Statutory Inspections · Mandatory · Pre-Construction
COSHH Assessment — Isocyanates / Spray Paint
A COSHH assessment for isocyanate-containing products including polyurethane spray paints, spray foam insulation, PU adhesives, and PU coatings. Isocyanates are powerful respiratory sensitisers — once sensitised, any future exposure triggers life-threatening occupational asthma. Air-fed RPE is mandatory for spraying operations. Annual spirometry health surveillance required. WELs: MDI 0.02 mg/m³ TWA; TDI 0.008 mg/m³ TWA.
Last reviewed: 2 April 2026 — This guide reflects UK law as of this date. COSHH Regulations 2002 remains current with no amendments enacted as of 2 April 2026. Next scheduled review: 2 April 2027.
| Document type | T6 — COSHH Assessment: Isocyanates / Spray Paint |
| Primary legislation | COSHH Regulations 2002, EH40 Workplace Exposure Limits |
| Where found | Polyurethane (PU) spray paints, spray foam insulation, PU adhesives and coatings, two-pack lacquers, PU sealants |
| Key hazard | Powerful respiratory sensitisers — once sensitised, any subsequent exposure (even trace amounts) can trigger life-threatening occupational asthma |
| WELs | MDI (methylene diphenyl diisocyanate): 0.02 mg/m³ TWA (8-hour). TDI (toluene diisocyanate): 0.008 mg/m³ TWA (8-hour) |
| RPE requirement | Air-fed RPE is MANDATORY for all spraying operations involving isocyanates. Filter-based RPE is NOT adequate for spraying |
| Health surveillance | Annual spirometry and respiratory questionnaire required for all exposed workers |
1. Why Isocyanates Require a Dedicated COSHH Assessment
Isocyanates are a family of highly reactive chemical compounds used as hardeners and cross-linkers in polyurethane products. On construction sites, they are encountered primarily in two-pack polyurethane spray paints, spray-applied foam insulation, PU adhesives, PU floor coatings, and two-pack lacquers. The defining hazard of isocyanates is respiratory sensitisation: they are among the most potent respiratory sensitisers encountered in any workplace. Once a worker becomes sensitised to isocyanates — a process that may take weeks, months or years of exposure — any subsequent exposure, even at concentrations far below the workplace exposure limit, can trigger severe occupational asthma that may be life-threatening.
Sensitisation is irreversible. There is no treatment that can desensitise a worker once the immune response has been established. A sensitised worker must be permanently removed from all isocyanate exposure, which in practice often means the end of their career in any trade that uses PU products. The HSE estimates that isocyanates are the second most common cause of occupational asthma in the UK, after flour dust. Construction workers — particularly spray painters, insulation installers, and flooring contractors — are among the most affected occupational groups.
The workplace exposure limits for isocyanates are extremely low, reflecting their potency. MDI has a WEL of 0.02 mg/m³ (8-hour TWA), and TDI has a WEL of 0.008 mg/m³ (8-hour TWA). During spraying operations, airborne isocyanate concentrations can exceed these limits by orders of magnitude within seconds if controls are inadequate. This is not a marginal hazard — it is an acute and serious risk that requires rigorous, documented controls and a dedicated COSHH assessment that addresses the specific exposure scenarios, control measures, health surveillance requirements, and emergency procedures for isocyanate-containing products.
Air-fed RPE is mandatory for spraying isocyanates
Filter-based respiratory protective equipment — including FFP3 disposable masks and half-mask respirators with organic vapour cartridges — is NOT adequate for spraying isocyanates. Isocyanate aerosol generated during spraying includes both particulate and vapour phases. Filter cartridges cannot provide reliable protection against the vapour phase, and their capacity is rapidly exhausted at the concentrations generated during spraying. Air-fed RPE (constant-flow airline breathing apparatus or positive-pressure full-face masks with an independent air supply) is the only acceptable form of respiratory protection for isocyanate spraying operations. This is not guidance — it is a mandatory requirement enforced by the HSE.
Isocyanates also present a significant skin hazard. Skin contact with uncured isocyanate products can cause both irritant contact dermatitis and skin sensitisation. Critically, skin absorption of isocyanates can contribute to respiratory sensitisation — meaning that a worker who repeatedly gets isocyanate products on their skin may develop occupational asthma even if their respiratory exposure is controlled. This dual exposure route makes skin protection an essential component of the COSHH assessment, not an afterthought.
2. Key Content of the COSHH Assessment
The COSHH assessment for isocyanates must address each stage of the exposure scenario in detail. The following table sets out the key content areas and the information that must be recorded in each.
