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Diss Factsheets

Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

The bioaccumulation potential of 2-methylundecanal has been assessed using a weight of evidence approach. The readily biodegradability status of 2-methylundecanal and an in vitro study showing that linear aldehydes are rapidly metabolised by trout liver S9 fractions, support a reduced BCF compared to estimates based only on log Kow. A regression QSAR corroborated with analogue data and models that take biotransformation into account all give BCF estimates below the REACH PBT B-criterion of 2000 L/Kg. The BCF estimates range from 34 to 778 L/kg. A cautious approach has been taken and the highest, valid estimate has been chosen as the key value for chemical safety assessment.

Key value for chemical safety assessment

BCF (aquatic species):
778 L/kg ww

Additional information

Reliable measured aquatic bioaccumulation data is not available for 2-methylundecanal. 

According to REACH Annex IX, information on bioaccumulation in aquatic species, preferable fish, is required for substances manufactured or imported in quantities of 100 t/y or more unless the substance has a low potential for bioaccumulation (for instance a log Kow ≤ 3). However, REACH Annex XI encourages the use of alternative information before a new vertebrate test, including fish, is conducted and according to the Integrated Testing Strategy (ITS, Figure R.7.10-2) in ECHA Guidance R.7c (Endpoint specific guidance), all available information should be assessed before further testing for bioaccumulation is performed.

For classification purposes, an experimentally derived high quality Log Kow value is suitable when a measured BCF on an aquatic organism is not available. 2-Methylundecanal has a measured log Kow of 4.9. This exceeds the CLP cut-off value of 4. Thus 2-methylundecanal is considered to have the potential to bioconcentrate for classification purposes.

For PBT/vPvB assessment purposes, predicted BCF estimates and log Kow may be used to provisionally assess bioaccumulation potential. A reliable estimated BCF value may also be used in the chemical safety assessment.

A consensus modelling approach was employed to predict bioconcentration factors in fish.The experimentally determined high quality Log Kow value of 4.9 was used as an input term in three commonly used and scientifically valid QSARs. The estimated BCF values ranged from 34 to 3095.

The BCF estimates of 3095L/kg and 2917 L/kg obtained respectively from the Arnot-Gobas BCF QSAR (assuming a biotransformation rate of zero) and the linear model developed by Veith et al (1979) are considered unrealistic maximum estimates. The Arnot-Gobas BCF QSAR (assuming a biotransformation rate of zero) was developed to fit upper bound BCF observations while the linear model developed by Veith et al (1979) is based on a limited data set of 56 chemicals, which are not expected to be metabolised.

2-Methylundecanal is readily biodegradable and it is generally accepted that readily biodegradable chemicals have a higher probability of being metabolised to a significant extent in exposed organisms than less biodegradable chemicals. In addition, an in vitro study for an analogue substance, dodecanal, shows that linear aldehydes are rapidly metabolised by trout liver S9 fractions. Therefore, 2-methylundecanal is expected to have a lower BCF than that predicted using models based only on log Kow.

The BCFBAF v3.01 regression model is based on log Kow and a series of correction factors if applicable. These correction factors were identified on an empirical basis but, in certain cases, may be rationalised on the basis of either known biotransformation reactions or likely reactivity. The “alkyl chain (8+ -CH2- groups)” correction factor is applied to 2-methylundecanal leading to a predicted BCF of 34 L/kg (log BCF 1.526). The use of this correction factor is supported by three compounds in the training set which share the “alkyl chain (8+ -CH2-groups)” structural feature: dodecane, 1,2-dibromodecane and 4-nonylphenol. All have significantly lower measured BCF values (240, 60 and 380 L/Kg respectively) compared to the predicted values (4897, 5130 and 2951 L/Kg) using the simple linear relationship between log BCF and log Kow. The BCF estimates for these analogues calculated from the regression model when the correction factor is applied are respectively, 209, 214 and 123 kg/L, and are much closer to the measured values indicating that this group of chemicals undergoes significant in vivo metabolism. The BCF values for 4-nonylphenol and dodecane are slightly under-estimated, equivalent to a log BCF estimation error of up to 0.49. Taking this worst-case deviation into account, the predicted BCF for 2-methylundecanal was adjusted to give a conservative estimated BCF of 104 (log BCF = 2.016). 

Since 2-methylundecanal is expected to be metabolised in fish, model predictions (including biotransformation rates) are also considered relevant. The Arnot-Gobas method predicts the whole body primary biotransformation rate (kM) for fish using a multiple-linear regression derived equation. The prediction is considered reliable for 2-methylundecanal as it falls within the descriptor domain and structural fragment domain of the kM model. The predicted kM is converted to a kM value for the typical body size of an upper trophic, middle trophic and lower trophic fish in the respective BCF algorithms. The resulting refined BCF estimates for 2-methylundecanal are 327, 642 and 778 L/kg respectively.

In summary, a regression QSAR corroborated with analogue data and models that take biotransformation into account give reliable BCF estimates in the range of 34 to 778 L/kg, well below the REACH B-criterion of 2000 L/Kg. Thus 2-methylundecanal is considered not to have potential to bioconcentrate for PBT/vPvB assessment purposes. The highest value of 778 L/kg has been chosen as a relevant and reliable conservative estimate for risk assessment purposes. Using this predicted BCF value, the chemical safety assessment does not show a need for further refinement.

Based on the above information, assessed together as a part of an overall WoE, it is concluded that there is no need for further investigation of aquatic bioaccumulation with fish.