(3S-cis)-3,6-dimethyl-1,4-dioxane-2,5-dione

Short-term toxicity to fish

Description of key information

In short-term toxicity tests of lactic acid in three different fish species mortality was observed resulting in estimated LC50 values ranging between 130 and > 320 mg/L (nominal) which can be attributed to a pH effect. Lactate as such is considered not exert any systemic effects on fish in the tested concentration range of up to 560 mg/L. Since dilactide undergoes immediate hydrolysis in contact with water, forming lactic acid, this is the moiety of interest by read-across.

Additional information

Since dilactide undergoes immediate hydrolysis in contact with water, forming lactic acid, this is the moiety of interest by read-across. In four studies on short-term toxicity of lactic acid to fish (three different species: Danio rerio, Lepomis macrochirus, Oncorhynchus mykiss) mortality was observed resulting in estimated LC50 values ranging between 130 and > 320 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.

An additional published study (Molony, 2001; Ikuta et al., 1992) is quoted, demonstrating the detrimental effect of low pH values on survival of rainbow trout (O. mykiss): The LC50 (24 h) was determined at pH 3.83. If hypothetically exposed over the guideline-compliant period of 96 hours, the LC50 may be expected to be higher, i.e. approximately pH 4. Accordingly, the LC50 of pH 3.83 found by Ikuta et al. (1992) is considered as a worst-case in the context of evaluating pH effects of acids tested according to accepted test guidelines.

The theoretical pH shift caused by lactic acid at the lowest identified LC50 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 mortality observed in the available short-term toxicity tests on fish 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 560 mg/L) lactic acid does not cause lethal effects on fish, except pH related mortality. A key value (LC50) for chemical safety assessment hence cannot be identified.