Workplace Exposure Limits: The EH40 Guide

A workplace exposure limit (WEL) is the maximum airborne concentration of a hazardous substance averaged over a defined reference period. In Great Britain, legally binding WELs are published in EH40, but staying below a number does not automatically prove that exposure is adequately controlled under COSHH.

WELs apply to inhalation exposure and must be used with the control hierarchy, good practice, other exposure routes and the substance's specific health effects.

What is EH40?

EH40 Workplace exposure limits is HSE's authoritative list of approved occupational exposure limits for Great Britain. HSE's WEL overview states that the limits in EH40 are legally binding under COSHH.

EH40 includes substance identity, CAS number where applicable, long-term and short-term limits, units and notes. Always use the current HSE edition and amendments rather than a copied table in an old assessment.

Northern Ireland dutyholders should check HSENI guidance and the legal framework applicable there.

What is an 8-hour time-weighted average?

An 8-hour TWA averages airborne concentration across an eight-hour reference period. It recognises that exposure can rise and fall during a shift.

A simplified calculation is:

TWA = sum of concentration × time for each period, divided by eight hours

For example, two hours at one concentration and six hours at another contribute in proportion to their duration. The arithmetic is only as reliable as the sampling strategy and task information.

A worker with a longer shift is not automatically permitted the same total dose. Adjustments and risk interpretation may be needed, particularly for unusual schedules, and should be handled by competent occupational hygiene advice.

What is a 15-minute short-term exposure limit?

A short-term exposure limit (STEL) is averaged over a 15-minute reference period. It controls brief peaks that may cause acute irritation, narcosis or other effects even when the full-shift average appears acceptable.

Where EH40 lists both limits, both must be met. A low eight-hour result can hide a high exposure during charging, cleaning, sampling or maintenance.

Where no STEL is listed, COSHH still requires control of exposure. The absence of a number is not permission for uncontrolled peaks.

What units are used?

EH40 commonly expresses gases and vapours in parts per million and milligrams per cubic metre, while dusts and aerosols are generally expressed by mass concentration. Conversions depend on temperature, pressure and molecular properties.

Do not compare a detector reading with an EH40 value until the units, averaging period and sampled fraction match. Inhalable and respirable dust measurements are not interchangeable.

Does a WEL mark the boundary between safe and dangerous?

No. A WEL is a statutory control benchmark, not a guarantee that no worker will experience an effect. Individual susceptibility varies, and exposure may occur through skin contact, ingestion or injection as well as inhalation.

COSHH requires employers to prevent exposure where reasonably practicable or otherwise control it adequately. HSE states that exposure to carcinogens, mutagens and asthmagens must be controlled to as low as is reasonably practicable as well as meeting any WEL.

Good control should therefore aim below the limit with a margin that reflects process variability and measurement uncertainty. Designing a process to sit exactly on the limit leaves no resilience for a busy shift, blocked filter or open door.

What do skin and other notations mean?

EH40 notes can signal important qualifications. A skin notation indicates that substantial uptake through the skin may occur, so air monitoring alone may underestimate total exposure.

Other entries may identify special measurement conventions or biological monitoring guidance. Read the notes and definitions in the publication rather than extracting only the number.

Skin notation does not simply mean “wear gloves”. The assessment should consider substitution, enclosure, splash control, contamination, glove material and breakthrough, removal technique, washing and surface cleaning.

When is exposure monitoring needed?

HSE's monitoring guidance says monitoring can be appropriate to show compliance with a WEL or to demonstrate that controls work adequately. It may also be needed where control failure could cause serious health effects or where the assessment cannot otherwise reach a sound conclusion.

Monitoring types include:

These methods answer different questions. An area monitor on a wall does not necessarily represent the breathing-zone exposure of someone leaning over an open vessel.

How should a monitoring survey be planned?

Define the decision before sampling. A useful brief states:

• Substance and applicable limit or benchmark.
• Workers and similar exposure groups.
• Tasks, duration and expected variability.
• Normal, worst credible and non-routine conditions.
• Existing controls and potential failure modes.
• Sampling method, laboratory and detection limit.
• Information that will be recorded during the shift.
• How uncertainty and repeat measurements will be handled.
• Actions for high or unexpected results.

Ask a competent occupational hygienist to design and interpret complex surveys. One result from a quiet day should not be presented as proof for every worker and season.

How do WELs fit into a COSHH assessment?

The assessment should identify whether a current WEL applies, describe expected exposure, select controls and state how their effectiveness will be verified. It should also address skin and other routes, substances without WELs and vulnerable workers.

A practical hierarchy is:

1. Eliminate or substitute the hazardous substance or process where reasonably practicable.
2. Enclose or automate the source.
3. Use suitable engineering controls such as local exhaust ventilation.
4. Apply reliable systems of work and restrict access.
5. Use suitable PPE for residual exposure.
6. Maintain, test, supervise and review every critical control.

The Safe Foundry workflow structures SDS and assessment information, while exposure estimates and monitoring still require site evidence and competence. See the features and help centre for product details.

How long are monitoring records kept?

COSHH regulation 10 requires personal exposure records representing identifiable employees to be retained for at least 40 years from the last entry. Other monitoring records are kept for at least five years.

Records should include enough context to interpret the result later: worker, task, controls, duration, method, conditions, result, assessor and actions. A laboratory number without the work context has limited value.

What should happen after a result exceeds a WEL?

Protect people first, investigate the source and review the assessment and controls. Consider whether other workers, products, shifts or locations are affected.

Do not solve the result only by issuing tighter respirators. Check elimination, enclosure, process condition, ventilation performance, maintenance, work method and supervision. Communicate relevant results to workers and verify that corrective action actually reduces exposure.

What is the practical rule?

Use EH40 as the authoritative legal benchmark, not as the whole control strategy. Good COSHH management understands the exposure pattern, keeps controls effective and aims for reliable performance below the limit rather than treating one passing sample as the finish line.