Environmental risks of chemicals are still often assessed substance-by-substance, neglecting mixture effects. This may result in risk underestimations, as the typical exposure is toward multicomponent chemical "cocktails". We use the two well established mixture toxicity concepts (Concentration Addition (CA) and Independent Action (IA)) for providing a tiered outline for environmental hazard and risk assessments of mixtures, focusing on general industrial chemicals and assuming that the "base set" of data (EC50s for algae, crustaceans, fish) is available. As mixture toxicities higher than predicted by CA are rare findings, we suggest applying CA as a precautious first tier, irrespective of the modes/mechanisms of action of the mixture components. In particular, we prove that summing up PEC/PNEC ratios might serve as a justifiable CA-approximation, in order to estimate in a first tier assessment whether there is a potential risk for an exposed ecosystem if only base-set data are available. This makes optimum use of existing single substance assessments as more demanding mixture investigations are requested only if there are first indications of an environmental risk. Finally we suggest to call for mode-of-action driven analyses only if error estimations indicate the possibility for substantial differences between CA- and IA-based assessments.

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Abstract-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 longterm 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.