Higher Order Uncertainties in Probabilistic Ecological Risk Assessment. T. Aldenberg, RIVM, Bilthoven, The Netherlands; and J. S. Jaworska, Procter & Gamble, Strombeek-Bever, Belgium.
Environmental Risk Assessment is centered on the analysis of the ratio of the Predicted Environmental Concentration (PEC) to the Predicted No Effect Concentration (PNEC). Due to uncertainties in both quantities, one may encounter several cases depending on how this uncertainty is accounted for. When the PEC is uncertain, while the PNEC is a fixed value, e.g. a quality objective, the analysis amounts to the estimation of the probability of exceeding the objective. This fixed value PNEC may be derived as a percentile of the Species Sensitivity Distribution (SSD) of a toxicant. By taking the uncertainty of the PNEC into account one needs to solve PEC/PNEC>1 where both variables are random and independent. The cumulative distribution function of the SSD quantifies the Fraction of Species Affected (FA). Instead of taking a percentile of FA to estimate the PNEC, one can use the entire FA function as a measure of the effect at given PEC. A point estimate leads to a single curve estimate of the FA. The estimation of the FA from a small sample leads however to an uncertain PDF for the FA, which is of a secondary nature. If the PEC is distributed, then FA curve acts as a transformation of the PEC distribution to FA distribution specific to this PEC. The result is a secondary FA distribution. If the PEC distribution is a secondary PDF due to variability in time and space and uncertainty, then the FA uncertainty will be tertiary. A systematic treatment of higher order uncertainties is proposed, both with respect to terminology and quantitative evaluation.
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