The employer is responsible. Not the safety officer, not the occupational health provider, not the employee wearing the mask.
That answer holds however the question is phrased, and it does not transfer. You can delegate the sampling, the analysis and the advice — you cannot delegate the duty.
Do you know whether any substance you use has a workplace exposure limit?
Dust, fume, vapour, solvent or gas
Who holds the duty
Section 2 of the Health and Safety at Work etc. Act 1974 places the duty to ensure employees' health on the employer. COSHH builds on that: regulation 7 requires the employer to ensure exposure is either prevented or adequately controlled.
Regulation 7(7) then defines what "adequate" means, and it is specific. Control is only adequate if:
- the principles of good practice in Schedule 2A are applied, and
- any workplace exposure limit approved for that substance is not exceeded, and
- for carcinogens, mutagens and asthmagens, exposure is reduced as low as is reasonably practicable
Nothing in that shifts to an adviser or a contractor. Bringing in an occupational hygienist is often the sensible thing to do — but if the limit is exceeded, it is the employer who has failed the duty.
What EH40 is
EH40 is the HSE publication listing approved workplace exposure limits for Great Britain. It is the reference point your COSHH assessment compares against, and it is periodically updated — an assessment written against an old edition may be measuring against a limit that has since been lowered.
You can read our summary of EH40 and how to use it.
Long-term and short-term limits
Most substances carry two limits, and they exist because substances harm in two different ways.
The two exposure limits
| Limit | Averaging period | What it guards against |
|---|---|---|
| Long-term exposure limit | 8 hours, time-weighted average | Cumulative harm from sustained exposure across a shift |
| Short-term exposure limit | 15 minutes | Acute effects from brief peaks — irritation, narcosis, immediate damage |
The eight hours in the long-term limit is the answer people are usually looking for: exposure is averaged over an eight-hour working day.
Averaging cuts both ways. A task producing very high concentrations for twenty minutes can average out below the eight-hour limit and still breach the short-term limit — and still harm someone. Meeting one limit is not evidence of meeting the other.
A WEL is a ceiling, not a target
This is the misunderstanding that does the most damage. A workplace exposure limit is the concentration that must not be exceeded. It is not a line below which exposure is safe and above which it is harmful.
Regulation 7(7)(a) makes this explicit by requiring the Schedule 2A principles of good practice as well as compliance with the limit. Designing a process to sit just under a WEL, when better control was reasonably practicable, does not satisfy COSHH.
For substances that cause cancer, heritable genetic damage or occupational asthma, the duty goes further still: exposure must be reduced as low as is reasonably practicable, whatever the limit says.
When is air monitoring actually required?
Not always — and this is where employers either overspend or skip something they needed.
Regulation 10(1) requires monitoring where the risk assessment indicates that it is requisite for ensuring the maintenance of adequate control, or otherwise requisite for protecting health.
In practice that means monitoring is needed when:
- You cannot otherwise demonstrate that control is adequate
- Failure of control would cause serious harm before it became obvious
- The substance has a limit and your controls sit close to it
- You need to verify that engineering controls such as extraction are performing
It is generally not needed where exposure is plainly trivial, or where a recognised control approach is being followed properly and the outcome is well established.
Monitoring is only worth commissioning if the sampling method suits the substance and the analysis is done by a competent laboratory. A badly designed sample produces a number that reassures you about nothing.
What to do with the result
A measurement is not the point — what you change because of it is.
- Compare against both the long-term and short-term limits in EH40
- If either is exceeded, control is inadequate by definition — act on the process, not on PPE first
- If you are close to a limit, treat that as a warning rather than a pass
- Record the finding in the COSHH assessment
- Re-monitor when the process, materials or controls change
Common mistakes
Assuming the safety officer owns it. The duty sits with the employer and does not move.
Treating the WEL as a safe level. It is a ceiling, and Schedule 2A applies regardless.
Checking only the 8-hour average. Short-duration tasks breach short-term limits while looking fine on a shift average.
Forgetting dust. Wood dust, flour and silica carry limits and are produced by the work, not bought in a labelled container.
Working from an old EH40. Limits change, and some have been lowered significantly.
Where this fits
See COSHH for the assessment this sits inside, the Approved Code of Practice at L5, and occupational health for health surveillance where exposure remains after your controls. Where respiratory protection forms part of the control, see RPE face fit testing.