COSHH Assessment for Isocyanate Paints: A Practical Guide for UK Contractors
Isocyanate paints — most commonly two-pack polyurethane (2K) coatings used in vehicle refinishing, industrial finishing and some construction applications — are among the most hazardous substances a tradesperson can encounter. Isocyanates are the single largest cause of occupational asthma in the UK. If you use or supervise the use of these products, a COSHH assessment under the Control of Substances Hazardous to Health Regulations 2002 is a legal requirement before work begins.
Why Isocyanates Demand Particular Attention
Isocyanates (typically HDI, MDI or TDI-based hardeners mixed into 2K paints) cause sensitisation. Once a worker is sensitised, even trace exposure can trigger a severe, permanent asthmatic response. There is no cure and no safe return to isocyanate work. This is why the HSE treats isocyanate work as a priority enforcement area and why your COSHH assessment must be thorough — not a copy-and-paste exercise.
Step-by-Step: Completing the COSHH Assessment
1. Identify the Substance and Gather Information
Start with the Safety Data Sheet (SDS) for every product you are using — the base coat, the isocyanate hardener and any thinners. The SDS (supplied under REACH/CLP) will state the isocyanate content, the Workplace Exposure Limit (WEL) and recommended controls.
Key WELs for isocyanates (as NCO group, EH40):
| Exposure limit | Value |
|---|---|
| Long-term WEL (8-hr TWA) | 0.02 mg/m³ |
| Short-term WEL (15-min STEL) | 0.07 mg/m³ |
These are among the lowest WELs in EH40, reflecting how dangerous these substances are.
2. Determine Who Is at Risk
Consider not just the spray operator, but anyone in the vicinity — other tradespeople, supervisors, people in adjacent rooms if the space is not isolated. Isocyanate aerosol and vapour travel. Bystander exposure is a real enforcement concern.
3. Assess the Exposure
Think about the task: How long will spraying last? What is the ventilation in the space? Is it airless spray, HVLP or brush/roller application? Spraying generates the highest aerosol concentration; brush application is significantly lower risk, though not risk-free. Use the SDS, your own knowledge and, where appropriate, air monitoring results to judge whether exposure is likely to exceed the WEL.
4. Apply the Correct Control Hierarchy
Under COSHH reg 7, you must apply controls in this order — do not jump straight to PPE.
Eliminate: Could you specify a water-based or isocyanate-free coating instead? Genuinely consider this at the design or procurement stage.
Substitute: If 2K is required, can you use a product with a lower isocyanate content or slower-reacting isocyanate that generates less vapour?
Engineering controls (prioritise these):
- Spray booths with adequate extract ventilation are the standard control for vehicle refinishing — they are not optional extras.
- For site or industrial work away from a fixed booth, use local exhaust ventilation (LEV) designed and positioned to capture aerosol at source.
- Enclose the work area and segregate it from other workers.
- Use application methods that reduce aerosol generation (e.g., brush or roller where the finish specification allows).
Administrative controls:
- Restrict access to the spray area with clear signage.
- Limit duration of exposure; rotate tasks where possible.
- Schedule isocyanate work when fewer people are on site.
- Ensure written procedures are followed — this forms part of your method statement if you are producing a RAMS for the job.
PPE — last in the hierarchy, but mandatory for isocyanate spraying:
- Respiratory protective equipment (RPE) for spraying must be a supplied-air respirator (airline) or a powered air-purifying respirator (PAPR) with a suitable hood or visor — a half-mask with an organic vapour filter is not adequate for spray application of isocyanates. The HSE is explicit on this point.
- For brush application with adequate ventilation, a full-face or half-face mask with a combined P3/ABEK filter may be sufficient — check the SDS and your exposure assessment.
- Chemical-resistant gloves, coveralls and eye protection are also required.
- RPE must be fit-tested (tight-fitting facepieces) and maintained in accordance with HSG53.
Health Surveillance
COSHH reg 11 requires health surveillance where workers are exposed to a substance known to cause occupational asthma — isocyanates are explicitly in this category. In practice this means:
- Pre-employment respiratory questionnaire.
- Regular in-work questionnaires and, where appropriate, lung function (spirometry) tests.
- Records kept for 40 years.
- Any worker developing symptoms must be removed from isocyanate exposure immediately and referred to an occupational health professional.
Health surveillance cannot be skipped on grounds of cost or inconvenience.
Recording Your Assessment
COSHH assessments must be recorded if you employ five or more people, but best practice is to record them regardless. Your written assessment should cover: the substance and its hazards, who is exposed and how, the controls in place, RPE specification, health surveillance arrangements and a review date. If this work is part of a wider job requiring a RAMS, the COSHH assessment feeds directly into the risk assessment section.
Review and Revalidation
Review your COSHH assessment whenever the product, task, location or workforce changes — and at least annually for ongoing isocyanate work. An out-of-date assessment offers no legal or practical protection.
Isocyanate paints are legitimate, high-performance products — but they require a COSHH assessment that reflects the real severity of the hazard. Getting the engineering controls and RPE right protects your workers' long-term health and keeps your business on the right side of HSE enforcement.