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(T/MF163) A Proposed Approach to Estimating Uncertainty Factors for the Ecological Assessment of New Substances in Canada. Bonnell, Mark1, Postlethwaite, Emma2, Atkinson, Andy 2, Hammond, Greg2, 1 2 ABSTRACT- Characterizing uncertainty is an important undertaking at all levels of ecological risk assessment for well known reasons. It becomes even more important at the screening level because typically fewer data are used and the extrapolations to the real world greater. Shortcomings of the 10-fold approach to using uncertainty factors (UFs) led Environment Canada to re-examine the basis for deriving UFs when assessing the toxicity of new substances in order to provide a more transparent and traceable approach. The proposed approach calculates a Total Certainty Factor (TCF) based on component Certainty Factors (CFs) for pivotal study quality, inter- and intra-specific variation, acute to chronic extrapolation, severe effects to no effects extrapolation and laboratory to field extrapolation. The TCF is compared to a maximum acceptable level of uncertainty of 1000 to determine the Residual Uncertainty Factor (RUF). The results of 11 new substance case studies, which compared risk quotients calculated using the RUF and traditional 10-fold UF approaches, suggests that the RUF approach provides a more comprehensive, transparent and sensitive UF calculation procedure without being hyper-conservative. Key words: uncertainty factor, safety factor, new substances, ecological risk |
