The exposome encompasses an individual's exposure to exogenous chemicals, as well as endogenous chemicals that are produced or altered in response to external stressors. While the exposome concept has been established for human health, its principles can be extended to include broader ecological issues. The assessment of exposure is tightly interlinked with hazard assessment. Here, we explore if mechanistic understanding of the causal links between exposure and adverse effects on human health and the environment can be improved by integrating the exposome approach with the adverse outcome pathway (AOP) concept that structures and organizes the sequence of biological events from an initial molecular interaction of a chemical with a biological target to an adverse outcome. Complementing exposome research with the AOP concept may facilitate a mechanistic understanding of stress-induced adverse effects, examine the relative contributions from various components of the exposome, determine the primary risk drivers in complex mixtures, and promote an integrative assessment of chemical risks for both human and environmental health.

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Here, recommendations to improve ecological and chemical status assessments in accordance with the European Water Framework Directive (WFD) are made on the basis of experience gained from the MODELKEY project database, linking existing biological and chemical monitoring data of 3 case study river basins (Elbe, Scheldt, and Llobregat). The data analysis within and across river basins revealed major obstacles to be tackled, including scarcity of matching ecological and chemical monitoring sites for cause-effect relationships as well as a general lack of stressor-specific metrics for single biological quality elements (BQE) to enable a comprehensive risk assessment of all predominant stressors, including toxicity. An example of such a metric, which is recommended for the BQE of benthic macroinvertebrates, is the trait-based species-at-risk index (SPEAR) that correlated well with a respective measure for toxic stress, referred to as toxic units, based on simple mixture toxicity concepts. Surprisingly, the assessment of chemical status of a total of 695 monitoring sites for 2000 to 2004 showed that environmental quality standards (EQSs) were exceeded for at least 1 of the currently 41 priority pollutants (PPs) in 92% to 98% of the cases in all 3 of the river basins, which, according to definition, indicates potential effects on ecological status. A comparison of compliance with EQSs for 41 PPs with a respective effect threshold (derived for benthic macroinvertebrates) revealed that the rather conservative concept of chemical status is most likely not protective in all cases. Furthermore, to account for the many other compounds that are detected frequently in European surface waters and that may also have ecotoxicological effects, we introduced a provisional predicted no-effect concentration that is in accordance with the EQS methodology and is suggested to identify potential emerging compounds for which no or insufficient toxicity data exist. In conclusion, this study aims to support the implementation of the WFD by drawing conclusions from the analysis of heterogeneous data sets of various member states and by introducing new tools to move toward an integrated European assessment of ecological and chemical status.

The European Union Water Framework Directive (WFD) requires a good chemical and ecological status of European surface waters by 2015. Integrated, risk-based management of river basins is presumed to be an appropriate approach to achieve that goal. The approach of focusing on distinct hazardous substances in surface waters together with investment in best available technology for treatment of industrial and domestic effluents was successful in significantly reducing excessive contamination of several European river basins. The use of the concept of chemical status in the WFD is based on this experience and focuses on chemicals for which there is a general agreement that they should be phased out. However, the chemical status, based primarily on a list of 33 priority substances and 8 priority hazardous substances, considers only a small portion of possible toxicants and does not address all causes of ecotoxicological stress in general. Recommendations for further development of this concept are 1) to focus on river basin-specific toxicants, 2) to regularly update priority lists with a focus on emerging toxicants, 3) to consider state-of-the-art mixture toxicity concepts and bioavailability to link chemical and ecological status, and 4) to add a short list of priority effects and to develop environmental quality standards for these effects. The ecological status reflected by ecological quality ratios is a leading principle of the WFD. While on the European scale the improvement of hydromorphological conditions and control of eutrophication are crucial to achieve a good ecological status, on a local and regional scale managers have to deal with multiple pressures. On this scale, toxic pollution may play an important role. Strategic research is necessary 1) to identify dominant pressures, 2) to predict multistressor effects, 3) to develop stressor- and type-specific metrics of pressures, and 4) to better understand the ecology of recovery. The concept of reference conditions to define the ecological status is hard to apply and tends to ignore the fact that ecosystems can be highly dynamic. A better understanding of ecosystem responses to changes as well as early warning systems and concepts sensitive to various stressors to discriminate disturbances from natural variation are required. Because ecosystems are closely interconnected, an integrated monitoring, diagnosis, and stressors-based management of the whole water, sediment, groundwater, soil, and air system is required considering land use and the interaction with a changing climate. Extending this holistic approach beyond a consideration of existing pressures by anticipating on future ones to use and protect the aquatic environment in a sustainable way is one of the big challenges.

The assessment of persistence (P), bioaccumulation (B), and toxicity (T) of a chemical is a crucial first step at ensuring chemical safety and is a cornerstone of the European Union’s chemicals regulation REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals). Existing methods for PBT assessment are overly complex and cumbersome, have produced incorrect conclusions, and rely heavily on animal-intensive testing. We explore how new-approach methodologies (NAMs) can overcome the limitations of current PBT assessment. We propose two innovative hazard indicators, termed cumulative toxicity equivalents (CTE) and persistent toxicity equivalents (PTE). Together they are intended to replace existing PBT indicators and can also accommodate the emerging concept of PMT (where M stands for mobility). The proposed “toxicity equivalents” can be measured with high throughput in vitro bioassays. CTE refers to the toxic effects measured directly in any given sample, including single chemicals, substitution products, or mixtures. PTE is the equivalent measure of cumulative toxicity equivalents measured after simulated environmental degradation of the sample. With an appropriate panel of animal-free or alternative in vitro bioassays, CTE and PTE comprise key environmental and human health hazard indicators. CTE and PTE do not require analytical identification of transformation products and mixture components but instead prompt two key questions: is the chemical or mixture toxic, and is this toxicity persistent or can it be attenuated by environmental degradation? Taken together, the proposed hazard indicators CTE and PTE have the potential to integrate P, B/M and T assessment into one high-throughput experimental workflow that sidesteps the need for analytical measurements and will support the Chemicals Strategy for Sustainability of the European Union.