Deriving predicted no-effect concentrations (PNECs) using a novel assessment factor method
Alexander I. Okonski, Drew B. MacDonald, Kelly Potter, Mark Bonnell
Abstract
Ecological risk assessments of substances with limited ecotoxicity datasets typically employ an assessment factor (AF) method to determine a predicted no-effect concentration (PNEC) needed for risk characterization. A PNEC is usually derived by dividing the lowest toxicity value in the substance’s dataset by a certain assessment factor. The AF method, in its various iterations, has been used for decades, and has been criticized for its uncertainty, inaccuracy, over-conservatism, high variability, and lack of transparency. A novel assessment factor method is proposed as an alternative to traditional AF methods. The proposed method offers novel features attempting to address these criticisms: it breaks the factor down into three components - endpoint standardization, extrapolation for species variation, and mode-of-action consideration (each of which has sub-components). This method also accommodates the inclusion of read-across and modeled data to fill data gaps for the substance. In addition, it better manages atypical datasets and atypical endpoints. Finally, and paramount to this endeavor, this novel method improves consistency among risk assessors, and can increase transparency of the entire PNEC derivation process to all stakeholders.