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Category name:
MDI Category

Justifications and discussions

Category definition:
All substances within the MDI category are considered to have sufficient chemical similarity as to be considered as a single group for read across assessment purposes. Not only do all substances contain the common functional group typical of MDIs, i.e. two isocyanate (NCO) reactive functional groups that are responsible for their chemical and biological reactivity, but they demonstrate a constant pattern in the changing of the potency of the properties across the group. More information on the category definition can be found as of page 58 of the Category Justification Document.
Category description:
Both structural descriptions as well as measured phys-chem. properties for all category substances confirm the predicted trends in these properties and justify the grouping. This is adequately documented in the CJD.
Category rationale:
1. CATEGORY HYPOTHESIS
Substances of the MDI category contain the common chemical feature of reactive NCO groups, present on at least two terminal aromatic rings joined together by a methylene bridge, that are responsible for the common chemical and biological reactivity exhibited across all substances of the MDI category. The presence of non-monomeric MDI constituents i.e. oligomers, their condensation products and their glycol adducts increases molecular weight and hydrophobicity, and decreases vapour pressure, water solubility and bioavailability of these non-monomeric MDI constituents causing the observed trends in properties across the category. A common feature of all substances of the MDI category is the very low vapour pressure and water solubility. In the presence of water reactive aromatic-bound isocyanate groups (NCO) are rapidly detoxified by hydrolysis forming poly-urea and, depending on the abundancy of water molecules, traces of transient, toxic amines can be formed. In the presence of biological macromolecules in the human body, MDI-bound isocyanate groups are also rapidly detoxified by reacting with those macromolecules. These aspects of exposure and fate are important considerations for both hazard identification and risk assessment of these substances.

2. CATEGORY JUSTIFICATION
The rationale for the selection of suitable and sufficient source substances upon which to perform the read-across to all target substances has been established consistent with the proposed hypothesis and for these “boundary substances” robust and consistent datasets of reliable studies have been established.

3. APPLICABILITY DOMAIN OF THE CATEGORY
The physico-chemical properties of the MDI category substances are very uniform and as expected from the chemical structures. This is a result of the relatively high concentrations of mMDI (4,4’- MDI or MDI Mixed Isomers) in all substances of the MDI category and the strict application of the category hypothesis and the exclusion criteria. Consistent with the category hypothesis the presence of the reactive NCO group and in this regard high levels of the most reactively available mMDI in all substances is the main driver for the human health and Environmental hazard identification and risk assessment. In broad terms, addition of the different building blocks (BB2, BB3 and BB4) into the chemical structures only further lowers the already low volatility and water solubility of the mMDI isomers making exposure risk even more unlikely to these modified MDI substances. The significance of the higher octanol-water partition coefficients for the non-monomeric MDI constituents in terms of hazard identification and risk assessment is questionable since they all rapidly react with water and are detoxified under relevant human health and environmental conditions.
Hence, as conclusion:
• All substances in the MDI category contain monomeric MDI as the majority constituent.
• All substances exclusively contain constituents that have at least two aromatic NCO groups that are electronically separated from other aromatic rings by at least a methylene bridge.
• The reactivity of the NCO groups in these constituents has been shown to be in a narrow range, monomeric MDI isomers having the highest reactivity.
• The category substances share the following common physico-chemical properties:
- a relative density (to water) above 1,
- a very low vapour pressure,
- high hydrophobicity and
- very low water “solubility”.
• With increasing molecular weight, there is a clear trend toward lower bio-availability of the non-monomeric MDI constituents in the category substances.
• Therefore, and as will be demonstrated in the environmental part and human health part, it can be concluded that all MDI substances belong to one category and will essentially be determined by the bio-available NCO in the category substances.

Details can be found in the Category Justification Document attached here and also included in IUCLID Section 13

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