Predictability of the toxicity of a multiple mixture of dissimilarly acting chemicals to Vibrio fischeri

A promising tool for the risk assessment of chemical mixtures is the predic tion of their toxicities from the effects of the individual components. For that purpose, concentration addition is uniformly regarded as valid for mi xtures of similarly acting chemicals. Whether this concept or the competing notion of independent action is more appropriate for mixtures of dissimila rly acting chemicals is still in dispute. Therefore, the presented study an alyzed and compared the predictive capabilities of both concepts for a mult iple mixture designed of strictly dissimilarly acting compounds. Experiment al investigations were conducted using a long-term bioluminescence inhibiti on assay with Vibrio fischeri. Results show an excellent predictive power o f independent action, while concentration addition overestimates the mixtur e toxicity. Thus, the precise prediction of mixture toxicities depends on a valid assessment of the similarity/dissimilarity of the mixture components . However, concentration addition underestimates the EC50 of the mixture on ly by a factor of less than three. As the similarity of components is often unknown for mixtures found in the environment, it is concluded that concen tration addition may give a realistic worst case estimation of mixture toxi cities for risk assessment procedures.