| Section | Content required |
|---|---|
| Product(s) assessed | Full product name, manufacturer, SDS reference number, isocyanate type (MDI, TDI, HDI, IPDI), concentration of isocyanate monomer and prepolymer. Every product containing isocyanates must be individually identified — generic assessments are not acceptable. |
| Task description | Specific task being assessed: spray painting, foam application, adhesive application, coating application, mixing, decanting. Include duration, frequency, location (indoor/outdoor/confined space), and whether the task is continuous or intermittent. |
| Persons exposed | Spray operatives, assistants, other workers in the vicinity, passers-by. Identify all persons who may be exposed, not just those directly handling the product. Include numbers and proximity to the spray operation. |
| Exposure assessment | Routes of exposure (inhalation of aerosol/vapour, skin contact, eye contact). Comparison of expected airborne concentrations against WELs. Reference to any air monitoring data. Assessment of whether exposure is adequately controlled. |
| Health effects | Respiratory sensitisation and occupational asthma (irreversible once established). Irritation of eyes, nose, throat and lungs. Skin sensitisation and dermatitis. Hypersensitivity pneumonitis. Note that sensitisation can occur at concentrations below the WEL. |
| Controls — Elimination | Can the isocyanate-containing product be eliminated? Can a non-isocyanate alternative be specified? Water-based paints, acrylic coatings, or non-PU adhesives may be viable substitutes for some applications. |
| Controls — Enclosure / LEV | Spray booths with dedicated extraction for paint spraying. Enclosed application systems where practicable. Local exhaust ventilation positioned to capture overspray and vapour at source. LEV must be examined and tested every 14 months under COSHH Reg 9. |
| Controls — Exclusion zone | Establish and enforce an exclusion zone around the spray area. No unprotected persons within the zone. Signage, barriers, and communication with other trades. Minimum exclusion distances based on product SDS and site conditions. |
| Controls — RPE | Air-fed RPE mandatory for spraying. Constant-flow airline apparatus or positive-pressure full-face mask with independent air supply. Air quality must meet BS EN 12021. Compressor intake must be sited in clean air. RPE for non-spraying tasks (e.g. brush/roller application of PU coatings): minimum half-mask with A2P3 filters, face-fit tested. |
| Controls — Skin protection | Chemical-resistant gloves (nitrile minimum 0.4 mm or butyl rubber). Full-coverage disposable overalls. Face/neck protection where spray mist is present. Barrier cream is NOT a substitute for gloves. Contaminated clothing must be removed immediately. |
| Health surveillance | Annual spirometry and respiratory questionnaire for all workers exposed to isocyanates. Baseline assessment before first exposure. Records retained for 40 years. Any decline in lung function triggers immediate review and possible removal from exposure. |
| Training | All workers handling isocyanate products must receive specific training on: the health hazards, correct use of controls and RPE, skin protection, emergency procedures, and the importance of reporting symptoms immediately. Training must be documented and refreshed periodically. |
| Emergency procedures | Spill containment and clean-up procedures. First aid for skin/eye contact and inhalation. Evacuation procedures for uncontrolled releases. Location of emergency equipment, eyewash stations, and emergency showers. |
| Assessor | Name, qualifications, and competence of the person who carried out the assessment. Date of assessment and review date. |
Skin absorption contributes to respiratory sensitisation
Isocyanates can be absorbed through the skin and contribute to respiratory sensitisation. A worker who repeatedly allows isocyanate products to contact unprotected skin may develop occupational asthma even if their inhalation exposure is below the WEL. This means that skin protection is not just about preventing dermatitis — it is a critical part of preventing respiratory sensitisation. Chemical-resistant gloves and full-coverage overalls are essential, and any skin contamination must be washed off immediately with soap and water.
3. Common Mistakes
Relying on filter-based RPE for spraying isocyanates
Filter-based respiratory protective equipment — including FFP3 disposable masks, half-mask respirators with organic vapour cartridges, and even powered air-purifying respirators with combination filters — is not adequate for spraying isocyanates. The aerosol generated during spraying contains both a particulate phase and a vapour phase. Filter cartridges cannot reliably capture the vapour phase, their capacity is rapidly exhausted at the high concentrations generated during spraying, and breakthrough can occur without warning. Air-fed RPE with an independent clean air supply is the only acceptable form of respiratory protection for isocyanate spraying. This is a mandatory requirement, not a recommendation, and the HSE will serve prohibition notices where filter-based RPE is being used for isocyanate spraying.
Not enrolling workers in annual spirometry health surveillance
COSHH Regulation 11 requires health surveillance for all workers exposed to substances that cause identifiable diseases — and isocyanate-induced occupational asthma is a clearly identifiable disease. Annual spirometry testing, combined with a respiratory symptom questionnaire, is the standard health surveillance protocol for isocyanate-exposed workers. The purpose is to detect early signs of respiratory sensitisation or declining lung function before the worker develops full-blown occupational asthma. Without spirometry, the first indication of sensitisation may be a severe asthma attack triggered by routine exposure — by which point irreversible damage has been done and the worker's career in that trade is effectively over.
5. Frequently Asked Questions
Does COSHH apply to 1K (single-component) PU products?▾
Yes. Single-component (1K) polyurethane products still contain isocyanates — typically MDI prepolymers. While the free monomer content may be lower than in two-pack (2K) systems, the sensitisation risk remains. 1K PU sealants, adhesives, and coatings release MDI vapour during application and curing, particularly in warm or poorly ventilated conditions. The COSHH assessment must cover all isocyanate-containing products, regardless of whether they are one-pack or two-pack formulations. The controls may differ in degree — brush or roller application of a 1K product may not require air-fed RPE if exposure is demonstrated to be below the WEL with other controls — but the assessment, health surveillance, and training requirements still apply in full.
Can I use a powered air-purifying respirator (PAPR) instead of air-fed RPE for spraying?▾
No. A PAPR draws ambient air through filters and relies on those filters to remove contaminants. For isocyanate spraying, the vapour phase of the aerosol can break through filter cartridges without warning, and cartridge capacity is rapidly exhausted at the concentrations generated during spraying. Only air-fed RPE — which supplies breathing air from an independent clean source (compressed air cylinder or remote compressor with appropriate filtration) — provides reliable protection for isocyanate spraying operations. PAPRs may be acceptable for non-spraying tasks involving isocyanates (such as brush application or mixing) where exposure is lower, but this must be justified by the COSHH assessment and supported by air monitoring data.
How quickly can sensitisation occur?▾
There is no predictable timeline. Some workers become sensitised after months or years of low-level exposure. Others have been sensitised after a single acute exposure event — for example, an accidental release or a failure of RPE during spraying. Once sensitisation has occurred, it is permanent and irreversible. The only way to prevent sensitisation is to prevent exposure, which is why the controls specified in the COSHH assessment must be followed rigorously every time isocyanate products are used, not just when convenient or when the task is expected to take a long time.
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