Exceeding a Workplace Exposure Limit (WEL) is a breach of COSHH Regulations 2002, and the consequences are serious. From unlimited fines, prohibition notices halting operations immediately, and up to two years' imprisonment for individuals found liable (HSE, 2026). For most hazardous substances, two limits apply at the same time: the Time-Weighted Average (TWA) and the Short-Term Exposure Limit (STEL). Staying within one does not mean you are within the other.
This article explains what each limit means, how they differ, and what UK employers need to do to comply with both.
Key Takeaways
- The TWA is the maximum average concentration of a hazardous substance permitted over an eight-hour working day.
- The STEL is the maximum concentration permitted during any single 15-minute period within a shift.
- Both limits are set by the HSE and published in EH40/2005 Workplace Exposure Limits.
- Breaching either limit is a criminal offence under COSHH Regulations 2002, regardless of compliance with the other.
- Around 500 substances currently have WELs listed in EH40 (HSE, 2026).
- Monitoring must cover both the full shift average and specific high-exposure tasks separately.
- Records of exposure monitoring must be kept for a minimum of five years, or 40 years for substances linked to serious disease.
What Are Occupational Exposure Limits?
Occupational exposure limits are the maximum concentrations of hazardous substances in workplace air that workers can be exposed to without unacceptable risk to health. In the UK, these are called Workplace Exposure Limits (WELs) and are set by the Health and Safety Executive (HSE) and published in EH40/2005 Workplace Exposure Limits (Safety Clarity, 2025).
WELs are legally binding under COSHH Regulations 2002. Exceeding them does not create a compliance gap that can be noted and corrected at the next review. It is a criminal offence. The HSE enforces WELs through air monitoring, workplace inspections, and enforcement notices, and employers are expected to reduce exposure as far below the limit as is reasonably practicable, not simply aim to stay just under it (Sevron, 2026).
Around 500 substances currently have WELs listed in EH40, although UK workplaces use many thousands of other substances. For substances without a WEL, employers must still assess the risk and implement appropriate controls under COSHH (HSE, 2026).
What Is a TWA?
The TWA, or Time-Weighted Average, is the average concentration of a hazardous substance measured over an eight-hour working day. It is designed to protect workers from the long-term, chronic health effects of repeated daily exposure to harmful substances over months and years, including conditions like occupational asthma, lung disease, and chemical sensitisation (Cambridge Sensotec, 2026).
The TWA allows for natural variation during a shift. A worker might face a brief period of higher exposure during a specific task and a long stretch of lower exposure during other parts of the day. As long as the average across the full eight hours stays within the TWA limit, the exposure is considered controlled. What matters is the cumulative daily dose, not any individual moment within it.
TWA limits are expressed in parts per million (ppm) or in milligrams per cubic metre (mg/m³). For example, under EH40 the TWA for aluminium dust is set at 10 mg/m³ and for wood dust (hardwood) at 1 mg/m³ over eight hours (Sevron, 2026).
What Is a STEL?
The STEL, or Short-Term Exposure Limit, is the maximum concentration of a substance allowed over any 15-minute period during a shift. It exists to protect workers from the immediate, acute effects of short bursts of high exposure: eye irritation, dizziness, respiratory damage, or worse, effects that can occur within minutes even when the eight-hour TWA remains well within its limit (EH40/2005, HSE).
A STEL applies regardless of how low the rest of the day's exposure is. If a worker is exposed to a concentration above the STEL during a 15-minute task, that is a breach of the WEL, even if the daily average comes in comfortably under the TWA. The two limits operate independently and must both be complied with at all times (Cambridge Sensotec, 2026).
For substances where a STEL is assigned but no eight-hour TWA limit is specified, total peak exposures above the eight-hour TWA value should be limited to one hour in any 24-hour period (HSE EH40/2005).
What Is the Difference Between STEL and TWA?
The two limits protect against different types of harm and operate independently of each other.
