Isocyanates are highly reactive industrial chemicals used to manufacture polyurethane products — foams, coatings, adhesives, sealants, and elastomers. Toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI) are the most commercially important. Isocyanates are the leading occupational cause of work-related asthma in many countries, including the UK. Once sensitisation occurs, even trace exposures can trigger severe asthmatic reactions. They are also potent skin and eye sensitisers.
Where it's found
Spray painting vehicles with isocyanate-containing automotive paints (the most significant cause of occupational isocyanate asthma in the UK). Manufacturing and processing polyurethane foam — furniture, mattresses, insulation panels. Spray foam insulation application in buildings. Manufacture of adhesives, sealants, and coatings for construction. Rubber and elastomer production. Mining and tunnelling (polyurethane grout injection). Two-component adhesives used in DIY woodworking. Isocyanate vapours are also released when polyurethane products are heated, cut, or burned — including foam furniture fires.
Routes of exposure
Inhalation of isocyanate vapours and aerosols during application and curing is the primary occupational route. Dermal contact with isocyanate-containing liquids contributes to sensitisation even without obvious respiratory symptoms — skin sensitisation can trigger respiratory reactions on subsequent inhalation exposure. Heating, cutting, sanding, or burning polyurethane products releases isocyanate vapours. DIY users of two-component adhesives and spray foam insulation can experience significant short-term exposure in enclosed spaces.
Health concerns
Isocyanates are the most frequent cause of occupational asthma in the UK, causing a substantial proportion of all new-onset occupational asthma cases reported to surveillance schemes. Once sensitised, an individual may react to vanishingly small concentrations — effectively ending their ability to work in any environment where isocyanates are present. Sensitisation is irreversible — there is no cure and no desensitisation protocol. In addition to asthma, isocyanates cause hypersensitivity pneumonitis (an immune-mediated inflammatory lung disease), skin and eye sensitisation, and at high acute exposures, pulmonary oedema and chemical pneumonitis.
Evidence
Isocyanate-induced occupational asthma and sensitisation are clinically well-established with decades of epidemiological and mechanistic evidence. UK SHIELD and SWORD occupational disease surveillance systems consistently identify isocyanates as a leading asthma cause. The irreversibility of sensitisation is documented and clinically confirmed. The mechanism of immune sensitisation and subsequent hypersensitivity is well understood. Regulatory bodies worldwide classify isocyanates as a priority occupational hazard.
Who's most at risk
Automotive spray painters are the highest-risk occupational group in the UK. Polyurethane foam manufacturers and insulation installers face significant exposure. Workers in construction using spray foam or polyurethane sealants in enclosed spaces. DIY users applying spray foam insulation or two-component adhesives in poorly ventilated spaces. People with pre-existing asthma or atopy (hay fever, eczema) may sensitise more readily, though sensitisation can occur in anyone.
Regulatory status
RegulationUK WEL for TDI is 0.02 mg/m³ (8-hour TWA) and 0.07 mg/m³ (15-minute STEL) — these are some of the lowest WELs set by the HSE, reflecting the potency and irreversibility of harm. COSHH Regulations require engineering controls (exhaust ventilation, enclosed application systems), RPE (supplied air respirators for spray application), health surveillance, and pre-employment and periodic lung function testing. The EU CMD sets a binding OEL for MDI and TDI. UK RIDDOR requires reporting of cases of occupational asthma confirmed in isocyanate-exposed workers.
How to reduce your exposure
Always use adequate respiratory protection — a P3 filter mask is insufficient for isocyanate spraying; supplied air respirators are required for spray application. Use enclosures, exhaust ventilation, and low-spray high-transfer efficiency equipment to minimise generation of aerosol. Wear protective gloves and coveralls — avoid all skin contact. Undergo pre-employment lung function testing and regular monitoring thereafter. Report any new respiratory symptoms immediately — early identification before full sensitisation is critical. Never allow heating, cutting, or burning of polyurethane foam without monitoring for isocyanate release. For DIY use of expanding foam products, work outdoors or with maximum ventilation and wear gloves.
The nutrition connection
Isocyanate occupational asthma is an entirely preventable disease — sensitisation only occurs through inadequately controlled exposure. Once sensitisation has occurred, the person affected typically cannot return to isocyanate-using work and may develop persistent asthma affecting all aspects of their life. Supporting respiratory health nutritionally — through anti-inflammatory dietary patterns, adequate vitamin D (linked to asthma severity), and omega-3 fatty acids — is meaningful for workers in high-risk industries, though it cannot substitute for primary exposure prevention.