Additivity in microbial toxicity of nonuniform mixtures of organic chemicals

Microbial toxicity of nonuniform mixtures of selected synthetic organic chemicals in several proportions is evaluated. Toxicity is quantified by the inhibition of oxygen uptake rate of a surrogate microbial test culture as measured by a respirometer. The joint toxic effects of the chemicals are analyzed for simple addition using toxic units (TU) and similarity parameters (λ). A new approach is proposed to assign acceptance limits to ΣTU₁ and λ to account for experimental errors and variances. Based on this approach, the joint toxic effects of 16 chemicals evaluated in this study in 14 different mixtures were found to be simply additive. Predictions of component concentrations based on simple additivity agreed with the measured values within an average factor of error of 1.4.