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Short-term toxicity to aquatic invertebrates

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Description of key information

The test substance is covered by the category approach of methylenediphenyl diisocyanates (MDI). The read-across category justification document is attached in IUCLID section 13.


Several ecotoxicity tests have been performed with invertebrates (D. magna) on the boundary substances and selected substances of the category. All studies were performed according to the OECD 202 guideline, with exposure solutions prepared according to OECD Guidance (OECD, 2019) in compliance with GLP and assigned a Klimisch score of 1 or 2. The aquatic toxicity of the tested substances was determined from exposures to a water accommodated fraction (WAF), as recommended in the OECD Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures (OECD, 2019). The invertebrate (D. magna) was much more sensitive to WAFs of the MDI substances than are the freshwater fish and algae. Therefore, testing according to OECD 202 was used to address the potential contribution of dissimilar structure features among the substances of the MDI category to the aquatic toxicity of the whole group. The daphnids were exposed to a range of effective loadings of the test substances dissolved in dilution water. The test substance was added to the medium by use of magnetic stirrer and stirred vigorously for 24 hours followed by filtration of particles by folded filters and aseptic filters. Observations were made on the swimming ability and the percentage immobilised after 24 and 48 hours of exposure. The content concentrations of the test item during the exposure period was verified by DOC-determinations as well as by substance specific analysis for the common compound MDA. Most recently, four selected category sub-group representatives have been further tested to determine the degree to which the presence of fine polyurea particles may have influenced the acute D. magna testing results by introducing potential physical toxicity (Neuland, 2021). The results of this repeated testing supported the finding that the presence of undissolved particles contributed to the effects observed in the initial runs of the studies. In all four cases, fine filtration of exposure solutions led to higher EL50 levels, i.e. lower toxicity of the tested substances, as compared to previous testing (Neuhahn and West, 2020). The tested substances showed results in the range EL50 (48h) of 3.7 to 31.7 mg/L. The measured MDA concentrations associated with these EL50 indicate that MDA is the main driver for ecotoxicity.


The ‘Monomeric MDI’ subgroup
Studies with and without humic acid were performed on 2,4'-MDI and 4,4'-MDI (Neuhahn, 2019) to determine if acute toxicity to the Daphnid is mitigated in the presence of dissolved organic matter (humic acid) such as occurs in natural water bodies.The addition of humic acid (5 mg/L) to the MDI exposures did not appear to mitigate the acute toxicity of MDI and MDA substance exposures to the Daphnid. However, only a brief time was allowed for dissolution and equilibration of the commercially-sourced humic acid into the exposure medium, which may not have provided a realistic representation of the physical/chemical state of natural organic matter occurring in the aquatic environment.


In the studies without humic acid, considered as key studies, the monomeric MDI isomers exhibited only slightly lower EL50 values compared to the other substances of the category. The 48-hour EL50 value for the boundary substance 4,4’-MDI was 9.0 mg/L and the lowest D. magna 48-hour EL50 value of 3.7 mg/L was obtained for 2,4’-MDI isomer. 


The ‘Oligomeric MDI’, ‘MDI and its condensation products’, ‘MDI, its condensation products and reaction products with glycols’ and ‘MDI and its reaction products with glycol’ subgroups
For the modified MDI substances belonging to all above mentioned subgroups, 48-hour EL50 values were between approximately 10 and 100 mg/L (based on whole substance loading rate) indicating that the glycol structure features do not significantly add to the aquatic toxicity potential exhibited by the mMDI. Except for the 4,4’-MDI Homopolymer substance, EL50 results in daphnids obtained are slightly below 10 mg/L (EL50 values of 7.4 mg/L and 8.9 mg/L respectively). The boundary substances pMDI and 4,4’-MDI/DPG/HMWP exhibit a 48h-EL50 value of greater than 10 mg/L (both Klimisch 1, Neuland (2021)). This observation is fully in line with the hypothesis on the aquatic toxicity of the category substances. As explained above, the effects observed are related to the amount of MDA that can be generated as side-product of hydrolysis from their common mMDI constituents. Accordingly, it is to be expected that category substances with a high content of mMDI, i.e. in particular the MDI isomers, show a lower EL50 value than category substances with rather low mMDI content, such as the modified MDI substances.


The 4,4’-MDI/DPG/HMWP - monomer-depleted
The hypothesis is also supported by effect values of 4,4’-MDI/DPG/HMWP – monomer-depleted substance, i.e. the substance from which the mMDI constituent was removed (by vacuum distillation). The 48-hour D. magna EL50 value was greater than 100 mg/L. This result provides evidence that the non-monomeric MDI constituents do not contribute to the acute aquatic toxicity.


The mono-urea of 4,4’-MDI and 4.4.’-MDA 
The studies described above have been supplemented by recently performed short-term daphnid toxicity studies (according to guideline OECD 202) on the (transient) transformation compounds, i.e. 4,4’-MDI- and 4,4’-MDA mono-urea (Neuhahn et al, 2019). Both compounds did not show toxic effects against daphnids (EL50 and EC50 values above 100 mg/L).


 


For the safety assessment however, a conservative approach has been followed and the lowest (worst-case) EL50 value of 3.7 mg/L is considered as key value for the PNECfreshwater derivation for all the MDI substances of the category.

Key value for chemical safety assessment

Fresh water invertebrates

Fresh water invertebrates
Dose descriptor:
EC50
Remarks:
an EL50 is reported instead
Effect concentration:
3.7 mg/L

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