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Short-term toxicity to aquatic invertebrates

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The acute toxicity of CBS to aquatic invertebrates (Dapnia magna) was tested according to OECD TG 202 "Daphnia sp. Acute Immobilisation test". After 48 hours of exposure an EC50 of 0.79 mg/L was obtained (National Institute of Technology and Evaluation, 1996).
CBS is rapidly degraded by hydrolysis and the main degradation products appearing during the time frame of acute tests are benzothiazole (BT) and benzothiazolone (BTon). The acute toxicity of BT to Daphnia magna was tested according to a guideline of U.S. Environmental Protection Agency (1975). The LC50 of 20 mg/l (nominal) and NOEC of 5.6 mg/l (nominal) were obtained (Analytical Bio-Chemistry Laboratories, Inc.,1978). According to OECD TG 202, the acute toxicity of BTon to Daphnia magna was determined to be a 48h-EC50 of 16.1mg/l (nominal). The nominal concentration was confirmed analytically (AKZO NOBEL, 2004).
There are some other benzothiazole derivatives as metabolites from CBS are considered as relevant for environmental exposure and risk assessment, such as MeBT, MBT, MBTS and MeSBT. The acute toxicities of MeBT and MeSBT to Daphnia magna were tested according to OECD TG 202 and the 48h- EC50 of 29.8 mg/l and 8.5mg/l (nominal) were determined, respectively, which were analytically confirmed (AKZO NOBEL, 2004). A 48h-LC50 of 4.1 mg/l of MBT to Daphnia magna was reported (Monsanto, 1979) as a nominal concentration with the application of acetone. For MBTS no acute toxic effect was observed to Daphnia magna up to its water solubility (Bayer AG, 1993).
Regarding the data summarized here, the described metabolites of CBS are less toxic to aquatic invertebrates than CBS itself.

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Additional information

A hydrolysis half-life time of CBS of 12.5 h and a complete hydrolysis of CBS in 24.9 h were observed in deionized water buffered at pH 7.0 and at 25°C (Monsanto, 1984). MITI online did not provide detailed information, whether the effect concentrations were nominal concentrations or from analytical monitoring. The observed effect is most probably a combined effect from both CBS and its hydrolysis products. Because the reported EC50 is higher than the water solubility of CBS (i.e. 0.32 mg/l), vehicle was most probably used in the study however detailed information regarding this issue is not available.

The LC50 reported in Monsanto 48 h acute Daphnia test is much higher than water solubility of CBS (i.e. 0.32 mg/l) with the help of co-solvent of acetone. Therefore, in 48 h of test CBS is expected to have been hydrolyzed and further dissolved in water and so on. Unfortunately this is not analytically confirmed. The observed effect (EC50 = 18 mg/l) is hence also a combined effect from both CBS and its hydrolysis products.