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EC number: 701-124-4
CAS number: -
The test substance is covered by the category approach of methylenediphenyl diisocyanates (MDI). Hence, data of the category substances can be used to cover this endpoint. The read-across category justification document is attached in IUCLID section 13.
The way in which MDI substances are mixed and reacted in the presence of water determines the extent to which the common transformation compound MDA is formed during conduct of numerous standardized acute aquatic toxicity tests for the MDI substances. Of all known and possible transformation products of the different constituents of the MDI category, it is only this transformation compound that has properties (water solubility, molecular weight) which could lead to meaningful potential for aquatic exposure. The physico-chemical properties and acute aquatic toxicity of the least hydrophobic and most bio-available representatives of the polyurea transformation compounds shows that these have very poor bioavailability and are essentially nontoxic. Differences in structural features among the expected polyurea transformation substances are of no consequence to this conclusion. Evidence from over twenty acute aquatic tests for nineof the sixteen substance of the MDI category and five common transformation compounds will be summarized to show that the invertebrates (i.e. Daphnia magna) are the most sensitive of species tested, and that MDA is the only relevant common transformation product of the category substances to which their aquatic toxicity potential is attributed.The acute aquatic toxicity potential of the substances of the MDI category is attributed to their potential to form MDA from transformation of their common mMDI constituents. The variation in observed median effective loading (EL50) values across the tested substances of the category can be related to variation in their mMDI contents (and associated distribution of the isomers) and variation with which the substances are dispersed into water during preparation of their wateraccommodated fractions. The mMDI isomers have exhibited the lowest EL50 values of the category substances tested, as they have the highest (essentially 100 %) mMDI contents. The apparent sensitivity of invertebrates (i.e. D. magna) relative to fish and algae for the category substances is also observed for other aniline type of substances. The potential contribution of structural differences among the category substances and their constituents to aquatic hazard potential is minimal. Only the monomeric MDI constituents can become sufficiently water-soluble through their transformation into amino-isocyanate and then to MDA. The variations in structure features, such as imparted by the different glycols or the condensation bonding features, make relatively small contributions to variation in hydrophobicity of the non-monomeric MDI constituents. These structure features are interconnected by chemical bonding elements such as urethane, urea, and imide linkages which are resistant to abiotic or biological transformations. Thus, the category substances belonging to the ‘Oligomeric MDI’, ‘MDI and its condensation products’, ‘MDI and its reaction products with glycols’ and the ‘MDI, its condensation products and reaction products with glycols’subgroups, all show uniformity in their acute aquatic hazard potential (EL50 values), especially when normalized to the MDA concentrations associated with their transformation under the defined testing conditions. When the mMDI constituents of such substances are intentionally removed, the acute aquatic hazard potential is also removed. The aquatic toxicity of the substances of the MDI category has been examined in acute studies using all three trophic levels. It should be noted that the way of media preparation according to current OECD testing guidelines and guidance documents promotes the generation of the common transformation product (MDA). Therefore, the results of these studies overestimate the exposure potential, and thus the aquatic toxicity potential of the MDI category substances. While neither fish nor algae showed adverse effects in studies with the three boundary substances (LL50 and EL50 values greater than 100 mg/L), toxic effects were observed in aquatic invertebrates. The measured MDA concentrations associated with these EL50 values indicate that MDA is the main driver for ecotoxicity. Due to the high reactivity of category substances with water, which rapidly results in the formation of inert polyurea, long-term aquatic studies are not deemed meaningful. The long-term effects of the common compound MDA have been adequately studied. The available data are reliable and therefore adequate for classification and risk assessment purposes.
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