Workplace Exposure Limits (WELs): COSHH Duties, Calculations and Compliance

If you’re responsible for managing hazardous substances at work in Great Britain, understanding workplace exposure limits is non-negotiable. These legally binding limits define the maximum concentration of dangerous substances permitted in workplace air, and getting them wrong can mean serious health problems for your workers—and serious legal consequences for your business.

Introduction to Workplace Exposure Limits

Workplace exposure limits (WELs) are UK legal limits for hazardous substances in workplace air, established under the Control of Substances Hazardous to Health Regulations 2002 (COSHH). They represent the maximum average concentration of a substance that a worker breathes over a set period during their working day.

The Health and Safety Executive sets and publishes WELs in the approved document EH40/2005, which is periodically updated to reflect new scientific evidence. The most recent consolidated edition lists limits for approximately 500 named substances.

WELs apply to gases, vapours, dusts, mists, fumes and fibres that are either used directly or generated as by-products during work activities. These limits are expressed as time weighted average concentrations—either over 8 hours (the long term exposure limit) or over a shorter period of 15 minutes (the short term exposure limit STEL).

Employers have a duty under COSHH regulations to keep exposure below the relevant WELs. However, the legal obligation goes further: you must reduce exposure to as low a level as is reasonably practicable, even if current levels are already within limits.

What Are Workplace Exposure Limits (WELs)?

A Workplace Exposure Limit is the legally defined maximum concentration of a hazardous substance in workplace air, averaged over a reference period. Think of it as a ceiling that must not be exceeded—but one that you should aim to stay well below.

Since Brexit, WELs are specific to Great Britain and operate independently from EU Occupational Exposure Limits (OELs) and Indicative Occupational Exposure Limit Values (IOELVs), though HSE still considers EU data when setting UK limits.

Key characteristics of WELs include:

• Scope: Approximately 500 named substances and groups are covered, including hardwood dust, respirable crystalline silica, isocyanates, and various welding fume components
• Units: Expressed in mg/m³ for particulates and some vapours, or ppm for gases and solvents, referenced to standard conditions (20°C and 101.3 kPa)
• Evidence basis: Set only where sufficient toxicological and epidemiological data exists to define a limit for inhalation exposure
• Notations: Many entries include special markers such as “Sk” (skin absorption risk), “Sen” (respiratory sensitiser), or “Carc” (carcinogen)

WELs Are Legal Limits – Not ‘Safe’ Levels

Here’s a critical point that many employers miss: a WEL is a statutory limit under COSHH, not a safe threshold limit value that you can comfortably work up to.

The legal limits exist as an upper boundary. Good control practice means controlling exposure well below this ceiling—guidance from BS EN 689:2018 suggests maintaining exposure at less than 10% of the WEL where practicable.

Important considerations:

• Legal maximum, not target: Treating a WEL as a goal to approach is the wrong mindset. Your aim should be adequate control at the lowest practicable level
• Not inherently safe: Some substances with WELs assigned are carcinogens, sensitisers, or reproductive toxins where no exposure level can be considered completely risk-free
• Duty extends beyond listed substances: Many hazardous substances have no WEL (process-generated fumes, proprietary mixtures, biological agents), yet COSHH still requires you to protect workers from exposure
• Enforcement reality: HSE has issued substantial fines for WEL breaches. One healthcare provider was fined £160,000 after chlorine gas exposure reached levels 8000 times the WEL, causing severe respiratory issues

How WELs Are Expressed: LTEL and STEL Time-Weighted Averages

Because exposure levels fluctuate throughout a shift—sometimes high during certain tasks, low or zero during others—WELs use time weighted average TWA calculations to reflect real-world conditions.

Exposure limits are typically given in two forms:

• Long-Term Exposure Limit (LTEL): The 8-hour time weighted average—the maximum average concentration permitted over a standard working day
• Short-Term Exposure Limit (STEL): The 15-minute time weighted average, specified for substances where brief peaks can cause acute adverse effects

When both an LTEL and STEL exist for a substance, the STEL always takes precedence for controlling short-term peaks. Even if your 8-hour average is compliant, exceeding the STEL during any 15-minute period is a breach.

For substances without a published STEL, HSE guidance often recommends treating 3× the LTEL as a provisional ceiling for short-term control, unless more specific guidance exists.

