Method for Assessing Additive Toxicity of Chemical Mixtures

Mixtures of chemicals or pesticides may produce unexpected effects; some are hazardous and some are beneficial. A method was divised in which individual toxic contributions of chemicals are summed, and the additive toxicity is defined by an index for two or more chemicals in combination. This linear index expresses the toxicity quantitatively: zero indicates simple additive toxicity, negative values indicate less than additive toxicity, and positive values indicate greater than additive toxicity. The significance of index values near zero is assessed by substituting values from the 95 percent confidence intervals into the formula to determine whether the range for the additive indices overlaps zero (simple additive toxicity). The range is derived by selecting values of the 95 percent confidence interval yielding the greatest deviation from the additive index. Mixtures of malathion and Delnav were found to be extremely toxic, and the additive index for the mixture against rainbow trout (Salmo gairdneri) was 7.20; that is, highly synergistic. Mixtures of copper, zinc, and nickel were additive in toxicity to rainbow trout. Mixtures of commercial synergizers sulfoxide or piperonyl butoxide with rotenone produced greater than additive toxicity against rainbow trout (indices = +2.13 and 2.36).