Application of QSARs in risk management of existing chemicals.

QSAR-estimates of aquatic toxicity of relatively unreactive (narcosis-type) organic chemicals for 19 species were used to predict "no-effect-levels" (NELs) at the ecosystem level by means of several extrapolation models which were recently discussed by the Organisation for Economic Cooperation and Development (OECD). It appeared that the differences between the numerical results of these extrapolation methods, despite their different statistical assumptions, were small. Equilibrium-partitioning theory for sediment and water as well as biota and water was used to derive NELs for aquatic sediments and whole-body residues. Simple equations are given, from which NELs for water, and subsequently for sediments and whole-body residues, can be predicted on the basis of only the octanol-water partition coefficient. Actual application of the models presented absolutely requires the availability of methods to recognize chemicals that act by the unspecific mechanism of narcosis. To this end, a classification scheme, based on molecular structure, was developed. This classification scheme was applied to the approximately 2000 so-called High Production Volume Chemicals (HPVCs) listed in Annex 1 of the Council Regulation on the Evaluation and the Control of the Environmental Risks of Existing Chemicals of the European Communities. From the classification of these 2000 HPVCs it is concluded that, for a large portion of all existing chemicals, reliable QSAR estimates of aquatic toxicity can be made; this considerably speeds up the process of priority setting and risk assessment of existing chemicals. The validity of this (dual) approach was tested by a comparison of estimated NELs with actual FCC values for all compounds from the US-EPA Water Quality Guidelines list that classify as narcosis-type compounds. This comparison shows that for many existing chemicals QSARs may be used to derive quality guidelines for water, aquatic sediments and residues in aquatic biota.