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A workplace exposure limit, or WEL, is the maximum concentration of a hazardous substance in the air that a worker can be exposed to, averaged over a set period. In Great Britain, WELs are listed in HSE’s publication EH40/2005, which is used with COSHH. The current edition was published in 2020 and HSE updates it from time to time, so always check the latest version.
Long-term and short-term limits
| Limit | Averaging period | What it protects against |
|---|---|---|
| Long-term exposure limit | 8 hour time-weighted average | Effects that build up over time, such as lung disease |
| Short-term exposure limit (STEL) | 15 minutes | Effects from brief high exposures, such as irritation or dizziness |
Limits are given in parts per million (ppm) for gases and vapours, and milligrams per cubic metre (mg/m³) for dusts, fumes and mists. For example, respirable crystalline silica has a long-term limit of 0.1 mg/m³.
The notations in EH40
- Sk: the substance can be absorbed through the skin, so air monitoring alone does not show the full exposure.
- Sen: the substance can cause occupational asthma or other sensitisation.
- Carc: the substance is a carcinogen.
- BMGV: a biological monitoring guidance value exists, for example from urine samples.
A WEL is not a safe level
Staying under a WEL is the minimum. For substances that are carcinogens, mutagens or that cause asthma, COSHH requires exposure to be reduced as low as reasonably practicable, not just below the limit. And many substances have no WEL at all, but must still be controlled. Our guide to COSHH assessment explains how to decide the controls.
How exposure is measured
Personal air monitoring uses a small pump worn by the worker, drawing air through a filter or tube near their breathing zone, which is then analysed in a laboratory. Direct reading instruments give real time readings for some gases. Monitoring should be planned by someone competent, usually an occupational hygienist.
How to read a safety data sheet
Suppliers of hazardous chemicals must provide a safety data sheet (SDS) with 16 standard sections:
| Section | What it tells you |
|---|---|
| 1 | Identification of the substance and supplier |
| 2 | Hazards identification, including hazard pictograms and statements |
| 3 | Composition and ingredients |
| 4 | First aid measures |
| 5 | Firefighting measures |
| 6 | Accidental release measures, such as spills |
| 7 | Handling and storage |
| 8 | Exposure controls and personal protection, including any WELs |
| 9 | Physical and chemical properties |
| 10 | Stability and reactivity |
| 11 | Toxicological information |
| 12 | Ecological information |
| 13 | Disposal considerations |
| 14 | Transport information |
| 15 | Regulatory information |
| 16 | Other information |
For a COSHH assessment, sections 2, 7 and 8 are usually the most useful. Our guide to hazard symbols explains the pictograms in section 2.
Training that helps
Our COSHH awareness training covers safety data sheets and exposure limits in practical terms. For managers and advisers, the NEBOSH National General Certificate covers WELs, monitoring and control, and the Level 6 NVQ in Occupational Health and Safety suits experienced practitioners working towards Chartered status.
Frequently asked questions
What is EH40?
HSE’s publication EH40/2005, Workplace exposure limits, which lists the legal exposure limits for use with COSHH in Great Britain.
What is the difference between a WEL and a STEL?
A long-term WEL is averaged over 8 hours. A short-term exposure limit (STEL) is averaged over 15 minutes.
Are workplace exposure limits legally binding?
Yes. Under COSHH, exposure must not exceed a WEL, and for carcinogens, mutagens and asthmagens it must also be reduced as low as reasonably practicable.
How many sections does a safety data sheet have?
Sixteen, in a set order. Section 8 covers exposure limits and personal protection.
Read the source
HSE EH40/2005 Workplace exposure limits (4th edition, 2020); the COSHH Regulations 2002, regulation 7; HSE L5, the COSHH ACoP; and the requirements for safety data sheets under GB REACH, Annex II. Facts checked on 6 October 2026.
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