Institute of Population Health
R. Samuel McLaughlin Centre for Population Health Risk Assessment


University of Ottawa
Categorical Default Approaches to Uncertainty Factor Development in Chemical Risk Assessment
Senate room (083) in Tabaret Hall, 550 Cumberland St

John C. Lipscomb PhD, DABT, F.A.T.S.
National Center for Environmental Assessment,
Office of Research and Development,
U. S. Environmental Protection Agency, Cincinnati, OH
 Thursday, October 30th, 2:00 PM

There have been a number of recent developments designed to incorporate increasingly data-informed approaches to consideration of uncertainty in human health risk assessment.  Traditional default uncertainty factors of 10 fold each for interspecies (animal to human) differences and intraspecies (human) variability have been parsed into finer constituents, to address toxicokinetic (TK) and toxicodynamic (TD) components as a basis for chemical specific adjustments.  An intermediate option between default and chemical specific adjustments, is categorical approaches, which draw on data from multiple chemicals as a basis to make and apply some generalizations.  For example, Renwick and colleagues have recently evaluated interspecies differences in the TK of compounds categorized according to the key enzymes in their metabolic pathways. 

This presentation will discuss two other methods to develop categorical default values for interspecies extrapolation for inhaled and ingested substances.  For inhaled substances, categorical default factors are based on measures of tissue exposure at the site of toxic insult – whether in the respiratory tract or in systemic tissues. For ingested substances, body weight scaling for interspecies based on allometric relationships may be used.  The application of these factors requires little data other than knowledge of route of administration and site of action and they represent one step in the increasingly data-informed continuum between default and chemical-specific factors. 
Dr. Lipscomb is a chemical manager developing risk assessment files for the US EPA Integrated Risk Information System (IRIS).  He is leading the efforts of the US EPA’s Risk Assessment Forum to produce guidance for Data Derived Extrapolation Factors, and is active also in developing methodology to address chemical mixtures and cumulative risk. He is an Associate Editor for Toxicological Sciences, and with Edward Ohanian edited Toxicokinetics and Risk Assessment (Informa, 2007).  He has also worked with the International Programme on Chemical Safety on initiatives for Chemical-Specific Adjustment Factors and Best Practices for Physiologically Based Pharmacokinetic Modeling for Risk Assessment. 

Bette Meek
McLaughlin Centre for Population Health Risk assessment, University of Ottawa
Introductory presentation

John C. Lipscomp
Presentation on Categorical default approaches for uncertainty factors in chemical risk assessment