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(M/MF136) Risk assessment of mixtures of parent compounds and transformation products in a multimedia fate model. Kooijman, Cornelis1, Fenner, Kathrin 1, Scheringer, Martin1, Hungerbuehler, Konrad1, 1 ABSTRACT- The current methodology for chemical risk assessment according to the Technical Guidance Document of the EU is suitable to describe the risk of single substances only. According to this methodology, risk is characterized by comparing a chemical's predicted or measured environmental concentration to its no-effect toxicity threshold. In reality, many chemicals are not directly mineralised, but are transformed to other substances, which might have a similar or even higher toxicity. In this case, two new aspects need to be considered in the risk assessment. First, the toxicity of the resulting mixture of the simultaneously available parent compound and transformation products needs to be assessed; second, a kinetic model has to be set up which allows one to calculate steady-state concentrations for the transformation products as they are formed out of the parent compound. In this study, we introduce a four-compartment level III multimedia model including a mathematical algorithm to calculate the steady-state concentrations of transformation products and parent compound (1), and compare the resulting concentrations to those obtained from a conventional single substance risk assessment. Next, we investigate possible definitions for the risk quotients of single transformation products as well as for the overall risk quotient of the resulting mixture of parent compound and transformation products, including mixture toxicity. The new methodology is applied to the case study of nonylphenol ethoxylate usage in Switzerland, and the applicability of the newly defined risk quotients is discussed. (1) Fenner, K.; Scheringer, M.; Hungerbuehler, K. Environ. Sci. Technol. 2000, 34, 1842-1850. Key words: transformation products, risk characterization |
