What a WEL Actually Is
A WEL is the maximum airborne concentration of a hazardous substance that a worker may be exposed to, averaged over a specific reference period, without it being considered an unacceptable risk to health — provided adequate controls are also in place.
Two time periods are used:
| Type | Reference Period | Purpose |
|---|---|---|
| 8-hour TWA (time-weighted average) | A full working shift | Controls chronic, long-term harm |
| Short-term exposure limit (STEL) | Any 15-minute period | Controls acute effects from brief peak exposures |
Both figures are expressed in parts per million (ppm) for gases and vapours, or milligrams per cubic metre (mg/m³) for dusts and particulates.
Example: The WEL for wood dust (hardwood) is 3 mg/m³ as an 8-hour TWA. For general inhalable dust with no specific WEL, a general limit of 10 mg/m³ (inhalable) and 4 mg/m³ (respirable) applies.
WELs Are a Ceiling, Not a Target
This is where many people misunderstand them. A WEL does not define a safe level. COSHH regulation 7 requires employers to reduce exposure to as low as reasonably practicable (ALARP) — even below the WEL. Breaching a WEL is a regulatory offence, but staying just beneath one is not automatically sufficient.
Think of a WEL as the legal line you must not cross, while ALARP is the standard you must actively pursue.
How Exposure Is Assessed
To know whether exposure is approaching or exceeding a WEL, you need to assess it. For most tradespeople, this involves:
- Identifying the substances present and their WELs via EH40 and safety data sheets (SDS).
- Estimating exposure — for many common tasks, the HSE provides COSHH essentials exposure bands and task-based guidance (e.g., HSG97 for dust from construction activities).
- Air monitoring where significant exposure is suspected and estimation isn't reliable. This typically involves personal sampling — a pump worn by the worker — analysed by an accredited laboratory (UKAS-accredited).
- Comparing results to the WEL and deciding whether controls are adequate.
Air monitoring is not always compulsory, but if you're working with substances assigned a WEL and your risk assessment cannot otherwise demonstrate adequate control, it's often the only defensible approach.
Controlling Exposure: The Hierarchy Applies
Under COSHH, once you've identified a risk, controls must follow the correct hierarchy:
- Eliminate the substance entirely if possible (e.g., design out a process that generates dust)
- Substitute with something less hazardous (e.g., water-based instead of solvent-based products)
- Engineering controls — local exhaust ventilation (LEV), on-tool extraction, enclosed systems
- Administrative controls — job rotation, reduced exposure time, restricted access
- PPE last — respiratory protective equipment (RPE) such as half-face masks with appropriate filters
RPE must be the right fit, the right class (the assigned protection factor must match the exposure level), and worn correctly. It is a supplement to other controls, not a substitute for them.
Substances With No WEL
Not every hazardous substance has a WEL — some lack sufficient toxicological data, and some risks are non-inhalation routes (skin absorption, ingestion). COSHH still applies; you must still assess and control exposure. Skin notation in EH40 (marked "Sk") flags substances where dermal absorption is a significant route.
Practical Takeaways
- Check EH40 and the substance's SDS to find the relevant WEL before starting work.
- Your COSHH risk assessment must address whether existing controls keep exposure below the WEL and as low as reasonably practicable.
- If you're using LEV or RPE, both need to be maintained, inspected, and — for LEV — thoroughly examined at least every 14 months under COSHH regulation 9.
- Keep records of any air monitoring results; COSHH requires records of monitoring carried out for substances with WELs to be retained for at least 5 years (or 40 years for processes that may expose workers to substances causing cancer or asthma).
Understanding WELs properly means you're not just ticking a box — you're making genuinely informed decisions about what your workers breathe in every day.