Workplace exposure limits explained

A workplace exposure limit, or WEL, is an approved limit for the concentration of a hazardous substance in the air a worker breathes over a specified period. It is a useful reference for assessing and controlling exposure. It is not a general declaration that a substance is harmless below the number, nor a replacement for preventing exposure where that is reasonably practicable.

This guide uses the Great Britain COSHH framework. HSE describes WELs as GB occupational exposure limits and publishes the current list in EH40. Northern Ireland uses its own enforcement and regulatory arrangements, so employers there should check the applicable local sources before treating this GB guide as operational advice. The article owns the WEL concept; it does not repeat the full COSHH assessment method or turn into a sales page for COSHH assessment software.

What does a WEL measure?

Under the COSHH Regulations 2002, a WEL concerns exposure to a substance in workplace air, over an approved reference period and by the approved method. The HSE explanation describes it as a concentration averaged over a set time. This matters because a single momentary reading, a room average and a worker's breathing-zone exposure may tell different stories.

An eight-hour time-weighted average reflects exposure across a standard reference period. Work may include a high-exposure task followed by lower-exposure time; the calculation considers the pattern rather than taking the highest reading alone. A short-term exposure limit, commonly shown in EH40 for substances where one is set, addresses a shorter reference period, often 15 minutes. A short high peak may matter even when the whole-shift average looks low. Check the current EH40 entry for the substance, its applicable reference period and any notation. Do not use an old value copied into a generic safety sheet.

Airborne concentration does not capture every route into the body. HSE notes that hazardous substances can also be taken up through skin contact, injection or swallowing. A worker's air exposure below a WEL does not settle a separate skin or contamination risk. The assessment must address the actual substance and work process.

What is EH40?

EH40/2005 Workplace exposure limits is HSE's current publication of approved GB WELs, updated from time to time. The publication name includes “2005,” but HSE's page records later changes, including revised limits introduced in January 2020. The HSE WEL page says EH40 contains all current GB WELs and is amended when new or revised limits are approved.

Check the HSE source at the time of the assessment. A chemical inventory, old SDS or training presentation may have a value that has been superseded. Make a record of the version and date used. This guide deliberately does not reproduce EH40's table of substances and numbers, because a copied table would become stale and could lose important footnotes or reference-period distinctions.

HSE says only around 500 substances have WELs, while many thousands are used at work. The absence of an EH40 entry does not make an exposure safe or exempt it from COSHH. Some hazardous materials also sit under specific regulatory schemes. The employer must identify the hazards and control the exposure under the law that applies to the task.

How does a WEL fit into COSHH?

Regulation 6 of COSHH requires a suitable and sufficient assessment of risk from hazardous substances before work liable to expose employees, including consideration of any relevant WEL. Regulation 7 requires exposure to be prevented or, where prevention is not reasonably practicable, adequately controlled. The regulation prioritises suitable processes and engineering controls, control at source and then suitable PPE where other measures cannot achieve adequate control.

The WEL therefore helps test part of the control picture. It is not a permit to expose workers up to the limit. HSE says exposures to substances classified as carcinogens, mutagens or asthmagens should be controlled to as low as reasonably practicable. A measured value below a WEL may still signal poor control if exposure could be prevented or reduced further by a practical method. Equally, a ventilation system should be checked for effectiveness, not assumed adequate because it was installed.

This owner stops at the role of limits. For a full task assessment, use the existing COSHH how-to guide and consider the COSHH checker as a question aid. A checker cannot sample air or decide that a control is legally sufficient. A substance used without a current safety data sheet needs its own assessment route; do not invent a WEL because a supplier document is missing.

When might exposure monitoring be needed?

The HSE monitoring guide explains that air monitoring can help show whether controls are effective and, where needed, whether exposure is below a WEL. Sampling should represent the worker's actual exposure during the relevant task and reference period. A measurement at a fixed wall position may miss exposure when a worker leans into a process or opens an enclosure.

Whether to monitor, how many samples to take and how to interpret them are technical decisions. Consider task variability, shifts, controls, abnormal activities and the substance's hazards. A competent occupational hygienist may be needed. Keep the method, date, operating conditions and result together so the number can be understood later. A result that lacks context is easily misused.

Monitoring is not the same as health surveillance. Air sampling measures exposure in the workplace. Health surveillance examines possible effects on people where the relevant criteria are met. HSE states that exposure monitoring has nothing to do with the current state of a worker's health. The two can be related in a wider control programme but answer different questions.

Example: a solvent-cleaning task

A workshop uses a solvent for a short cleaning task. The assessment identifies the substance and checks the current EH40 entry, including any eight-hour and short-term reference periods. The team examines how the product is dispensed, whether a safer substitution is practical, whether the work can be enclosed or ventilated and whether skin contact is possible. A competent person decides whether exposure sampling is needed and designs it around the worker's actual task. The workshop then checks that the control works during normal use and maintenance.

The example does not identify a particular solvent or assign a numeric limit. It shows why an EH40 value is one input, alongside process design, control performance and other exposure routes.

If evaluating a way to organise COSHH records and actions, see Complys COSHH assessments and ask for a demonstration of current functionality.