EH40 Workplace Exposure Limits Explained
If you work with chemicals, dust, fumes or vapours, you've probably come across the term workplace exposure limit (WEL). These limits are the backbone of controlling airborne hazards on UK sites, and they're set out in a document called EH40 — published by the HSE and given legal weight through the Control of Substances Hazardous to Health Regulations 2002 (COSHH).
Understanding EH40 isn't just a tick-box exercise. Get it wrong and you're exposing workers to substances that cause occupational asthma, lung disease, or worse.
What Is EH40?
EH40 (Workplace Exposure Limits) is the HSE's official document listing the legal maximum airborne concentrations for hundreds of hazardous substances. The current edition is EH40/2005 (4th edition, updated 2020) and it is legally referenced by COSHH regulation 7, which requires that exposure is either prevented or, where that isn't reasonably practicable, adequately controlled — including by ensuring WELs are not exceeded.
EH40 is a freely available HSE publication. If you're writing COSHH assessments, you need to be familiar with it.
What a WEL Actually Means
A WEL is expressed as an airborne concentration of a substance — typically in parts per million (ppm) or milligrams per cubic metre (mg/m³) — averaged over a specific time period. There are two types:
| Type | Abbreviation | Averaging period |
|---|---|---|
| Long-term exposure limit | LTEL | 8-hour time-weighted average (TWA) |
| Short-term exposure limit | STEL | 15-minute TWA |
The LTEL reflects what a worker could be exposed to over a normal working day without suffering harm. The STEL caps short bursts of higher exposure — important for substances that can cause acute effects quickly, such as irritation or dizziness.
Not every substance has both values. Some only have an LTEL; others have only a STEL; a few have neither listed but are still hazardous and require control under COSHH.
WELs Are a Legal Ceiling, Not a Safe Target
This is the most common misunderstanding on site. A WEL is the maximum permitted concentration — not a target to aim for. COSHH regulation 7(1) requires that exposure is reduced as low as is reasonably practicable (ALARP), even when you're below the WEL.
Some substances are also assigned Skin or Sen notations in EH40:
- Skin — significant absorption can occur through the skin, so airborne control alone is insufficient.
- Sen — the substance is a known respiratory or skin sensitiser (e.g. isocyanates in spray paints, wood dust). Once a worker is sensitised, any further exposure can trigger a severe reaction. There is no safe threshold for a sensitised individual.
Common WELs Tradespeople Should Know
A few examples from EH40 that come up regularly in trade environments:
- Wood dust (hardwood) — LTEL 3 mg/m³ (hardwood is also classified as a human carcinogen)
- Wood dust (softwood) — LTEL 5 mg/m³
- Respirable crystalline silica (RCS) — LTEL 0.1 mg/m³ (stone, concrete, mortar, sandstone cutting)
- Welding fume (general) — LTEL 1 mg/m³ (and classified as a carcinogen since 2019)
- Isocyanates — LTEL 0.02 mg/m³ NCO (respiratory sensitiser, Sen notation)
These aren't arbitrary numbers. They're derived from toxicological and epidemiological data and they reflect real-world disease outcomes.
How to Use EH40 in a COSHH Assessment
Under COSHH regulation 6, you must carry out a suitable and sufficient assessment of the risk from hazardous substances before work begins. EH40 feeds directly into that process:
- Identify the substance and find its WEL in EH40 (or check the Safety Data Sheet — SDSs reference WELs under Section 8).
- Estimate exposure — consider how long workers are exposed, how much of the substance is airborne, and how many people are affected.
- Apply the hierarchy of controls: eliminate the substance if possible; substitute for something less harmful; use engineering controls (LEV, on-tool extraction, enclosure); administrative controls (job rotation, reduced shift length); RPE as a last resort or supplement.
- Check you're below the WEL — and then go further, reducing exposure ALARP.
- Record, review, and monitor — for substances like silica and hardwood dust, air monitoring may be needed to verify control measures are working.
RPE and WELs
Respiratory protective equipment (RPE) is often misused as a primary control. It should supplement engineering controls, not replace them. If you're relying on RPE to stay below a WEL, the assigned protection factor (APF) of the RPE must be sufficient to bring the wearer's exposure below the WEL. RPE selection guidance is covered in HSE document RPE at Work (HSG53).
Key Takeaway
EH40 is not optional background reading — it is the legal reference document for airborne hazard control under COSHH. Know the WELs for the substances your workers encounter, apply controls in the right order, and remember that a WEL is a ceiling, not a comfort zone. Occupational lung disease is largely preventable, and EH40 exists precisely to help you prevent it.