Understanding Workplace Exposure Limits (WELs) for UK Businesses

What Are Workplace Exposure Limits?

A Workplace Exposure Limit (WEL) is the maximum concentration of a hazardous substance in the air that a worker can be exposed to, averaged over a reference period. WELs are set by the Health and Safety Executive and published in EH40/2005 (Fourth edition, 2020) — the official UK table of workplace exposure limits.

WELs exist for hundreds of substances, from common solvents and dusts to specific industrial chemicals. They are legally binding under Regulation 7 of the Control of Substances Hazardous to Health Regulations 2002: you must ensure that exposure does not exceed the WEL, or if it does, you must take immediate action to reduce it.

A WEL is not a "safe" level. It is the maximum permissible concentration. The COSHH regulations require you to reduce exposure to as low as is reasonably practicable, regardless of whether a WEL exists. For substances that cause cancer, genetic damage, or respiratory sensitisation, the duty is even stricter — you must minimise exposure, not just stay below the limit.

TWA vs STEL: Two Types of Limit

Most substances in EH40 have one or both of two types of exposure limit:

8-Hour Time-Weighted Average (TWA)

The TWA is the average concentration of the substance in the air over an 8-hour working day. It accounts for the fact that exposure varies throughout the day — you might be exposed to higher levels during certain tasks and lower levels (or zero) during breaks and other work.

Example: The 8-hour TWA for wood dust (all species) is 3 mg/m³. If a joiner is exposed to 6 mg/m³ during 4 hours of active cutting and sanding, and 0 mg/m³ during the other 4 hours, the TWA is 3 mg/m³ — right at the limit. In practice, you would need to reduce exposure during the cutting period to build in a safety margin.

15-Minute Short-Term Exposure Limit (STEL)

The STEL is the maximum concentration over any 15-minute period. It protects against short bursts of high exposure that the TWA alone would not catch.

Example: Ammonia has a STEL of 35 ppm. Even if average exposure over the full shift is well below the 8-hour TWA of 25 ppm, a 15-minute period where a worker is exposed to 50 ppm (e.g., during a spill clean-up or when opening a concentrate) would exceed the STEL and require action.

Not every substance has a STEL. Where one is listed in EH40, it applies alongside the TWA. Where no STEL is specified, the HSE guidance recommends that short-term exposure should not exceed three times the TWA over a 15-minute reference period.

Common WELs for Substances SMEs Encounter

Here are the workplace exposure limits for substances that small and medium-sized businesses encounter frequently. All values are from EH40/2005 (Fourth edition, 2020). Check the HSE website for the latest edition, as WELs are periodically updated.

Dusts

Substance	8-Hour TWA	15-Min STEL	Common industries
Wood dust (all species)	3 mg/m³	—	Joinery, carpentry, furniture making
Flour dust	10 mg/m³ (inhalable)	30 mg/m³ (inhalable)	Bakeries, food manufacturing
Silica dust (respirable crystalline)	0.1 mg/m³	—	Construction, stone cutting, concrete work
Inhalable dust (general, PNOC)	10 mg/m³	—	All workplaces with dusty operations
Respirable dust (general, PNOC)	4 mg/m³	—	All workplaces with dusty operations

Silica dust at 0.1 mg/m³ is one of the lowest dust WELs and one of the hardest to stay below without proper controls. If your workers cut concrete, stone, or brick, this limit demands serious engineering controls — wet cutting, LEV, or both.

Solvents

Substance	8-Hour TWA	15-Min STEL	Common uses
Acetone	500 ppm	1,500 ppm	Nail salons, adhesive removal, cleaning
White spirit	25 ppm	—	Painting, decorating, degreasing
Toluene	50 ppm	—	Adhesives, paints, lacquers
Isopropanol (IPA)	400 ppm	500 ppm	Cleaning, sanitising, printing
Xylene (all isomers)	50 ppm	100 ppm	Paints, varnishes, adhesives

Other common substances

Substance	8-Hour TWA	15-Min STEL	Common uses
Ammonia	25 ppm	35 ppm	Cleaning, refrigeration, hair salons
Chlorine	0.5 ppm	1 ppm	Water treatment, from bleach misuse
Glutaraldehyde	0.05 ppm	0.05 ppm (ceiling)	Healthcare disinfection
Isocyanates (as NCO)	0.02 mg/m³	0.07 mg/m³	Spray painting, foam manufacturing, adhesives
Formaldehyde	2 ppm	2 ppm (ceiling)	Salon keratin treatments, mortuaries, laboratories
Carbon dioxide	5,000 ppm	15,000 ppm	Breweries, cellars, welding

Isocyanates deserve particular attention. The WEL is extremely low (0.02 mg/m³ TWA), and isocyanates are the leading cause of occupational asthma in the UK. Any business using isocyanate-containing products — spray paints, polyurethane foams, certain adhesives — needs rigorous controls and health surveillance.

When Is Exposure Monitoring Required?

The COSHH Regulations (Regulation 10) require exposure monitoring in certain circumstances:

Mandatory monitoring: Some substances have specific monitoring requirements listed in Schedule 5 of the COSHH Regulations. These include vinyl chloride, isocyanates (in specific processes), and certain other substances.
Where there is uncertainty: If you cannot be confident that exposure is below the WEL based on the controls in place, you need monitoring to find out.
After changes: If you change your process, products, or controls, monitoring verifies that exposure is still adequately controlled.

