Assessment groups for the evaluation of mixture toxicity

Retrospective environmental impact assessments, e.g. under the EU Water Framework Directive (WFD), are based on statutory environmental quality standards (EQS). These in turn are based on hazard assessments for direct toxicity to primary producers, invertebrates and vertebrates, direct toxicity to sediment organisms, secondary toxicity to piscivorous birds and mammals, and effects on human health following consumption of drinking water and fishery products. The lowest of these values is chosen as the final EQS.

To assess the risk for a substance, the measured environmental concentrations are compared with the EQS. The Ecotox Centre has developed an approach for assessing the toxicity of mixtures in which the resulting risk quotients are added up for all substances in a mixture that are relevant for an assessment group. So far, this approach has been applied to direct effects on primary producers, invertebrates and vertebrates and to secondary poisoning. In this case study, the approach will be extended to all WFD endpoints (i.e. toxicity to sediment organisms, human health via the consumption of drinking water and human health via the consumption of fishery products). To illustrate the approach, it will be applied to monitoring data and compared with predictions based on concentration addition, which are based on the underlying data in the EQS dossiers.

The project results may be useful for the risk assessment of mixtures under the WFD. A risk assessment for mixtures based on assessment groups is more scientifically sound and easier to communicate than the general mixture EQS for pesticides of 500 ng/l from the current WFD proposal and also takes into account the contribution of the other priority substances to the mixture risk.