A workplace exposure limit (WEL) is the maximum concentration of an airborne substance, averaged over a set period, that workers may be exposed to. WELs are published by the HSE in a document called EH40, and they are legally binding under COSHH.
The most important thing about a WEL is what it is not: it is not a safe level. It is a legal ceiling. Regulation 7 requires you to reduce exposure so far as is reasonably practicable — being under the limit does not mean you have finished.
What EH40 actually is
EH40 is the HSE’s published list of approved workplace exposure limits. It is revised periodically, so always work from the current edition rather than a figure someone wrote into a document years ago.
Its status matters: the limits in EH40 are approved under COSHH, which makes exceeding them a breach of Regulation 7 rather than merely poor practice.
The two types of limit
| Type | Averaged over | Guards against |
|---|---|---|
| Long-term exposure limit (LTEL) | 8 hours, time-weighted average | Chronic effects from repeated daily exposure |
| Short-term exposure limit (STEL) | 15 minutes, time-weighted average | Acute effects from brief high peaks |
Time-weighted average is the concept people miss. An 8-hour TWA means the average across the whole shift — so a short burst well above the limit can still average out to compliance. That is precisely why STELs exist: some substances cause harm through short spikes that an 8-hour average would hide.
Not every substance has both. Some have only a long-term limit, some only a short-term one.
How limits are expressed
- ppm — parts per million by volume. Used for gases and vapours.
- mg/m³ — milligrams per cubic metre of air. Used for dusts, fumes and mists, and also quoted for gases.
The two are not interchangeable without converting, and the conversion depends on the substance’s molecular weight and on temperature and pressure. Do not mix them up when comparing a monitoring result to a limit.
Some limits worth knowing
Two examples, both from EH40, both 8-hour time-weighted averages:
- Respirable crystalline silica — 0.1 mg/m³. Generated by cutting, grinding or drilling concrete, stone, brick, tiles and engineered stone. A very small number, and easily exceeded by dry cutting without suppression.
- Hardwood dust — 3 mg/m³. Hardwood dust is also a recognised carcinogen, which triggers the stricter “as low as reasonably practicable” duty.
Check the current EH40 for any substance you actually work with — limits change between editions, and quoting an outdated figure is worse than looking it up.
For silica specifically, see our Silica Dust Awareness course.
Substances with no WEL
Most substances hazardous to health have no published limit. That does not mean they are safe or unregulated. Where there is no WEL, COSHH still requires exposure to be prevented or adequately controlled — you simply have no number to measure against, so you rely on good control practice and the Schedule 2A principles.
Note also that dust with no specific WEL is still a COSHH substance once it reaches 10 mg/m³ inhalable or 4 mg/m³ respirable as an 8-hour TWA. Those thresholds come from the definition in Regulation 2, not from EH40.
Carcinogens, mutagens and asthmagens — the limit is not the target
For these substances, COSHH Regulation 7 requires exposure to be reduced to as low as is reasonably practicable, not merely to below the WEL.
The reasoning is that for a genotoxic carcinogen there is no threshold below which risk is known to be zero. Sitting at 90% of the limit and declaring compliance misses the duty entirely.
In EH40 these substances carry notations. Two worth recognising:
- Sen — capable of causing respiratory sensitisation (occupational asthma)
- Sk — can be absorbed through the skin, meaning air monitoring alone will not tell you the true dose
The Sk notation is the one people overlook. If a substance is skin-absorbed, you can be comfortably below the airborne limit and still overexposing someone through their hands.
Do you have to monitor?
Not always. Regulation 10 requires air monitoring where the risk assessment shows it is needed to protect health, where you cannot otherwise demonstrate adequate control, or for the substances and processes listed in Schedule 5.
Where you do monitor:
- Sampling must be representative of what people actually breathe — personal sampling in the breathing zone, not a static box in the corner
- It must reflect the real work pattern, including the worst tasks, not a quiet Tuesday
- Records relating to identifiable employees must be kept 40 years; other records 5 years
Monitoring proves whether controls work. It is not itself a control — a result is a measurement, not a protection.
What to do if you are over the limit
- Act immediately — stop or modify the task; do not schedule a review meeting and continue
- Improve controls, working up the hierarchy: can it be eliminated, substituted, enclosed, extracted?
- Use RPE as an interim measure while engineering controls are put right — with face-fit testing, not just issue
- Re-assess and re-monitor to confirm the fix worked
- Consider health surveillance for those exposed
- Record everything — the finding, the action, the re-test
Further reading
Related training
- COSHH Awareness — substances hazardous to health, exposure routes and the hierarchy of control
- Silica Dust Awareness — respirable crystalline silica, where it comes from and how to control it
Related reading: What is COSHH? · COSHH employer duties · What does COSHH stand for?
£9 per course. CPD accredited, 100 % online, verifiable e-certificate on completion. Pay for 2, get 3 — any 3 courses for £18.
Please note: this is a CPD-accredited awareness course and general information, not legal advice. It is not an Ofqual-regulated qualification. Exposure limits are revised between editions of EH40 — always check the current edition for the substances you work with, and take competent occupational hygiene advice where monitoring is required.