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Environmental fate & pathways

Phototransformation in air

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Description of key information

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 atmospheric half-lives for all representative MDI constituents are predicted to range from 0.4 to 7.8 hours using the AOPWIN v1.92a model, when the background hydroxyl radical concentration of 1.5 x 10^6 molecules/cm3 is assumed. 


Photo-transformation of the category substances in water is not an important process affecting fate of the MDI category substances in the environment. It can be assured that this is a negligible removal mechanism, as the rate of hydrolysis would predominate over the rate of any photolytic degradation as exposure to sunlight in surface waters would also involve exposure to water. The lack of experimental or predicted photo-transformation in water data is not considered as being a deficit for the assessment of potential environmental exposures to the MDI substances.


The current knowledge around the reactions of aromatic diisocyanates in the atmosphere have been recently summarized in a report (Plehiers, 2020). The environmental fate of aromatic diisocyanates in the atmosphere has been summarized by Allport et al. (2003) and Tury et al. (2003). Two main reaction paths had been investigated: i) hydrolysis by water vapour, and ii) oxidation by hydroxyl radicals [.OH] generated by photochemical processes. Although the constituents of the MDI category substances do not react to any appreciable extent with water vapour in the air, they are not persistent in the troposphere, being readily degraded by reaction with hydroxyl radicals. For this reaction process, the mechanisms of both hydroxyl radical addition to aromatic ring carbons and hydrogen abstraction from aliphatic carbons are included in the predicted overall second-order reaction rate constants. The atmospheric half-lives for all representative MDI constituents are predicted to range from 0.4 to 7.8 hours when the background hydroxyl radical concentration of 1.5 x 10^6 molecules/cm3 is assumed. The substances would be ultimately converted to carbon dioxide, water, and inorganic nitrogen species unless otherwise deposited as aerosol particles. The differences in predicted reaction rate constants across the different constituents of the MDI category can be attributed to the differences in the number of aromatic rings (radical addition to ring carbons) and in number of aliphatic carbons (hydrogen-abstraction). Thus, the mMDI constituents which have the least number of aromatic rings and aliphatic carbons are the slowest to degrade. These predicted half-lives derived from the AOPWIN software are based on correlation of measured hydroxyl radical reaction rate constants with relevant structure features of the selected amount of representative constituents. Whereas the measured reaction rate constants involved substances which were studied in the vapour phase, it can be expected that similar reaction rate constants can be applied to the MDI category constituents which would occur as almost exclusively as condensation aerosols or adsorbed to particulates in the atmosphere. This atmospheric degradation of the MDI substances with hydroxyl radicals raises concerns that the process might lead to ozone or smog formation in the lower atmosphere. However physical deposition of diisocyanate from such low atmospheric concentrations should be very low. Any such deposition of aerosols of MDI substances onto surface water or soil is considered of negligible consequence because of the reactivity of these compounds in the aquatic environment. Losses of MDI substances to air are very low. This is partly because of their low volatility and also because of careful control of all aspects of their lifecycle: manufacture, transport, use, and disposal. Emissions of the MDI substances at any stage in their lifecycle, using current technology, are not expected to have an adverse impact on the environment.

Key value for chemical safety assessment

Half-life in air:
8 h

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