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Characterization of Data Variability and Uncertainty: Health Effects Assessments in the Integrated Risk Information System (IRIS)

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In response to a Congressional directive contained in HR 106-379 regarding EPA's appropriations for FY2000, EPA has undertaken an evaluation of the characterization of data variability and uncertainty in its Integrated Risk Information System (IRIS) health effects information database. Through consultation with EPA's Science Advisory Board, EPA developed and implemented a systematic plan to select a representative sample of chemical assessments in IRIS to be evaluated in-depth by an independent panel of experts for the extent to which EPA has documented uncertainty and variability. EPA conducted a screening evaluation on 10% percent of the IRIS summaries of chemical assessments completed during the period of 1988-1994 (52 of 522 pre-Pilot assessments) and all 15 Pilot/post-Pilot IRIS summaries and Toxicological Reviews (completed after 1995) for overall documentation of data variability and uncertainty. An EPA contractor then selected 16 assessments (IRIS summaries and support documents) for in-depth examination from the screening sample (8 of 52 pre-Pilot and 8 of 15 Pilot/post-Pilot). The contractor selected six independent experts (outside EPA) in the field of human health risk assessment, who performed this in-depth review. In general, the outside experts concluded that the characterization of data variability and uncertainty varied across the assessments they reviewed. While the documentation of data variability and uncertainty has generally improved since the IRIS Pilot's introduction of Toxicological Reviews to substantiate IRIS summaries, the reviewers found that the quality of the characterization of data variability and uncertainty varied among the Pilot/post-Pilot assessments. The reviewers also suggested ways to describe uncertainty and variability, and a number of scientific improvements, especially the need to update older assessments with more recent scientific data and risk assessment methods. This study supports EPA's goal to make the scientific bases for risk assessment conclusions more transparent. EPA will continue to look into ways to improve the characterization and documentation of data variability and uncertainty in future IRIS assessments.

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Attachments Include: EPA Screening Evaluation Report: Presentation and Discussion of Uncertainty and Variability in IRIS Assessment (PDF version 162K) Versar Report: Characterization of Data Uncertainty and Variability in IRIS Assessments, Pre-Pilot vs. Pilot/post-Pilot (PDF version 64K) Appendix A of Versar Report : Individual reports of experts assembled by Versar, Inc. (PDF version 574K)

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