Long-Term Exposure Limit (LTEL)

The LTEL represents the 8-hour time weighted average concentration that workers may be exposed to on any working day without suffering chronic health effects from prolonged exposure.

This limit primarily protects against conditions that develop over time:

• Occupational asthma
• Chronic obstructive pulmonary disease (COPD)
• Occupational cancers
• Systemic toxicity affecting organs like the liver, kidneys, or nervous system

Current LTEL examples from EH40/2005:

Substance	LTEL (8-hour TWA)	Primary Health Concern
Respirable crystalline silica	0.1 mg/m³	Silicosis, lung cancer
Hardwood dust	3 mg/m³	Nasal cancer, respiratory sensitisation
Toluene	50 ppm	Neurological effects

For extended shifts (10 or 12 hours), exposure must still be referenced back to an 8-hour TWA for compliance—which typically means on-shift concentrations must be correspondingly lower.

Short-Term Exposure Limit (STEL)

The term exposure limit STEL is the maximum 15-minute average concentration permitted for substances where short bursts of exposure can cause immediate harm—irritation, narcosis, or acute toxic effects.

Current STEL examples:

Substance	STEL (15-minute TWA)	Acute Effect
Hydrogen sulphide	10 ppm	Respiratory paralysis, collapse
Carbon monoxide	200 ppm	Hypoxia, loss of consciousness
Nitrogen dioxide	1 ppm	Severe respiratory irritation
Ammonia	35 ppm	Eye and respiratory tract burns

The STEL applies to any 15-minute period during the shift—it’s not an average of all peak exposures. Exceedance requires urgent action even if the 8-hour LTEL remains compliant.

Examples of Substances with WELs and Typical Health Risks

WELs cover a broad range of industry-relevant chemicals, vapours, and dusts. Whether you’re in construction, manufacturing, laboratories, food production, or healthcare, you’ll likely encounter substances subject to these legal limits.

Common substances and their associated risks:

• Flour dust (bakeries, food processing): WEL 10 mg/m³ (8-hour TWA, total inhalable dust). Causes occupational asthma and rhinitis—flour dust is one of the most common causes of work-related respiratory disease in the UK
• Isocyanates (spray painting, foam production): Very low WELs (e.g., 0.02 mg NCO/m³ 8-hour TWA for some species). Potent respiratory sensitisers that can trigger severe occupational asthma after even brief exposures
• Welding fume components: Including manganese (neurotoxicity risk) and hexavalent chromium (carcinogen). Recent HSE guidance reinforced that all welding fume is now considered a potential carcinogen
• Carbon monoxide (boilers, engines, confined spaces): WEL 30 ppm (8-hour TWA), STEL 200 ppm. Causes hypoxia, headaches, confusion, and at high levels, rapid collapse and death
• Respirable crystalline silica (construction, stone cutting): WEL 0.1 mg/m³. Causes silicosis and lung cancer—exposure levels in uncontrolled cutting operations can exceed the WEL by 10-fold or more

WEL entries include notations that affect how you design control strategies. A “Sk” notation means the substance can be absorbed through skin contact injection or intact skin, requiring controls beyond just respiratory protection. A “Sen” notation indicates a respiratory sensitiser where even sub-WEL exposures may trigger sensitisation in susceptible individuals.

Calculating Exposure Against WELs

Employers must be able to compare measured concentrations against the relevant WEL using time-weighted average calculations. This means measuring or monitoring exposure during work activities and performing the correct arithmetic.

The general TWA formula:

TWA = (Σ (concentration × time)) ÷ reference time

Where:

• Concentration is measured in mg/m³ or ppm
• Time is in hours or minutes
• Reference time is 8 hours for LTEL or 15 minutes for STEL

In practice, a worker’s shift often involves several tasks at different exposure levels. Each period must be captured and weighted appropriately.

Simple 8-Hour TWA Example

Scenario: A worker is exposed to 0.2 mg/m³ of a dust continuously for an 8-hour shift.

Calculation:

• TWA = (0.2 mg/m³ × 8 hours) ÷ 8 hours
• TWA = 0.2 mg/m³

Interpretation:

• If the substance has an LTEL of 1.5 mg/m³, the exposure (0.2 mg/m³) is approximately 13% of the limit—compliant, and reasonably well controlled
• If the LTEL were 0.05 mg/m³, the same exposure would be 4× the limit—a breach requiring immediate control improvements

Partial-Shift Exposure Example

Scenario: A worker is exposed to 0.25 mg/m³ for 3 hours during a specific task, then has zero exposure for the remaining 5 hours.

