In nature, individuals are frequently exposed to mixtures of chemical and non-chemical stressors. Estimating the interactive effects of single and multiple chemicals with non-chemical stressors is a challenge, which aims to integrate human health and environmental effect assessment for mixtures, both within xenobiotics and between those and natural stressors. Among natural stressors, ultraviolet (UV) light has lately increased in relevance, as concentrations of stratospheric ozone are reduced leading to increasing intensity of UV-B radiation reaching Earth's surface and stressing the biota.
The current European regulatory standard for evaluating the environmental fate of xenobiotics is the multimedia model EUSES. EUSES lacks a functionality to relate total compound concentrations to matrix-specific effective concentrations that are governing the resultant risk potential, and proper models for predicting abiotic and biotic degradation rates as well as prevalent metabolites from molecular structure. Also, the employment of generic compartments without a spatial resolution of climate and media-specific parameters, as well as the inability to address temporal variations, put serious limitations to the current EU risk assessment of toxic substances, in particular the risks associated with the interaction of chemicals-natural stressors. New developments are needed for realistic exposure and for measurement of combined exposures.
The foremost aim of this proposal is to develop and use improved assessment tools and novel models, aiming at reducing uncertainty in current risk assessment and screening methodologies, e.g. by improving the scientific basis for setting safety factors in the current risk assessment methodologies. In order to contribute to the development of more accurate predictive conceptual models, mandatory requirement for risk assessment with a scientific basis, we will use a key bioindicator species in aquatic ecosystems, Daphnia magna, as a model organism and undertake mechanistic and experimental studies with the purpose of understanding the toxicity mechanisms for the ultraviolet (UV) radiation stress. These mechanisms will be considered in the assessment of the effects of cumulative exposure to UV radiation and chemical substances. We are particularly interested in the impact of non-lethal doses of UV radiation, which are currently relevant due to the slow increases in the levels of UV radiation crossing the protective stratospheric ozone layer, under depletion for some time. One of the aims of the project is to understand the effect of non-lethal doses of UV and chemical compounds on the organism's morphology, physiology and reproduction in single exposures. The contribution of different DNA repair mechanisms will also be evaluated. Secondly, the production of reactive oxygen species, triggered by UV exposure, will be assessed, as well as the interplay between oxidative stress and DNA repair. After having characterized the toxicity mechanisms for UV and chemical exposure, this will be considered in the assessment of the effects of cumulative exposure to UV radiation and chemical substances. A conceptual model to predict effects of UV exposure on chemicals toxicity will be developed and validated. Conclusions inferred from this approach will be used to generate test rules for risk assessment of combined action of different stressors. These hypotheses for rules may then be used in cumulative risk assessments conducted for exposure scenarios of relevance, leading to a reduction in uncertainty in the process of risk assessment.
