John Wiley & Sons, Ltd.

Critical issues in using the common mixture toxicity models CA or RA on species sensitivity distributions: A theoretical approach

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The risk of chemical mixtures to ecosystems is often assessed by applying the model of concentration addition (CA) or response addition (RA) combined with species sensitivity distribution curves (SSD). However, the mixture effect predictions were shown to be consistent only when these models are applied for a single species, and not with several species simultaneously aggregated to SSDs. The more stringent procedure for mixture risk assessment would hence be to apply first the CA or RA models to each species separately and, in a second step, to combine the results to construct an SSD for a mixture. Unfortunately, this methodology is not applicable in most cases because it requires large data sets usually not available. Based on theoretical data sets generated, we aim to characterize the difference that can exist between these two methodologies. Results show that the use of CA on SSD directly may lead to underestimations of the mixture concentration affecting 5% or 50% of species, especially when substances present a large standard deviation in ecotoxicity data constructing their SSD. The application of RA can lead to over‐ or underestimations, depending mainly on the slope of the dose‐response curves of the individual species. When assessing the risk of mixtures, one has therefore to keep in mind this source of error when applying CA or RA to SSDs directly. Environ Toxicol Chem © 2013 SETAC

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