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Bioaccumulation: aquatic / sediment

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Description of key information

Due to the high reactivity of the substances of the MDI category with water, bioaccumulation tests can in principle not be performed with these substances. However, one bioaccumulation test (OECD 305E) with 4,4'-MDI (CERI, 2000) and a supporting mesocosm study with pMDI with an indication of bioaccumulation potential have been performed (Heimbach, 1993). As no analytical measurements were done, it cannot be determined if the values are truly related to MDI. However, based on the available information and the reactivity of MDI substances of the category approach, no new bioaccumulation study is deemed necessary.

Key value for chemical safety assessment

BCF (aquatic species):
200 dimensionless

Additional 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. It is recognised that there is a measured log Pow value of 4.52 for monomeric MDI (determined with MDI mixed isomers) (Yakabe et al., 2000). This log Pow value is only of theoretical interest as MDI is very unstable in aqueous media (hydrolysis half-life T1/2 under homogeneous conditions < 5 min.), see Section 5.1.2: Hydrolysis). The determination was by an HPLC method and comparison to reference chemicals.

A BCF study (CERI, 2002) carried out according to guideline principles with [14C] radiolabelled 4,4’-MDI and Cyprinus carpio, derives BCF values of 92 and 200 for concentrations of 0.8 and 0.08 µg MDI/L. The reported BCF values are highly questionable given the known rapid hydrolysis of MDI under these conditions. Liquid scintillation counting was the method of analysis for all water and fish samples and no analyses of 4,4’-MDI itself were carried out during the course of the study. A BCF value of 200 indicates low potential for bioaccumulation, but it must be recognised that this value probably reflects the bioconcentration of water soluble hydrolysis products which most likely include 4,4’-MDA and low molecular weight urea. So it can be concluded that degradation products of MDI do not bioaccumulate.

MDI reacts with water to form predominantly high molecular weight, inert polyureas and trace amounts of 4,4’-methylenedianiline (MDA). This MDA is considered as the only degradation product of significance, and it has a log Pow value of 1.55. Its measured BCF is < 14 (at 0.2 mg/L) in Cyprinus carpio. So, it is concluded that degradation products of MDI do not bioaccumulate.

There is a mesocosm study on pMDI (Heimbach 1993) study, carried out over 112 days, at a loading of 10,000 mg pMDI/L. MDI was detected neither in the water (detection limit 0.005 mg/L) nor in fish (detection limit 0.51 mg/kg). A numeric BCF can not be calculated from these non-detects, but clearly these results show in a practical way that MDI does not accumulate in fish.

Lastly, a BCF value of 439 for 4,4’-MDI may be estimated using the BCFBAFTM (v 3.00) model, a part of the Estimation Program Interface Suite (v.4.00, 2009) of QSAR tools (Tury, 2010). An estimated BCF value of 439 for 4,4’-MDI suggests a low potential for bioaccumulation. This is in line with the low bioaccumulation value found in the bioaccumulation study and in the mesocosm study.

These studies have been supplemented by an additional in-vitro fish liver metabolism study recently performed on a (transient) transformation compound, i.e. diamino-monourea of MDA, the most water-soluble and lowest molecular weight polyurea species. In a recent in-vitro fish liver metabolism study (OECD 319A) (Ebersbach and Windisch, 2019) performed with the transient transformation compound diamino-monourea of MDA, a similar low bioaccumulation potential was observed.

To summarise, a new bioaccumulation study is not deemed necessary due to rapid hydrolysis (resulting in unlikely exposure) of substance of the MDI category. Supporting data including a mesocosm study, modelling and assessment of degradation products, all indicate no potential for significant bioaccumulation in the aquatic environment.