Reaction mass of (E)-3-methyl-4-(2,6,6-trimethylcyclohex-2-en-1-yl)but-3-en-2-one and (E) -1-(2,6,6-trimethylcyclohex-2-en-1-yl)pent-1-en-3-one and (E) -1-(2,6,6-trimethylcyclohex-1-en-1-yl)pent-1-en-3-one

Toxicity to aquatic algae and cyanobacteria

Description of key information

Based on valid algal inhibition studies for two closely related structural analogues, the toxicity to freshwater green algae of the registered substance has been estimated:72h ErC50 value of >5.1 and <= 7.47 mg/L and NOEC of 1.68 to 2.3 mg/L. The EC50 value of > 5.1mg/L represents the fact that only 33% inhibition was observed at the highest test concentration of 5.1mgL (source study 1). Therefore the determined value of 7.47 mg/L (source study 2) has been chosen as the key value for chemical safety assessment. Out of the two reliable NOEC values, the lowest and more conservative NOEC of 1.68 mg/L has been selected.

Key value for chemical safety assessment

EC50 for freshwater algae:

7.47 mg/L

EC10 or NOEC for freshwater algae:

1.68 mg/L

Additional information

A study to assess the toxicity to aquatic algae and cyanobacteria is not available for the registration substance. However, valid algal inhibition studies exist for two analogue substances; Methyl Ionone Alpha Iso 60 (CAS 127-51-5) and Methyl Ionone (CAS 1335-46-2, trade quality Raldeine A GD). Both studies are GLP compliant, conducted to OECD 201 guidelines, covered the required exposure duration of 72 hours and included sufficient dose levels to enable the relevant determination of potency. The ErC50 value determined from each study was respectively >5.1 mg/L (Methyl Ionone Alpha Iso 60, study reliability 1) and 7.47 mg/L (Raldeine A GD, study reliability 1). The respective NOECS are 2.3 and 1.68 mg/L.

A detailed justification for the proposed read-across in-line with the ECHA RAAF guidelines is provided in the target endpoint record. This shows that the aquatic toxicity of the target substance and the two source substances, which consist of the same isomers but in different ratios, are expected to be similar on the basis that all individual isomers present in these substances will exhibit similar aquatic toxicity due to their structural similarity, same mode of action and similar lipophilicity and that, therefore, any variation in the ratio of isomers between substances will not significantly influence their aquatic toxicity properties.  This is supported by the above effect results for toxicity to green algae for both source substances and also the similar toxicity to daphnia (see endpoint 6.1.3).

Therefore, used in a weight-of-evidence approach the above green algae results are considered to give a reliable estimate of the toxicity to freshwater green algae of Reaction mass of (E)-3-methyl-4-(2,6,6-trimethylcyclohex-2-en-1-yl)but-3-en-2-one and (E)-1-(2,6,6-trimethylcyclohex-2-en-1-yl)pent-1-en-3-one and (E)-1-(2,6,6-trimethylcyclohex-1-en-1-yl)pent-1-en-3-one. Furthermore, the algae EC50 values and algae NOEC values are within the same environmental classification band and as such are considered adequate for regulatory purposes. The EC50 value of > 5.1mg/L represents the fact that only 33% inhibition was observed at the highest test concentration of 5.1mgL. Therefore the determined value of 7.47 mg/L has been chosen as the key value for the purposes of classification and labelling. Out of the two reliable NOEC values, the lowest and more conservative NOEC of 1.68 mg/L has been selected.