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Short-term toxicity to aquatic invertebrates

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The acute toxicity of MBT 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 0.71 mg/L was obtained (National Institute of Technology and Evaluation, 1999).
MBT can be rapidly degraded by direct photolysis in water and the main degradation products appearing during the time frame of acute tests are benzothiazole (BT) and benzothiazolone (BTon) (Brownlee, 1992). 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 MBT are considered as relevant for environmental exposure and risk assessment, such as MBTS and MeSBT. For MBTS no acute toxic effect was observed to Daphnia magna up to its water solubility (Bayer AG, 1993). The acute toxicities of MeSBT to Daphnia magna was tested according to OECD TG 202 and the 48h- EC50 of 29.8 mg/l (nominal) was determined, which was analytically confirmed (AKZO NOBEL, 2004).
Regarding the data summarized here, the described metabolites of MBT are less toxic to aquatic invertebrates than MBT itself.

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Additional information

MBT can be rapidly degraded by direct photolysis with a half-life time of ca. 30 mins (CMA, 1989) and can be rapidly oxidized in water by atmospheric oxygen with 60% conversion in 2 hours (Hansson & Agrup, 1993). 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 MBT and its hydrolysis products like BT and oxidation product like MBTS, however BT and MBTS have much lower acute toxicities to aquatic invertebrates than MBT.