The USEtox model was developed in a scientific consensus process involving comparison of and harmonization between existing environmental multimedia fate models. For life cycle impact assessment, USEtox may be used as a comparative tool for ecosystem and human toxicity. As a
characterization model, it covers the entire impact pathway transforming a chemical emission into potential impacts on freshwater ecosystems based on quantitative modeling of fate, exposure and ecotoxicity effects.
Taken together, these are represented as chemical-specific characterization factors (CFs). In the case of freshwater ecotoxicity, impacts are measured as potentially affected or disappeared species [PAF m3-day / kgemitted].
Through analysis of the freshwater CFs of over three thousand organic chemicals, this work provides insight into the chemical properties that most strongly influence freshwater ecosystem toxicity for a variety of
emission scenarios. Furthermore, the analysis addresses the influence of chemical properties along the emission-fate-exposure-impact chain of events. The main trends are identified using results for the entire dataset
of chemicals, and typical patterns are illustrated for a small selection of chemicals with characteristic combinations of properties. For an emission directly to water, the effect factor, which is obtained from laboratory
measurements of substance toxicity to different trophic levels, strongly controls toxicity. Multimedia transfer affects the CF for these emissions by less than two orders of magnitude. However, for emission to air or soil,
intermedia transfer and degradation may decrease the CF by up to 10 orders of magnitude. This result shows the importance of the Henry's law constant, the organic carbon and octanol-water partitioning coefficient, the degradation half-life in various media, and the treatment of intermittent rain in the model. The interplay between these parameters and the model, which assumes a typical ratio of water to land surface area, shows that direct air to water transfer is less important for many hydrophilic
chemicals than might be suspected. As a result, for some compounds, second-order transfers, eg., from air to soil to water, are relatively more important. USEtox addresses some of the pressing problems in current life cycle impact assessment of chemical emissions by providing a consensus
model that can calculate transparent chemical-specific characterization