Isocyanate Exposure at Work: Preventing Occupational Asthma and Protecting Employees

Isocyanates are widely used across industry in products such as paints, foams, adhesives and coatings. They are essential to many manufacturing and construction processes, particularly in polyurethane production.

However, despite their widespread use, isocyanates present one of the most significant chemical health risks in modern workplaces.

Why are isocyanates are a major workplace health concern?

Isocyanates are highly reactive chemicals used in a wide range of industrial applications including:

• Vehicle spray painting and refinishing
• Insulation foam installation
• Industrial coatings and varnishers
• Polyurethane manufacturing

Common types of industrial isocyanates include:

• Methylene Diphenyl Diisocyanate (MDI)
• Toluene Diisocyanate (TDI)
• Hexamethylene Diisocyanate (HDI)

The greatest danger arises when chemicals become airborne during spraying, heating, or mixing processes, allowing workers to inhale contaminated aerosols.

Statistics on occupational asthma

Occupational asthma is one of the most common work-related respiratory diseases in the UK.

According to the Health and Safety Executive:

• 1,500 to 3,000 new cases of occupational asthma are estimated to occur in the UK each year.
• If asthma that is worsened by work is included, this figure rises to around 7,000 cases annually.
• Occupational asthma accounts for roughly 1 in 6 cases of adult-onset asthma.

Among workplace causes, isocyanates are consistently identified as one of the leading triggers of occupational asthma in Great Britain.

Certain industries face a particularly high risk. For example, vehicle spray painters have significantly higher rates of occupational asthma compared with the general working population due to exposure to isocyanate containing paints and coatings.

Health hazards associated with isocyanates

Exposure to isocyanates can occur through inhalation, skin contact, or eye exposure, with inhalation posing the most serious health risk.

Occupational Asthma

The most significant health effect linked to isocyanates is occupational asthma, these chemicals act as respiratory sensitisers, meaning repeated exposure can cause the immune system to become hypersensitive.

Once sensitisation occurs, even very small amounts of exposure can trigger severe asthma attacks.

Common symptoms include:

• Wheezing
• Shortness of breath
• Chest tightness
• Persistent coughing
• Flu-like symptoms

In many cases, affected workers must leave their occupation to prevent further exposure.

Skin and eye effects

Common symptoms include:

• Skin irritation
• Dermatitis
• Eye inflammation

Skin exposure is particularly important because it may contribute to respiratory sensitisation, increasing the risk of developing occupational asthma.

High risk work activities

Although isocyanates are used across many sectors, some tasks present significantly higher exposure risks.

These include:

• Spray painting and coating
• Vehicle repair and refinishing
• Foam insulation spraying
• Polyurethane manufacturing

Spray applications are especially hazardous because they produce fine airborne particles that are easily inhaled.

Control Measures: Managing the risk

Elimination or substitution

Where possible, organisations should consider:

• Using non isocyanate products
• Substituting them with less hazardous alternatives
• Using lower concentration formulations

While elimination is often difficult in industrial settings, substitution should always be considered during risk assessments.

Engineering controls

Engineering controls are critical for preventing airborne exposure.

Examples include:

• Local exhaust ventilation (LEV) systems
• Enclosed spraying booths
• Dedicated spray booths with filtration
• Automated coating process

Maintain spray booths or rooms with adequate extraction systems that create negative pressure. This prevents paint vapours escaping into workshop areas and contaminating the wider workplace. All local exhaust ventilation (LEV) systems should be thoroughly examined and tested at least once every 14 months.    

Administrative controls

Administrative measures help reduce the likelihood of exposure through safe working practises.

These include:

• Worker training and competency assessments
• Clear safe systems of work
• Restricted access to spray areas
• Proper storage and handling procedures
• Safe clearance procedures

Display measurable clearance times clearly for all workers to see. Workers must not remove respiratory protection until they are safely outside the spray area, or the required clearance time has fully elapsed.

Employers must comply with the Control of Substances Hazardous to Health Regulations 2002 (COSHH), which requires employers to prevent or adequately control exposure to hazardous substances.

Personal Protective Equipment (PPE)

Where exposure cannot be controlled by other methods, suitable PPE must be provided.

Typical protective equipment includes:

• Air fed respiratory protective equipment (RPE)
• Chemical resistant gloves
• Protective overalls
• Eye and Face protection

Respiratory protective equipment should be properly fitted, maintained and regularly tested. Filtering respirators do not provide enough protection against paint mist and vapours during spray operations. Breathing apparatus should ideally be full-visor type, but half-mask type with appropriate eye protection is acceptable with more frequent biological monitoring.

Monitoring and testing

Air monitoring

Workplace air monitoring may include:

• Personal exposure sampling
• Static monitoring of spray booths
• Real time measurement where available

These tests help determine whether exposure exceeds workplace exposure limits (WEL) set out in EH40/2005.

Biological Monitoring

For isocyanates, biological monitoring is often considered the most reliable method of assessing exposure.

This typically involves urine testing to detect breakdowns products of isocyanates in the body, providing an indication of recent exposure.

Biological monitoring helps organisations:

• Confirm whether workers have been exposed
• Identify failures in control measures
• Verify that respiratory protection and ventilation are effective

The importance of health surveillance

Workers exposed to isocyanates should be included in health surveillance programs

This may involve:

• Respiratory health questionnaires
• Lung function tests
• Occupational health assessments

Early detection of symptoms can prevent long term respiratory damage and allow intervention before the condition becomes severe.

Summary

Isocyanates remain essential to many industries, but their health risks should never be underestimated. Occupational asthma caused by isocyanate exposure can be life-changing and, in many cases, career-ending.

However, with proper risk assessment, engineering controls, monitoring and health surveillance, the risks can be significantly reduced.

For organisations handling isocyanates, the key to protecting workers lies in recognising the hazards early and ensuring that robust control measures are consistently applied.

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