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Short-term toxicity to aquatic invertebrates

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In short-term toxicity test of lactic acid in water fleas immobilisation was observed resulting in estimated EC50 values ranging between 130 and 750 mg/L (nominal) which can be attributed to a pH effect. Lactate as such is considered not exert any systemic effects on daphnids in the tested concentration range of up to 750 mg/L. Since dilactide undergoes immediate hydrolysis in contact with water, forming lactic acid, this is the moiety of interest by read-across.

Key value for chemical safety assessment

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In three studies on short-term toxicity of lactic acid to water fleas immobilisation was observed resulting in estimated EC50 values ranging between 130 (measured) and 750 mg/L (nominal). All studies are relatively poorly documented and therefore reliable only with restrictions. However, they can be used in a weight-of-evidence approach. In one of the studies (Bowmer et al., 1998) the observed mortality was explicitly attributed to pH effects: pH at the test concentrations of 320 and 560 mg/L was measured to be 4.1 and 3.5.

The theoretical pH shift caused by lactic acid at the lowest identified EC50 of 130 mg/L (= 1.443 mmol/L), considering a pKa of 3.9, would result in a final pH of 3.43. This compares quite well to the measured pH values of 4.1 and 3.5 at test concentrations of 320 and 560 mg/L in a slightly buffered system (Bowmer et al., 1998).

Therefore, the immobilisation observed in the available short-term tests on water fleas can undoubtfully be attributed to a pH shift instead of being a toxic effect of the lactate ion/molecule. Accordingly, since any toxic effects of the lactate are of interest instead of acidification it can be concluded that within the tested concentration range (up to 750 mg/L) lactic acid does not cause adverse effects on invertebrates, except pH related toxicity. A key value (EC50) for chemical safety assessment hence cannot be identified. Since dilactide undergoes immediate hydrolysis in contact with water, forming lactic acid, this is the moiety of interest by read-across.