In evaluating PMNs, the Environmental Effects Branch attempts to estimate levels of a given compound which, if met or exceeded in the environment, could cause adverse effects. The data set from which such concentrations are estimated may run the gamut from fairly complete packages, containing both acute and chronic test data, to only the structure of the chemical itself. The purpose of this position document is to describe the procedure and rationale followed to estimate environmental concentrations of concern for a particular chemical. The methodology described should identify (at least 95% of the time) concentration concern levels for TSCA chemicals which may cause adverse environmental effects.