Ethanethiol

Toxicity to aquatic algae and cyanobacteria

Description of key information

Effect concentrations for the toxicity of ethanethiol to Pseudokirchneriella subcapitata were: EbC50 72 hr biomass 0.75 mg/l; ErC50 72 hr growth rate 3.0 mg/l. The NOEC for both biomass and growth rate was 0.83 mg/l.

Key value for chemical safety assessment

EC50 for freshwater algae:

3 mg/L

Additional information

Thiebaud (2000) study is GLP compliant and has been according to OECD guideline but was not considered to be relevant for risk assessment purpose by reviewers in 2009. The test substance was not maintained within 80% of nominal concentrations, but a geometric mean value was calculated using the initial and final concentrations. The NOEC for both biomass and growth rate were determined using a statistical analytical method (Dunnett test), and the EC₁₀and EC₅₀ were determined by Probit analysis. The EC₁₀and EC₅₀ for biomass and the EC₁₀

for growth rate were extrapolated below the measured concentrations, and therefore caution should be taken when using these values. Nevertheless, the study has been considered reliable, with a Klimisch score of 2 by the IUCLID dataset 30.09.2009 was used as a critical study for the OECD SIDS Initial Assessment Report C2 -C4 Aliphatic Thiols for SIAM 30 (April 2010). A GLP-compliant study has also been conducted according to OECD TG 201 with an analogous substance, propane-2-thiol (isopropylmercaptan, CAS 75-33-2). Again, measured test substance concentrations were below 80% of nominal. The high loss of test item is most likely explained by adsorption effects and the high volatility of the test item. Although the test was conducted in closed vessels, losses could have occurred during media preparation and sampling. Recoveries from the freshly prepared test media samples were also <80% of nominal and test substance was not detected at the three lowest test concentrations at 0 hours. Based on initial measured concentrations, the 72 h EC₅₀for growth rate was 21.9 mg/l and the NOEC was 3.2 mg/l (Kley, 2011).

Although there are some methodological deficiencies with the available data for toxicity to algae, Thiebaud (2000) together with supporting data for the analogous substance propane-2-thiol (Kley, 2011), indicates that algae are less sensitive than invertebrates to ethanethiol. Further testing for this endpoint is therefore not considered to be necessary since acute toxicity to invertebrates is the critical value for determination of the Predicted No Effect Concentration.