How a Fabrication Facility Reduced Isocyanate Exposure to Undetectable Levels in Paint Spraying Operations

Sysco Environmental Ltd was commissioned to assess employee exposure to hazardous substances at a metal fabrication site in Cambridgeshire. A key focus of this assessment was the potential exposure to isocyanates used in the on-site spray painting processes. Employees were observed carrying out paint mixing, spraying, and clean-up activities using two-pack polyurethane paints, known to contain various diisocyanates.

The site operates a dedicated paint shop, equipped with a mixing station, spraying tables, and mechanical extraction systems. Employees were provided with air-fed respirators during tasks. This case study evaluates the airborne concentration of isocyanates, their use in paints, and outlines recommended control measures.

Understanding Isocyanates in Paints

What Are Isocyanates?

Isocyanates are a group of highly reactive chemicals used primarily in the production of polyurethane-based materials such as foams, coatings, adhesives, and paints. Common industrial isocyanates include:

• Toluene Diisocyanate (TDI) – Used in flexible foam and spray applications.

• Methylene Diphenyl Diisocyanate (MDI) – Common in rigid foams and potting compounds.

• Hexamethylene Diisocyanate (HDI) – Widely used in automotive and industrial coatings.

• Isophorone Diisocyanate (IPDI) – Used in high-performance paints and sealants.

Why Are Isocyanates Used in Paints?

Isocyanates are a key ingredient in two-component polyurethane paints, where they act as curing agents (hardeners). When mixed with a polyol resin, isocyanates initiate a cross-linking chemical reaction that produces a durable, chemical-resistant, and UV-stable coating.

This makes isocyanate-based paints ideal for:

• Automotive and aerospace finishes

• Structural steel coatings

• Protective coatings for harsh environments

However, during spraying, airborne droplets or vapours can be inhaled or come into contact with skin, presenting serious health risks.

Health Effects of Isocyanates

Isocyanates are classified as respiratory sensitisers and are among the leading causes of occupational asthma in the UK.

Acute Exposure Effects

• Eye and nose irritation

• Throat inflammation

• Coughing and breathlessness

Chronic Exposure Risks

• Occupational asthma (often irreversible)

• Long-term lung inflammation

• Severe sensitisation: minute future exposures can trigger asthmatic reactions

Exposure may occur via:

• Inhalation of overspray or vapours during spraying

• Dermal contact with uncured paint or contaminated surfaces

Monitoring Methodology

Monitoring followed MDHS 25/3 guidelines for organic isocyanates in air. Personal and static samples were taken using piperazine-treated filters and analysed via high-performance liquid chromatography (HPLC).

Isocyanates Monitored:

• Hexamethylene Diisocyanate (HDI)

• Isophorone Diisocyanate (IPDI)

• Methylenediphenyl Diisocyanate (MDI)

• Toluene Diisocyanate (TDI)

Workplace Exposure Limit (WEL):

• Long-Term (8-hour TWA): 0.02 mg/m³

• Short-Term (15-min STEL): 0.07 mg/m³

Monitoring Results

Conclusion: All isocyanate levels were below the detection limit, indicating that the control measures in place (especially use of air-fed RPE) were highly effective.

Key Recommendations for Ongoing Risk Control

1. Ventilation

• Status: Fixed extraction was functioning and serviced within the past year.

• Recommendation: Maintain a 14-month service cycle. Fit airflow indicators on spray booths to allow users to verify performance instantly. Maintain a LEV logbook as per HSG258.

2. Respiratory Protective Equipment (RPE)

• Status: Air-fed Versaflo M-107 units in use.

• Recommendation: Maintain mandatory use of RPE for all spraying tasks. Provide clean storage for non-disposable units to avoid contamination. Train staff in RPE inspection and maintenance.

3. Health Surveillance

• Status: No programme in place.

• Recommendation: Implement a formal respiratory health surveillance programme in line with HSE asthma guidelines for isocyanate users.

4. Employee Training

• Status: Informal only.

• Recommendation: Deliver annual COSHH refresher training focused on:

  • Symptoms of respiratory sensitisation

  • Correct mixing and spraying procedures

  • Emergency response to RPE failure

  • Importance of consistent PPE use

Conclusion

Sysco Environmental Ltd’s assessment confirmed that isocyanate exposure during paint spraying operations at the Cambridgeshire facility is well controlled, with airborne concentrations below detectable levels. The use of air-fed RPE and functioning extraction systems plays a critical role in ensuring safety.

This case study illustrates the importance of:

• Proactive exposure monitoring

• Maintenance of engineering controls

• Formalised health surveillance and training