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arbitrarily selected as a conservative threshold for adverse <br />effects. <br />Results <br />Toxicant Solutions <br />Chemical analysis confirmed accuracy of exposure <br />concentrations. Measured copper and zinc concentrations averaged <br />92% (SE = 4.8) and 84% (SE = 0.50) of nominal, respectively. <br />Because measured and nominal concentrations were in close <br />agreement, statistical analyses were based on nominal <br />concentrations. <br />Behavioral Assay <br />Logistic regression showed that olfactory inhibition <br />increased as a function of toxicant concentration after 24-h <br />exposure to copper and zinc, and 96-h exposure to copper, but not <br />after 96-h exposure to zinc (Table 2). The EC50s and 95% <br />confidence limits (in parentheses) were 36.1 (22.6, 57.9) µg/L <br />copper and 1640 (805, 3360) µg/L zinc for 24-h exposures, and <br />59.0 (40.2, 86.5) µg/L copper for 96-h exposure. The ECls were <br />0.888 (0.155, 5.12) µg/L copper and 135 (16.1, 1150) µg/L <br />zinc for 24-h exposures, and 15 (5.69, 43.6) µg/L copper for 96-h <br />exposure. These results suggest that copper is a more potent <br />olfactory toxicant than zinc. They also suggest that both <br />toxicants had greater inhibitory effects after 24-h exposure than <br />10 <br />