For many SME situations, you can make a reasonable judgement without formal air monitoring. A cleaner using a bottle of bleach in a well-ventilated bathroom is unlikely to approach the chlorine STEL. A hairdresser mixing small quantities of colour in a ventilated salon is unlikely to exceed the ammonia TWA.

But some situations demand monitoring:

Workers using volatile solvents in poorly ventilated or enclosed spaces
Dust-generating processes (woodworking, concrete cutting, bakery work) where significant airborne dust is visible
Any process involving isocyanates
Where workers report symptoms (headaches, throat irritation, coughing, skin reactions) that suggest overexposure

Exposure monitoring must be carried out by a competent person — typically an occupational hygienist — using calibrated sampling equipment. Results must be recorded and kept for at least 5 years (40 years for substances that cause cancer or asthma).

How WELs Fit Into Your COSHH Assessment

Every COSHH assessment should include a check against relevant WELs. The process, covered in detail in our COSHH assessment guide, works like this:

Identify whether your substance has a WEL. Check EH40, or look at Section 8 of the Safety Data Sheet — it will list any applicable WELs for the product or its components.
Assess whether exposure could approach or exceed the WEL. Consider the quantity used, the duration of use, the ventilation available, and the nature of the task. For a worked example of this assessment, see our COSHH risk assessment examples.
If exposure is clearly well below the WEL: Document your reasoning in the assessment. "Used in small quantities (50ml per application) in a well-ventilated room with open windows. Exposure expected to remain well below the WEL of [X]. No monitoring required at this time."
If exposure could approach the WEL: Arrange professional exposure monitoring to get actual data. In the meantime, review and strengthen your controls.
If monitoring shows exposure above the WEL: Take immediate action (see below).

What to Do If Exposure Exceeds the WEL

If exposure monitoring shows that a WEL has been exceeded, or if there are signs of overexposure (worker symptoms, visible dust clouds beyond what controls should allow), you must act:

Immediate steps

Stop the activity or take immediate measures to reduce exposure (increase ventilation, move to a better-ventilated area, provide RPE as an interim measure).
Investigate the cause. Was a control measure not working? Was the process changed? Was PPE not worn correctly?
Inform affected workers that the WEL was exceeded and what you are doing about it.

Corrective actions

Review and improve control measures. Work through the hierarchy of control: can you eliminate or substitute the substance? Can you improve engineering controls (LEV, enclosure)? Can you change the process to reduce exposure time?
Repeat monitoring after implementing changes to confirm that exposure is now below the WEL.
Update the COSHH assessment to reflect the new controls and the monitoring results.
Arrange health surveillance for affected workers if not already in place — particularly for substances that cause asthma or dermatitis.

Record keeping

Record the exceedance, the investigation findings, the actions taken, and the follow-up monitoring results. Keep these records for at least 5 years (40 years for carcinogens and asthmagens).

Repeated or persistent WEL exceedances are a serious compliance failure. The HSE can issue enforcement notices and prosecute where employers fail to control exposure adequately.

Where to Find WELs

EH40/2005 (as amended) is the definitive reference. It is published by the HSE and available as a free PDF from hse.gov.uk. The table lists substances alphabetically with their 8-hour TWA and STEL values.

Safety Data Sheets (Section 8) will also list the WELs for the product or its ingredients. However, always cross-check against EH40 — an SDS from an overseas manufacturer might list non-UK exposure limits.

EH40 is updated periodically as new scientific evidence becomes available and exposure limits are reviewed. Make sure you are using the current version.

Common Misconceptions About WELs

"If we are below the WEL, we are fine"

Not necessarily. The COSHH Regulations require you to reduce exposure to as low as is reasonably practicable, not merely to stay below the WEL. For carcinogens, mutagens, and respiratory sensitisers, the duty is to minimise exposure — the WEL is a ceiling, not a target.

"WELs only matter for factories"

WELs apply to every workplace. A hairdresser mixing ammonia-based colour, a baker surrounded by flour dust, a cleaner using solvents in an enclosed space — all face potential WEL-relevant exposures. Small scale does not mean low risk, especially in poorly ventilated environments.

"We do not need monitoring because we use small quantities"

Quantity is only one factor. A small quantity of a volatile solvent in an enclosed space can produce high airborne concentrations. Duration matters too — a bakery worker exposed to flour dust for 8 hours has a very different risk profile from someone who walks through the bakery for 5 minutes.

"General ventilation is always enough"

For some substances and situations, general ventilation (opening a window, running a fan) is adequate. For others — isocyanates, respirable silica dust, welding fume — it is not. The SDS and HSE guidance will specify when local exhaust ventilation or other engineering controls are required.

Making WELs Part of Your COSHH Process

WELs are a practical tool, not a bureaucratic hurdle. They give you a measurable standard to assess whether your controls are working. When you build WEL checks into your COSHH assessments, you move from subjective risk judgements to evidence-based decisions.

COSHHmate is being built to help you manage this. The guided assessment builder will prompt you to check for relevant WELs, record your exposure judgement, and flag when monitoring might be needed — so nothing gets overlooked.

If you want COSHH assessments that stand up to scrutiny, join the waitlist to be first to know when COSHHmate launches.