MDHS 25/4 Organic Isocyanates in Air

Measuring Airborne Isocyanates in the Workplace

LEVCentral Expert Commentary

For occupational hygienists involved in spray painting, polyurethane manufacture or any process using isocyanates, MDHS 25/4 is one of the most important analytical methods ever published by HSE.

Unlike general airborne contaminant sampling methods, MDHS 25/4 was specifically developed to address one of the greatest challenges in occupational hygiene—accurately measuring the wide range of organic isocyanates that may exist as vapours, aerosols or mixtures of both. These compounds are potent respiratory sensitisers and remain one of the leading causes of occupational asthma in the UK.

The method describes how airborne organic isocyanates should be sampled, stabilised and analysed so that results can be compared directly with the UK Workplace Exposure Limits (WELs), which are expressed as Total Reactive Isocyanate Groups (TRIG) rather than individual compounds. This makes MDHS 25/4 the recognised reference method for demonstrating compliance with UK exposure limits.

For LEV engineers, MDHS 25/4 is significant because it provides the analytical evidence that demonstrates whether spray booths, extraction systems and other engineering controls are actually protecting workers from exposure—not simply whether the ventilation system is moving sufficient air.

View HSE Guide

HSE Guide

Key Learning Points

MDHS 25/4 explains:

Why isocyanates require specialist sampling techniques.
The chemistry of organic isocyanates.
Measuring Total Reactive Isocyanate Groups (TRIG).
Sampling vapours.
Sampling aerosols.
Combined vapour and aerosol sampling.
Derivatisation using 1-(2-methoxyphenyl)piperazine (1,2-MP).
Filter sampling.
Impinger sampling.
Sample handling and preservation.
Laboratory analysis using High Performance Liquid Chromatography (HPLC).
Identification of isocyanate derivatives.
Quantification of exposure.
Quality assurance.
Interpretation of results.
Limitations of the analytical method.

Why MDHS 25/4 is Different

Unlike many workplace contaminants, isocyanates cannot simply be collected on a filter and analysed later.

They are highly reactive and rapidly change chemically after sampling. MDHS 25/4 overcomes this by causing the isocyanates to react immediately with a derivatising reagent (1,2-MP) during sampling, creating stable compounds that can subsequently be analysed accurately in the laboratory.

This is one of the reasons the method has become the internationally recognised benchmark for occupational exposure assessment.

Source Document Information

Organisation: Health and Safety Executive (HSE)

Document: MDHS 25/4 – Organic Isocyanates in Air

Series: Methods for the Determination of Hazardous Substances (MDHS)

Document Type: Analytical Sampling Method

Primary Topics: Organic Isocyanates, Occupational Hygiene, Air Sampling, Exposure Monitoring, HPLC Analysis, Spray Painting, Occupational Asthma.

Audience: Occupational Hygienists, Analytical Laboratories, LEV Engineers, Health & Safety Professionals, Motor Vehicle Repair Specialists, Polyurethane Manufacturers and Exposure Monitoring Consultants.

LEVCentral Perspective

Although this document is primarily written for occupational hygienists and analytical laboratories, every LEV engineer should understand what it is measuring and why.

LEV commissioning and Thorough Examination & Testing demonstrate that an extraction system is operating as intended.

MDHS 25/4 demonstrates whether workers are actually being exposed to isocyanates.

These are complementary activities rather than alternatives.

One particularly important feature of the method is its ability to measure Total Reactive Isocyanate Groups (TRIG) rather than simply selected monomeric isocyanates. This is essential because many industrial products—particularly two-pack spray paints and polyurethane coatings—contain mixtures of monomers, oligomers and prepolymers. Measuring only one component could significantly underestimate exposure.

The method also highlights the complexity of isocyanate exposure assessment. Depending on the process, contaminants may exist as vapour, aerosol or both, requiring different sampling arrangements. This reinforces why isocyanate monitoring should only be undertaken by suitably competent occupational hygienists using validated methods.

It is also worth noting that the derivatising reagent used in MDHS 25/4 (1,2-MP) is a controlled substance in the UK. Consequently, organisations carrying out this sampling require the appropriate authorisations. BOHS operates a Group Authority Licence (GAL) that allows eligible members of the Faculty of Occupational Hygiene to work within the required legal framework.

Further Resources

Recommended Learning

Thought Leadership

MDHS 25/4 demonstrates an important principle that applies throughout occupational hygiene:

Good engineering should always be capable of being verified by good measurement.

An LEV system may appear to perform well, airflow measurements may meet specification and the spray booth may pass its Thorough Examination and Test. However, the ultimate question remains:

Are workers still being exposed?

MDHS 25/4 provides one of the most robust ways of answering that question for isocyanates.

From a LEVCentral perspective, the document also highlights the close relationship between LEV engineering and occupational hygiene. Ventilation engineers design and verify the control system; occupational hygienists verify the resulting exposure. Together, these complementary disciplines provide the evidence needed to demonstrate that hazardous substances are being controlled effectively under COSHH.

It is this combination of competent engineering and competent exposure assessment that provides the strongest assurance that workers are genuinely protected.