Workplace Exposure Limits (WELs) Explained

Workplace Exposure Limits (WELs) are the maximum concentration of hazardous substances in air that workers can be exposed to without unacceptable risk to health. They're set by the Health and Safety Executive and published in EH40. If you use substances with WELs, you must ensure exposure stays below these limits through effective control measures and monitoring.

What are Workplace Exposure Limits?

Workplace Exposure Limits (WELs) are legally binding maximum concentrations of hazardous substances in workplace air, measured in parts per million (ppm) or milligrams per cubic meter (mg/m³). They represent the airborne concentration at which it's believed nearly all workers can be repeatedly exposed without adverse health effects.

WELs are set for substances where there's sufficient evidence of health effects related to airborne exposure, including:

• Chemicals and solvents (acetone, toluene, xylene)
• Dusts (wood dust, flour dust, silica)
• Metals and metal compounds (lead, chromium, nickel)
• Gases (carbon monoxide, hydrogen sulfide, ammonia)
• Welding and hot process fumes
• Pesticides and herbicides

Types of Workplace Exposure Limits

The UK uses two types of exposure limits to control both sustained and short-term exposures:

Long-term exposure limit (8-hour TWA)

The 8-hour Time-Weighted Average is the maximum average exposure over an 8-hour working day. It accounts for variations during the shift—periods of high and low exposure are averaged out.

How it works:

• Exposure is measured or estimated at different times during the shift
• Higher exposures are permitted for short periods if balanced by lower exposures at other times
• The weighted average must not exceed the WEL

Example: If a substance has an 8-hour TWA of 100 ppm:

• You could have 2 hours at 200 ppm and 6 hours at 67 ppm (average = 100 ppm) ✓
• You cannot have continuous exposure at 120 ppm (exceeds limit) ✗

Short-term exposure limit (15-minute reference period)

The 15-minute STEL is the maximum average exposure over any 15-minute period during the working day. It prevents acute health effects from short, intense exposures.

Why STELs matter:

• Some substances cause immediate harm at high concentrations (irritation, dizziness, unconsciousness)
• Brief exposures above the 8-hour TWA might seem acceptable when averaged out, but could be dangerous in the moment
• STELs protect against peak exposures that occur during specific tasks

Restrictions on STELs:

• Should not be exceeded at any time during the day
• Exposure at the STEL should not occur for more than 15 minutes at a time
• Should not be repeated more than 4 times per day
• At least 60 minutes should elapse between successive exposures at the STEL

How to find Workplace Exposure Limits

EH40: The official source

The Health and Safety Executive publishes EH40/2005 Workplace Exposure Limits, updated annually. It lists:

• All substances with assigned WELs in the UK
• 8-hour TWA limits (where applicable)
• 15-minute STELs (where applicable)
• Additional notes on carcinogens, sensitisers, and skin absorption

How to access EH40:

• Download free from the HSE website
• Search for specific substances by name or CAS number
• Check the notes column for special requirements

Safety Data Sheets

Section 8 of every Safety Data Sheet must state if a substance or its components have Workplace Exposure Limits. Look for:

• UK WEL — The limit that applies in the UK (from EH40)
• EU OEL — European occupational exposure limits (may differ from UK)
• Other national limits — US OSHA PEL, ACGIH TLV (for reference)

The SDS should clearly state the limit value and whether it's an 8-hour TWA, 15-minute STEL, or both.

If the SDS doesn't mention WELs: Check EH40 yourself. Not all suppliers provide UK-specific information, especially for imported products. Some substances may have WELs for individual components even if the product as a whole doesn't.

Substance-specific guidance

For common hazardous substances, the HSE publishes specific guidance:

• COSHH essentials sheets (substance-specific control advice)
• Guidance notes (detailed technical information)
• Industry-specific codes of practice

Your duties when WELs apply

Under the COSHH Regulations 2002, if a substance has a Workplace Exposure Limit you must:

1. Prevent exposure or reduce it below the WEL

Implement control measures to ensure worker exposure does not exceed the WEL. Follow the hierarchy of controls:

• Eliminate — Stop using the substance if possible
• Substitute — Use a safer alternative
• Engineering controls — Enclose the process, install local exhaust ventilation
• Administrative controls — Reduce exposure time, rotate workers
• PPE — Respiratory protection as a last resort or supplementary measure

2. Monitor exposure to ensure compliance

Where there's a WEL, you generally need to measure actual workplace exposure to prove you're complying. Monitoring should be:

• Representative — Reflects typical working conditions
• Personal or static — Measures what workers actually breathe
• Conducted by a competent person — Usually an occupational hygienist
• Compared to the WEL — Results assessed against limits in EH40
• Used to improve controls — Not just a paper exercise

