Draft article for BOHS newsletter - Measurement of exposure to isocyanates

There is a government target to reduce by 30% the incidence of occupational asthma by 2010. Isocyanates are recorded as the chief cause of occupational asthma, and exposure through spraying isocyanate-based surface coatings is an important contributor. This note comments on monitoring isocyanate exposure under Regulation 10 of COSHH. Monitoring is needed to prove that control measures are effective.

Since a review in 1981, the maximum exposure limits (MELs) for isocyanate have been 0.02 mg/m3 (8-hour time-weighted average, TWA) and 0.07 mg/m3 (15-minute TWA). These limits are expressed as total isocyanate and apply to the isocyanate functional group, whether present as monomer, polymer or partially cured aerosol. The limits reflect the fact that isocyanate groups are chemically and toxicologically active in whatever form, and capable of causing asthma. Isocyanate-based paints contain polymeric isocyanate, often with less than 1% monomer present.

As discussed at a workshop session in the 2004 British Occupational Hygiene Society (BOHS) conference, HSE has grounds for believing that not all occupational hygienists and analytical laboratories understand fully the requirements for sampling and analysing airborne isocyanates. A letter from Mark Piney and others in the BOHS Newsletter (Vol. 15(4), November 2002) warned about the problems that can occur, and provided some guidance. Consultants and analytical laboratories need to know the right way of measuring exposure. Where deficiencies occur, HSE intends to take action.

Air sampling

Recently, several of HSE’s occupational hygiene surveys for isocyanate exposure showed high results for sprayers and others. Workers were exposed significantly to isocyanate aerosol. However, consultants’ surveys and reports of isocyanate in air in the same premises had shown concentrations much lower than HSE’s results, indicating no grounds for concern. The spraying companies had been misled - workers’ health was at risk.

In the cases investigated, the main reasons the consultants measured much lower isocyanate concentrations in air were because they did not use the correct sampling train, and perhaps more importantly, the analysts measured only monomeric isocyanate derivatives.

The method for the determination of total organic isocyanate in air is detailed in MDHS 25/3 (HSE Books, ISBN 0717616681). The Health and Safety Laboratory runs the WASP scheme (Workplace Analytical Scheme for Proficiency – contact hslinfo@hsl.gov.uk). A series of derivitised isocyanate monomer samples are circulated for participating laboratories to measure.

HSE believes that proficiency in determining total isocyanate is required for laboratories that analyse airborne isocyanate samples. It expects analytical laboratories to report total isocyanate and consultants to assess control of exposure to total isocyanate. HSE will ask HSL to include polymeric isocyanate derivatisates in the WASP scheme, and is prepared to fund analytical laboratories’ participation in this new scheme for the first year.

Biological monitoring

Biological monitoring is a relatively new and very useful technique for measuring personal exposure and demonstrating that adequate control of isocyanate has been achieved. As a monitoring tool, biological monitoring falls under Regulation 10 of COSHH (2002), referred to in para.192 of the Approved Code of Practice and Guidance, fourth edition (ISBN 0717625346).

The sample is simple to collect and details of the monitoring assay are available from the HSL website. General advice on biological monitoring is given in HSE guidance HSG167.

HSE intends to use biological monitoring as part of an initiative in motor vehicle repair, to reduce workplace exposures to isocyanates. A positive result in a urine sample, taken shortly after spraying ends, shows that inhalation of isocyanate occurred. Following a considerable amount of field-testing, it is becoming clear to HSE that a ‘none detected’ result for urinary isocyanate metabolite is achievable for most sprayers. This is not a formal Guidance Value, but it is an indicator of what HSE views as ‘reasonably practicable’ for many sprayers and other workers.