The risk evaluation of difficult substances in USES 2.0 and EUSES. A decision tree for data gap filling of Kow, Koc and BCF

The decision tree gives reasonable worst case estimates for the Risk Characterisation Ratios of substances of which no Kow value can be estimated or measured. The decision tree only asks for more a detailed analysis of both sorption and bioaccumulation, when the predicted environmental concentrations are sufficiently high to make a difference for the outcome of the risk assessment. The report suggests to use the Kow value of the neutral molecule of acids and bases even when these molecules are present as anions or cations at pH 7.

The guidelines and suggestions in this report were based on the evaluation of a large data set with the Kow, soil sorption coefficient (Koc) and bioconcentration factor (BCF) of pesticides. It is argued that these guidelines and suggestions are valid for a wide range of "difficult" substances. The uncertainties encountered with the estimation of Koc and BCF from Kow for different classes of "difficult" substances were estimated and their influence on the outcome of the risk evaluation with USES 2.0 or EUSES was analysed.