3. Keep records

Exposure monitoring records must be kept for:

• At least 5 years for most substances
• At least 40 years for substances that can cause serious diseases with long latency (carcinogens, mutagens, asthmagens)

Records should include:

• Date and duration of monitoring
• Name of substance monitored
• Work process involved
• Results of measurements
• Comparison to WEL
• Names or identification of employees monitored
• Name of person conducting monitoring

4. Take action if the WEL is exceeded

If monitoring shows the WEL is being exceeded:

• Stop the work immediately if there's serious risk to health
• Identify why the limit was exceeded (control failure, process change, incorrect use)
• Improve or repair controls before work resumes
• Conduct repeat monitoring to verify the problem is resolved
• Inform affected employees of the exceedance and actions taken
• Provide health surveillance if appropriate

When is air monitoring required?

Not all work with substances requires formal air monitoring, but if a WEL applies, you should strongly consider it. Monitoring is particularly important when:

Situations requiring monitoring:

• A WEL exists and you're not certain exposure is well controlled
• Exposure levels are variable (different tasks, times, or conditions)
• High quantities of substance are used
• Control measures might fail (ventilation breakdowns, equipment wear)
• Work methods change frequently or are difficult to control
• Employees report symptoms (headaches, dizziness, irritation)
• You're working close to the WEL (little margin for error)
• Initial verification after installing new controls

When monitoring may not be needed:

• Exposure is obviously negligible (very small quantities, infrequent use, enclosed process)
• Robust, well-maintained controls are in place with clear evidence they work
• Extensive industry data exists showing similar processes achieve low exposure
• You've used a validated control approach (e.g., COSHH Essentials guidance)

How air monitoring is conducted

Workplace air monitoring is a technical process that should be carried out by a competent person—typically an occupational hygienist accredited by the Faculty of Occupational Hygiene.

Types of monitoring:

Personal monitoring:

• Air sampling pump worn by the worker
• Sampler positioned in the breathing zone (near nose/mouth)
• Measures actual exposure during normal work
• Most accurate for assessing individual risk

Static monitoring:

• Fixed air sampler at specific locations
• Useful for checking general area concentrations
• Less accurate for individual exposure
• Can help identify problem areas or sources

Real-time monitoring:

• Electronic monitors providing instant readings
• Useful for identifying peak exposures
• Some substances can't be measured this way
• May be less accurate than laboratory analysis

The monitoring process:

1. Planning — Identify what to measure, when, and who to monitor
2. Sampling — Collect air samples during representative work
3. Analysis — Laboratory analysis of samples (for most substances)
4. Calculation — Determine 8-hour TWA and compare to WEL
5. Reporting — Document findings with recommendations
6. Action — Implement any necessary control improvements

Carcinogens and the "ALARP" principle

For substances that cause cancer, genetic damage, asthma, or reproductive harm, meeting the WEL is not enough. You must reduce exposure to as low as reasonably practicable (ALARP), even if it's already below the limit.

Why ALARP matters:

• No safe threshold exists for carcinogens—any exposure carries some risk
• Cumulative effect — Lower exposures over longer periods still increase risk
• Precautionary principle — We may not fully understand all health effects yet

Applying ALARP in practice:

1. Implement the highest level of control feasible:

   • Full enclosure rather than partial
   • On-tool extraction rather than general ventilation
   • Automation rather than manual handling

2. Continuously improve controls:

   • Don't accept "good enough" if better is possible
   • Review new technology and methods regularly
   • Benchmark against best practice in your industry

3. Demonstrate what you've considered:

   • Document alternatives considered
   • Explain why certain controls weren't adopted (cost, practicality, effectiveness)
   • Show that you've balanced risk reduction against feasibility

Substances with "Skin" notation

Some substances in EH40 have a "Skin" notation next to the WEL. This means the substance can be absorbed through intact skin in significant amounts, potentially contributing to overall exposure and health risk.

What "Skin" means for you:

• Inhalation exposure alone doesn't tell the full story — Skin exposure adds to the dose
• Air monitoring may underestimate total exposure — You're also absorbing through skin contact
• Dermal (skin) exposure must be prevented or minimized
• Appropriate gloves and protective clothing are essential
• Good hygiene practices are critical (washing, no eating/drinking in work areas)

Examples of substances with "Skin" notation:

• Many solvents (toluene, xylene, acetone)
• Pesticides and herbicides
• Phenol and derivatives
• Aniline and related compounds

Understanding the numbers in EH40

WELs can seem confusing because they're expressed in different units. Here's how to read them:

Common units:

Parts per million (ppm):

• Used for gases and vapours
• Represents volume concentration
• Example: "50 ppm" means 50 parts of substance per million parts of air

Milligrams per cubic meter (mg/m³):

