UNIFORM SYSTEM FOR THE EVALUATION OF SUBSTANCES .2. EFFECTS ASSESSMENT

This is the second article in the series on USES, the Uniform System f or the Evaluation of Substances. In USES, hazards are expressed as PEC /NEC ratios, the Predicted Environmental Concentration, divided by the No-Effect Concentration. This article describes the extrapolation met hods used to derive concentrations that are assumed to be ''safe'' (No -Effect Concentrations, NECs, or No-Effect Levels, NELs) for the group s to be protected: humans, aquatic ecosystems and terrestrial ecosyste ms including top predators, and sewage treatment plant functioning. Fo r ecosystems, extrapolation factors of 10, 100 or 1000 are used (accor ding to a modification of the EPA method), but if at least 4 experimen tal No-Observed Effect Concentrations (NOECs) are known, a statistical method is used (Aldenberg and Slob). Alternatively, the extrapolation factors as proposed in the EC Guidance Document for new substances ca n be used. For humans, usually no extrapolation is carried out, and th e experimental test data of mammals are compared directly to the expos ure level to create Margin Of Safeties (MOS). This procedure leaves mo re room for expert judgement than a rigid extrapolation factor approac h. For hazard ranking, No-Effect Levels for humans is derived, making use of fixed extrapolation factors of 10, 100, 1000 or 10000, dependin g on the type of data available. The NECs and NELs will be applied in the hazard and risk characterization stage of USES.