• Concentration addition;
  • Independent action;
  • Mixture toxicity;
  • Combined effects;
  • Vibrio fischeri


A promising tool for the risk assessment of chemical mixtures is the prediction of their toxicities from the effects of the individual components. For that purpose, concentration addition is uniformly regarded as valid for mixtures of similarly acting chemicals. Whether this concept or the competing notion of independent action is more appropriate for mixtures of dissimilarly acting chemicals is still in dispute. Therefore, the presented study analyzed and compared the predictive capabilities of both concepts for a multiple mixture designed of strictly dissimilarly acting compounds. Experimental investigations were conducted using a long-term bioluminescence inhibition assay with Vibrio fischeri. Results show an excellent predictive power of independent action, while concentration addition overestimates the mixture 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 only 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 concentration addition may give a realistic worst case estimation of mixture toxicities for risk assessment procedures.