• Used for dusts, fumes, mists, and some vapours
• Represents mass concentration
• Example: "10 mg/m³" means 10 milligrams of substance per cubic meter of air

Dust fraction specifications:

For dusts, the WEL may specify:

Inhalable dust:

• Larger particles that can be breathed into the nose and mouth
• Includes particles that deposit in the upper respiratory tract
• Less stringent limit (higher number)

Respirable dust:

• Smaller particles that reach deep into the lungs
• Greater health concern (can cause silicosis, pneumoconiosis)
• More stringent limit (lower number)

Example: Hardwood dust

• Inhalable WEL: 3 mg/m³ (8-hour TWA)
• Respirable WEL: 1 mg/m³ (8-hour TWA) Both must be complied with.

Record keeping requirements

Proper record keeping is a legal requirement when conducting exposure monitoring:

What to record:

For every monitoring exercise:

• Date and time of monitoring
• Name and CAS number of substance
• Description of work process being monitored
• Location of monitoring
• Type of monitoring (personal/static)
• Duration of monitoring
• Sampling and analytical methods used
• Results (concentration measured)
• Calculation of 8-hour TWA
• Comparison to the WEL
• Assessment of whether exposure is controlled
• Recommendations for action
• Name and qualifications of person conducting monitoring

For personal monitoring:

• Names or identification codes of employees monitored
• Job roles and tasks performed during monitoring

Following exceedances:

• Immediate actions taken
• Investigation findings
• Control improvements implemented
• Verification monitoring results

Retention periods:

• Minimum 5 years for most substances
• Minimum 40 years for:
  • Carcinogens (substances that cause cancer)
  • Mutagens (substances that cause genetic damage)
  • Respiratory sensitisers (substances that cause asthma)
  • Any substance that can cause serious disease with long latency

Respiratory Protective Equipment and WELs

When engineering controls cannot reduce exposure below the WEL, Respiratory Protective Equipment (RPE) may be necessary as an additional or interim measure. However, PPE should never be the first or only control.

Selecting appropriate RPE:

The level of protection needed depends on the degree of exceedance:

Assigned Protection Factor (APF): The APF indicates how much the RPE reduces exposure. For example:

• APF 10 = Reduces exposure by a factor of 10
• APF 20 = Reduces exposure by a factor of 20
• APF 40 = Reduces exposure by a factor of 40

Calculation: If measured exposure is 100 mg/m³ and the WEL is 10 mg/m³:

• Exposure is 10 times the WEL
• You need RPE with APF of at least 10

RPE is not a substitute for control:

• Use only as additional measure or when engineering controls aren't feasible
• For carcinogens, RPE alone is never adequate—you must control at source
• Maintain engineering controls even when using RPE
• Provide training on correct use, limitations, and maintenance
• Conduct fit testing annually for tight-fitting respirators
• Replace filters according to manufacturer's schedule or more frequently if needed

Common mistakes with WELs

Avoid these frequent errors:

1. Assuming no WEL means no risk

Many hazardous substances don't have assigned WELs, either because:

• They haven't been studied sufficiently
• Evidence is insufficient to set a specific limit
• They're controlled by other means

You must still assess and control exposure, even without a WEL. Follow COSHH hierarchy of controls and aim for exposure as low as reasonably practicable.

2. Using WELs from other countries

US OSHA PELs, ACGIH TLVs, or EU OELs may differ from UK WELs. Always use the UK limits from EH40 for compliance purposes. Other countries' limits can be informative but aren't legally binding in the UK.

3. Not considering the "Skin" notation

Air monitoring alone doesn't capture skin absorption. For substances with "Skin" notation, ensure comprehensive skin protection and good hygiene practices.

4. Averaging exposures across different workers

WELs apply to individual workers, not team averages. If one worker is exposed above the WEL but others aren't, you're still in breach. Monitor highest-exposed workers.

5. Treating the WEL as a target

The WEL is a maximum limit, not a safe target. Always aim to reduce exposure as low as reasonably practicable, especially for carcinogens, mutagens, and asthmagens.

6. Not monitoring after changes

Any change to the process, substance quantity, work method, or control measures can affect exposure. Always conduct verification monitoring after changes.

Frequently asked questions

Next steps

To understand COSHH assessment requirements in detail:

COSHH Assessment Requirements →

Learn how to read Safety Data Sheets for exposure information:

Safety Data Sheets: How to Use Them →

Understand how to store hazardous substances safely:

Safe Storage of Hazardous Substances →

Use our COSHH checker to identify your obligations:

COSHH Checker Tool →

Related articles:

• What is a COSHH Assessment?
• What is COSHH?
• Safety Data Sheets (SDS)

Useful tools:

• COSHH Checker
• Risk Assessment Tool