STEL vs TWA: At a Glance
| Feature | TWA | STEL |
|---|---|---|
| Time period | 8-hour shift average | Any 15-minute window |
| Type of harm prevented | Long-term chronic effects (e.g. lung disease, sensitisation) | Acute immediate effects (e.g. dizziness, respiratory damage) |
| Applies when | Across the full working day | During any high-exposure task |
| Expressed as | ppm or mg/m³ | ppm or mg/m³ |
| Compliance requirement | Must stay within the daily average | Must stay within the peak at all times |
| Monitoring method | Full-shift personal air sampling | Task-specific sampling during peak-exposure periods |
Both limits apply simultaneously to any substance that carries both limits. A worker can be well within the daily TWA for a solvent and still breach the STEL during a 15-minute cleaning task in a confined space. That is a compliance failure regardless of how the rest of the shift looks.
What Happens When a WEL Is Exceeded?
Exceeding a WEL is a breach of the COSHH Regulations 2002. The HSE can:
- Issue an improvement notice, requiring corrective action within a set timeframe.
- Issue a prohibition notice, halting operations immediately where there is a risk to health.
- Pursue criminal prosecution, resulting in unlimited fines and up to two years' imprisonment for individuals found liable (Rockall Safety, 2025).
Company directors and managers can be held personally liable where negligence is established. Beyond regulatory penalties, workers exposed above WEL limits face real health consequences: occupational lung disease, chemical sensitisation, neurological damage, and, in serious cases, conditions that are irreversible.
The HSE has tightened limits and increased enforcement activity recently. WEL compliance is a higher operational priority now than it was five years ago (Al Salama Safety, 2026).
How Do Employers Comply With Both Limits in Practice?
Under the COSHH Regulations 2002, employers must prevent or control exposure to hazardous substances and keep it below the WELs set out in EH40. This obligation applies to both the TWA and the STEL for every substance that has both limits assigned.
COSHH Compliance Checklist
Step 1: Identify every hazardous substance used or produced in the workplace.
Step 2: Refer to EH40 for the applicable TWA and STEL values for each substance.
Step 3: Identify who is exposed, during which tasks, and for how long.
Step 4: Apply the hierarchy of controls: elimination, substitution, engineering controls, administrative controls, PPE.
Step 5: Carry out personal air sampling across full shifts and during high-exposure tasks.
Step 6: Keep records for a minimum of five years, or 40 years for substances linked to serious disease.
Step 7: Review controls frequently enough to detect problems before they cause harm.
If a substance's SDS does not clearly state UK WEL values, please check EH40 directly. Not all suppliers provide UK-specific exposure limit information, particularly for imported products (Safety Clarity, 2025).
Keeping your SDS library current means the hazard and exposure limit data your team relies on for COSHH assessments always reflects the latest supplier information.
Final Thoughts
STEL and TWA each address a distinct exposure risk. The TWA governs long-term cumulative doses. The STEL caps what is permissible during any single peak-exposure task. Both are legally binding and must be met at all times.
Monitoring that covers only the full-shift average misses the STEL risk entirely. Monitoring only during tasks misses the broader daily picture. A complete exposure programme accounts for both, with records kept and controls reviewed on a schedule that catches problems before they become enforcement issues.
Frequently Asked Questions
What is the difference between STEL and TWA?
The TWA is the average exposure over an eight-hour shift, designed to prevent long-term chronic health effects. The STEL is the maximum exposure permitted over any 15-minute period, designed to prevent acute effects from short bursts of high concentration. Both can apply to the same substance and must be complied with independently.
Where can I find STEL and TWA values for specific substances?
The HSE publishes WEL values for both STEL and TWA in EH40/2005 Workplace Exposure Limits, which is available on the HSE website. The SDS for a substance should also list applicable WEL values, though not all suppliers provide UK-specific limits.
Do I need to comply with both STEL and TWA?
Yes. Where both limits apply to a substance, compliance with one does not mean compliance with the other. A worker can be within the daily TWA and still breach the STEL during a high-intensity task.
What happens if exposure exceeds a WEL?
Exceeding a WEL is a breach of COSHH Regulations 2002 and can result in HSE enforcement action, unlimited fines, prohibition notices, and criminal prosecution. Employers must act immediately to bring exposure back within limits and review their control measures.
Mehreen Iqbal
Started with a Bachelors in Microbiology, then a Masters in Public Health; Currently a Workplace Safety Expert.