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MIBK possesses a 96-hour LC50 in freshwater fish > 179 mg/L, and has been shown to possess a 96-hour EC50 in daphnia magna > 200 mg/L. The NOEC (21 d) of MIBK in the Daphnia sp. reproduction test was reported to be 78 mg/L (nominal) or 30-35 mg/L (measured). The growth inhibition test in freshwater Lemna gibba was > 146 mg/L. The toxicity to microorganisms is predicted to possess a 3 hour EC50 greater than 100 mg/L in activated sludge of a predominantly domestic sewage system, based on the read-across substance MIBC. Metabolic data demonstrate that methyl i-butyl carbinol is rapidly and extensively converted to the methyl i-butyl ketone via oxidation of the alcohol functional group by alcohol dehydrogenase in the liver. Methyl i-butyl ketone is subjected to further oxidative metabolism (hydroxylation) by the hepatic microsomal mixed function oxidase system to produce 4-hydroxymethyl-4-methyl-2-pentanone (i.e., diacetone alcohol), which is the major metabolite formed from both methyl i-butyl carbinol and methyl i-butyl ketone exposures. Thus, methyl i-butyl ketone may be used as an appropriate surrogate for methyl i-butyl carbinol and vice versa considering that the alcohol is rapidly metabolized to the ketone and that exposure to either substance ultimately results in the rapid formation of diacetone alcohol.The substance should therefore not be classified on the basis of absence of toxic effect whatever is the trophic level considered.

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