Calculation:

• TWA = ((0.25 × 3) + (0 × 5)) ÷ 8
• TWA = (0.75 + 0) ÷ 8
• TWA = 0.094 mg/m³

Key insight: Low-exposure or zero-exposure periods count towards the 8-hour average and can bring overall exposure within the WEL. However, this doesn’t eliminate the need to review peak task exposures against any applicable STEL.

Extended Shift and STEL Priority Example

Extended shift scenario: A worker on a 10-hour shift is exposed to 10 mg/m³ throughout.

Calculation:

• 8-hour TWA = (10 mg/m³ × 10 hours) ÷ 8 hours
• 8-hour TWA = 12.5 mg/m³

If the LTEL is 10 mg/m³, this exposure exceeds the limit—despite the on-shift concentration appearing to equal the limit value. Extended shifts require adjusted calculations or reduced on-shift concentrations.

STEL scenario: During a 15-minute cleaning task, average exposure reaches 45 mg/m³. The STEL for the substance is 40 mg/m³.

Outcome: This is a breach requiring corrective action—regardless of whether the 8-hour LTEL is met. STEL exceedances indicate immediate risk of acute effects and must be addressed urgently.

Monitoring and Assessing Exposure Against WELs

COSHH Regulation 10 requires exposure monitoring in specific situations. This isn’t optional—monitoring exposure becomes mandatory when:

• Working with certain governing bodies’ listed carcinogens or biological agents
• Handling respiratory sensitisers
• Exposure levels could foreseeably approach or exceed WELs
• Controls depend on correct functioning of engineering measures

Monitoring approaches:

• Personal sampling: Pumps with sampling heads positioned in the worker’s breathing zone—the gold standard for assessing what the worker breathes
• Static sampling: Fixed-position samplers that measure general workplace air concentrations—useful for identifying emission sources but less representative of personal exposure
• Real-time instruments: Direct-reading devices like photoionisation detectors (PIDs) or electrochemical sensors for gases—provide immediate feedback but may require validation against laboratory methods

Practical guidance:

• Design monitoring programmes with competent persons (occupational hygienists) using recognised standards like HSG173 and BS EN 689:2018
• Compare results directly with LTEL and STEL values from the current EH40/2005
• Repeat monitoring at intervals based on risk level and process stability—high-risk processes may need quarterly or more frequent assessment

Identifying Hazardous Substances and Relevant WELs

Before you can monitor, you need a complete picture of what substances hazardous to health are present. This means measuring isn’t the first step—identification is.

Step-by-step identification process:

1. Inventory all substances: Include purchased chemicals, products, and process-generated materials (fumes from welding, dusts from cutting, vapours from cleaning)
2. Obtain Safety Data Sheets: Check Section 8 for exposure limits. SDSs from suppliers should list the UK WEL where one exists
3. Cross-reference EH40/2005: Verify substance names and CAS numbers against the latest published list. SDSs can be outdated
4. Note special hazard classifications: Identify carcinogens (Carc), sensitisers (Sen), and substances with skin absorption risk (Sk)
5. Assess mixtures: Products containing multiple components may have several WELs to consider—for example, a solvent blend with three different substances each having distinct limits

Interpreting Monitoring Results

Once you have sampling results, comparing them against WELs requires more than simple arithmetic. You need to understand what the numbers mean for compliance and worker health.

Interpretation framework:

Result vs WEL	Status	Required Action
<10% of WEL	Good control	Maintain current measures; continue good practice
10-50% of WEL	Adequate	Monitor regularly; improve where reasonably practicable
50-100% of WEL	Marginal	Review controls urgently; increase monitoring frequency
≥100% of WEL	Exceeded	Non-compliant; immediate corrective action required

Additional considerations:

• Measurement uncertainty: Laboratory results have confidence intervals. A result of 95% WEL may actually exceed the limit when uncertainty is factored in
• Worst-case conditions: Were samples taken during normal operations or during unusual high-exposure events? Both matter for comprehensive assessment
• Multiple substances: Where workers are exposed to several substances affecting the same target organ, additive effects may apply—consult HSE guidance on mixed exposures

Controlling Exposure: COSHH Duties Beyond Just Meeting WELs

Meeting the WEL is the minimum legal requirement—not the end of your duties. COSHH establishes a clear hierarchy of control that employers must follow to protect workers health.

