Isocyanates: Exposure Limits Across Jurisdictions

Isocyanates are among the most important substance groups in occupational hygiene. They are potent sensitisers of the respiratory tract and a leading cause of occupational asthma worldwide. For occupational hygienists, navigating the various exposure limits for isocyanates across different jurisdictions is a recurring challenge. This article provides an overview of the current landscape.

Introduction: Why Isocyanates Demand Attention

Isocyanates are reactive chemicals widely used in the production of polyurethane foams, coatings, adhesives, and elastomers. The most common isocyanates encountered in occupational settings include toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), and hexamethylene diisocyanate (HDI).

Exposure to isocyanates can cause severe respiratory sensitisation, occupational asthma, and other respiratory effects at very low concentrations. Once sensitised, a worker may react to concentrations far below the exposure limit. This makes proper exposure control essential, and it makes the choice of the right limit value particularly important.

European Classification

Under CLP, 23 isocyanate substances have a harmonised classification (CLH) that includes respiratory sensitisation (Resp. Sens. 1). In addition, a significant number of isocyanates have been notified to ECHA with self-classifications for respiratory sensitisation.

The EU has also introduced mandatory training requirements for the use of diisocyanates under REACH Restriction Entry 74, which came into effect in August 2023. This restriction applies to substances containing diisocyanates at concentrations above 0.1% and requires workers to complete specific training before handling these products.

ECHA RAC Proposed OELV

The ECHA Risk Assessment Committee (RAC) has proposed an occupational exposure limit value (OELV) for diisocyanates as a group. In DOHSBase, this proposed limit is tracked under pseudo-CAS WGM-0312, which represents the group of diisocyanate monomers.

The RAC opinion is significant because it proposes a single limit value applicable to all diisocyanates, expressed as NCO (the reactive isocyanate group). This group-based approach reflects the understanding that the respiratory sensitisation hazard is driven by the NCO functional group rather than by the specific molecular structure of individual diisocyanates.

For occupational hygienists, the RAC proposed OELV provides a reference point when no national legal limit is available, and it sits at a defined level in the DOHSBase limit value hierarchy.

Dutch Health Council: Di- and Tri-Isocyanates

The Dutch Health Council (Gezondheidsraad) has issued health-based recommended occupational exposure limits for di- and tri-isocyanates. In DOHSBase, these recommendations are tracked under pseudo-CAS WGM-0309.

The Health Council’s approach covers both diisocyanates and triisocyanates, providing a broader scope than the ECHA RAC proposal. The recommended limits are health-based, meaning they are derived from toxicological and epidemiological evidence without consideration of technical or economic feasibility. This places them at a high level in the limit value hierarchy — they represent the concentration below which adverse health effects are not expected.

In the Netherlands, the Health Council recommendation serves as the basis for the legal limit value set by the Ministry of Social Affairs and Employment (SZW). However, the legal limit may differ from the Health Council recommendation if socio-economic factors are taken into account.

UK HSE: All Isocyanates as a Group

The UK Health and Safety Executive (HSE) takes the broadest approach, setting a workplace exposure limit (WEL) for all isocyanates as a group. In DOHSBase, this is tracked under pseudo-CAS WGM-0308.

The UK WEL applies to all isocyanates — mono-, di-, and tri-isocyanates — expressed as NCO. This is the most inclusive group definition among the jurisdictions covered in DOHSBase. The UK approach simplifies compliance monitoring: regardless of which specific isocyanate is present in the workplace, the same limit applies.

Hierarchy of Limit Values: Netherlands vs Other EU

For occupational hygienists working in the Netherlands, the hierarchy of applicable limit values for isocyanates typically follows this order:

Dutch legal limit value (SZW) — the legally binding limit for specific isocyanates listed by SZW
Dutch Health Council recommendation (WGM-0309) — the health-based limit for di- and tri-isocyanates as a group
ECHA RAC proposed OELV (WGM-0312) — the proposed EU-wide limit for diisocyanates
UK HSE WEL (WGM-0308) — the UK group limit for all isocyanates

The DOHSBase limit value hierarchy helps users navigate this complexity by presenting the most relevant limit value first, while still providing access to all available limits for comparison.

For isocyanates that have individual substance-specific limits (such as TDI or MDI), these take precedence in the hierarchy. The group limit values (WGM-0308, WGM-0309, WGM-0312) serve as fallback values for isocyanates that do not have their own substance-specific limit, and as reference points for comparison.

Practical Implications

When assessing isocyanate exposure in the workplace, occupational hygienists should:

Identify the specific isocyanate(s) present and check for substance-specific limit values
Consult the group limit values when no substance-specific limit is available
Consider the NCO measurement approach since group limits are expressed as NCO
Review the limit value hierarchy to understand which limit applies in your jurisdiction

DOHSBase Online makes this process straightforward by presenting all applicable limit values — both substance-specific and group-based — in a single overview for each isocyanate substance.

Explore isocyanate data in DOHSBase Online and learn more about the limit value hierarchy in our knowledge base.