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.

EU: Binding Limit from Directive 2024/869

Following an opinion of ECHA’s Risk Assessment Committee (RAC), the EU set a binding limit for all diisocyanates as a group with Directive (EU) 2024/869 of 13 March 2024, in Annex I of the Chemical Agents Directive 98/24/EC: 6 µg NCO/m³ as an 8-hour average and 12 µg NCO/m³ for short-term exposure, with a skin, dermal and respiratory sensitisation notation. Until 31 December 2028 a transitional limit of 10 µg NCO/m³ and 20 µg NCO/m³ applies, because of measurement feasibility and the time downstream sectors in particular need for risk management measures. Member states had to transpose the directive by 9 April 2026. In DOHSBase, the group limit for diisocyanate monomers is tracked under pseudo-CAS WGM-0312.

The limit is 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.

The Netherlands transposed the directive in Stcrt. 2025, 37861. Since 9 April 2026, annex XIII of the Working Conditions Regulation sets a statutory limit for diisocyanates (measured as NCO) of 0.010 mg/m³ (8-hour TWA) and 0.020 mg/m³ (15-minute); from 1 January 2029 this becomes 0.006 mg/m³ and 0.012 mg/m³. The reduction is already in the same publication.

Dutch Health Council: Di- and Tri-Isocyanates

On 28 November 2018 the Dutch Health Council (Gezondheidsraad) issued a health-based recommended limit for di- and triisocyanates: 0.1 µg NCO/m³. In DOHSBase, this recommendation is tracked under pseudo-CAS WGM-0309.

The advice covers both diisocyanates and triisocyanates, a broader scope than the EU limit. It is health-based, without consideration of technical or economic feasibility. Because no safe threshold can be identified for sensitisation, the value is risk-based: the committee limits the additional risk of allergic respiratory complaints to 1%. The Council also notes that skin exposure can contribute to developing those complaints.

The statutory limit is 60 (from 2029) to 100 (now) times higher than this recommendation. The difference comes from the feasibility assessment that a statutory limit includes and a health-based recommendation does not.

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: 0.02 mg/m³ (8 hours) and 0.07 mg/m³ (15 minutes), with a sensitiser notation (EH40/2005). 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 statutory limit (annex XIII): the group limit for diisocyanates (measured as NCO), 0.010 mg/m³ (8-hour TWA) since 9 April 2026, 0.006 mg/m³ from 1 January 2029
Dutch Health Council recommendation (WGM-0309): the health-based value of 0.1 µg NCO/m³ for di- and tri-isocyanates
EU limit from Directive 2024/869 (WGM-0312): transposed in the Netherlands and Belgium; for other member states the binding ceiling for the national limit
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.

Belgium: statutory limit since June 2026

With the royal decree of 25 May 2026, published in the Belgian Official Journal on 3 June 2026, Belgium added a statutory limit for diisocyanates to its Codex on well-being at work. It is the same value, on the same timetable, as the EU limit and the Dutch statutory limit above, expressed as NCO:

10 µg NCO/m³ as an 8-hour average and 20 µg NCO/m³ for short-term exposure;
from 1 January 2029, 6 µg NCO/m³ and 12 µg NCO/m³.

Source: FPS Employment. Anyone assessing a Belgian workplace therefore checks against this statutory value. How DOHSBase shows the Belgian Codex values is set out on DOHSBase for Belgium.

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.