The COSHH control hierarchy:

1. Prevent exposure: Eliminate the substance or substitute with something less hazardous
2. Control exposure: Where prevention isn’t practicable, use engineering and organisational controls
3. PPE as last resort: Respiratory and personal protective equipment supplements other controls but should never be the primary defence

Practical control measures by type:

• Substitution: Replace high-vapour-pressure solvents with water-based alternatives; use pre-mixed materials instead of powder forms
• Engineering controls: Local exhaust ventilation (LEV) for welding, soldering, flour handling; enclosed systems for isocyanate spraying and chemical transfers
• Administrative controls: Job rotation to limit individual exposure duration; restricted access to high-exposure areas
• PPE/RPE: Appropriate respiratory protective equipment selected for the substance and concentration; fit-tested and maintained

Validation requirements:

Control strategies must be proven to work. LEV systems require thorough examination and test (TExT) at least every 14 months—often more frequently for high-risk processes. This isn’t just advice; it’s a legal requirement under COSHH Regulation 9.

Ongoing Review, Health Surveillance and Record-Keeping

Reducing exposure isn’t a one-off exercise. COSHH demands ongoing vigilance through regular review, health surveillance where appropriate, and comprehensive record-keeping.

When to review COSHH assessments:

• Processes, substances, or workloads change
• New scientific evidence emerges
• HSE publishes updated WELs
• Monitoring reveals exposure levels approaching limits
• Health surveillance identifies concerning trends

Health surveillance requirements:

Health regulations under COSHH require surveillance where there’s reasonable likelihood of disease developing. This includes:

• Lung function testing for workers exposed to flour, isocyanates, RCS, or welding fumes
• Skin checks for those handling known skin sensitisers
• Biological monitoring for substances with established biological exposure indices

Record retention:

Record Type	Minimum Retention Period
Risk assessments	Duration of work + reasonable period
Monitoring results (general)	5 years minimum
Monitoring results (carcinogens/sensitisers)	40 years
Health surveillance records	40 years from last entry
LEV examination reports	5 years

These records demonstrate compliance and provide crucial evidence for epidemiological follow-up if workers develop occupational disease years later.

Staying Up to Date with Workplace Exposure Limits

WELs are not static. They evolve as new toxicological data emerges, as certain governing bodies issue updated guidance, and as HSE responds to emerging health evidence.

Recent update history:

HSE has historically updated WELs through revised editions or supplements to EH40/2005. The August 2018 changes, for example, introduced new or revised limits for around 31 chemicals following transposition of EU Directive requirements.

Practical steps for staying current:

• Check the latest EH40: Always download the current version from the HSE website rather than relying on older PDFs that may be superseded
• Review supplier SDSs regularly: Suppliers should update Section 8 when limits change, but verify against official sources
• Update COSHH assessments promptly: When a WEL is tightened (lower limit) or newly introduced, existing controls may become inadequate
• Monitor HSE communications: Subscribe to HSE bulletins and review relevant British or European standards updates

Building a compliance routine:

Establish an annual review cycle that includes:

• Scanning for regulatory updates from the safety executive
• Verifying current EH40 against your substance inventory
• Reviewing monitoring programme adequacy
• Updating safe handling procedures and training materials

Understanding workplace exposure limits WELs and maintaining compliance is an ongoing responsibility. The substances you work with, the limits wels that apply, and the control strategies you deploy will all need periodic reassessment.

Key takeaways:

• WELs are legal limits set by the Health and Safety Executive—not safety thresholds to aim for
• Exposure must be controlled to as low a level as reasonably practicable, not just below the limit
• Both LTEL (8-hour) and STEL (15-minute) limits must be met where both exist
• Monitoring exposure using appropriate sampling methods is essential for demonstrating compliance
• Control measures follow a hierarchy: eliminate, substitute, engineer, administer, then protect with PPE
• Records must be maintained—up to 40 years for carcinogens and sensitisers

Your next step: review your current COSHH assessments against the latest EH40/2005 and confirm that your monitoring and control programmes reflect current legal requirements. If you’re unsure whether your exposure levels are compliant, engage a competent occupational hygienist to conduct a thorough assessment.