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The most sensitive acute toxicity of MBT is to aquatic algae (Selenastrum capricornutum) tested according to OECD TG 201 "Alga, Growth Inhibition Test". After 72 hours of exposure, an ECr50 of 0.5mg/L was obtained. Besides the results from acute tests, the ecotoxicity of MBT was also determined in long-term tests to three trophic levels. Anembryo-larval test to Oncorhynchus mykiss carried out in a flow-through system resulted in a NOEC of 0.041 mg/l, which is the most sensitive one in three trophic levels and hence used for further risk assessment. For the assessment of microorganisms in biological treatment plants, Tomlinson (1966) studied the inhibition of the first nitrification step (oxidation of NH4 to NO2) and obtained after 2-4 h exposure an EC75 value of 3mg/l for non-adapted sludge. It was concluded in chapter 4.1 that MBT can be rapidly degraded by photolysis in water, the direct photolysis in water has a half-life of ca.30 mins. It is expected that in static aquatic toxicity tests the observed effects were caused by the parent substance and the degradation products. Analytical measurements reveal that in a flow-through system the influence of photolysis could be minimized. If the present quality criteria are strictly applied, the static acute tests, where no monitoring of the test concentrations occurred, should be regarded as not valid. The main degradation products appearing during the time frame of acute tests are mercaptobenzothiazole disulfide (MBTS), benzothiazole (BT) and benzothiazolone (BTon). MBTS, BT, BTon are much less toxic than MBT. Thus the results of the static tests with nominal concentrations can be considered as the upper limit for the actual effect value.