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Validation of toxicity data and risk limits for soils: final reportNo Observed Effect Concentrations of species or processes are used to derive statistical sensitivity distributions. Hazardous Concentrations (HC5 and HC50) derived from these distributions define the ecotoxicological risk limits. The project 'Validation of Toxicity Data and Risk Limits for Soils' was set up to evaluate the ecological relevance of (1) laboratory toxicity data and (2) ecotoxicological risk limits, and to identify which factors introduce uncertainties. Toxic effects of zinc and other metals were studied on selected species (Trifolium pratense, Folsomia candida, Enchytraeus crypticus, Eisenia andrei) and on microbial degradation activities. Differences in bioavailability between soil types were found to be of prime importance in laboratory-to-field extrapolation of toxicity data. Studies with mixtures suggested that each contaminant should be taken into account in risk assessment. Variable exposure conditions appeared to modulate toxicity moderately. Toxic effects of zinc on community endpoints were determined at an experimental field plot and a heavy-metal contaminated field site using enchytraeids, nematodes and micro-organisms. No or weak responses were found at the HC5 level, while measurable effects were present at the HC50 level. This was confirmed by data from the literature. The scientific underpinning of HC values may be improved if toxicity-modulating factors such as bioavailability are taken into account in the interpretation of laboratory toxicity data and if long-term effects of contaminants are focused on.