Overview of the Quantitative Risk Assessment for the Current Review of the National Ambient Air Quality Standards (NAAQS) for PM

ZACHARY PEKAR (1), Neal Fann(2), Harvey Richmond (3), Ellen Post (4), Bryan Hubbell (5), Beth Hassett-Sipple (6), Pradeep Rajan (7)

(1,2,3,5,6,7) US EPA, (4) Abt Associates, Inc.

     Abstract Number: 239
     Last modified: November 9, 2009

Abstract
As part of its review of the national ambient air quality standards (NAAQS) for particulate matter (PM), the U.S. EPA has developed a human health risk assessment designed to characterize the nature and magnitude of human health impacts associated with exposure to PM$_(2.5). For this poster presentation, we will provide an overview of the approach used in conducting the PM NAAQS risk assessment, with emphasis on enhancements incorporated into the current analysis, reflecting our improved understanding of factors related to PM$_(2.5) risk. The PM NAAQS risk assessment focuses on modeling risk for a set of 15 urban study areas within the continental U.S., with these study areas being chosen to provide coverage for attributes related to PM$_(2.5) exposure and risk (e.g., differences in PM$_(2.5) sources and composition, differences in population demographics and behavior). While this analysis builds on methodology used in earlier PM NAAQS risk assessments, enhancements have been incorporated into the current risk assessment to increase the utility of risk estimates to decision makers. Specifically, the current risk analysis incorporates the latest multi-city epidemiological studies as sources of the effects estimates used in estimating both long-term mortality impacts and short-term mortality and morbidity impacts. We also have incorporated the results of geochemical global modeling into the characterization of policy-relevant background (PRB), which provides an improved scientific basis for the estimates for this important input. In simulating ambient PM$_(2.5) levels under alternative (lower) standard levels, for a subset of the study areas, we have simulated local emissions control strategies in addition to the regional controls, reflected in the proportional roll-back approach which as been used in previous PM NAAQS risk assessments. For this analysis, we have framed the treatment of uncertainty using a four-tiered approach outlined by the World Health Organization. Our qualitative analysis of uncertainty now includes identification of potentially important sources of uncertainty along with a ranking (high, medium or low) with regard to the potential impact of each source on risk estimates. Our quantitative analysis of uncertainty is based on single- and multi-element sensitivity analyses designed not only to identify sources of uncertainty having potentially significant impacts on risk estimates, but also to provide an additional set of reasonable risk estimates that can help frame the core set of (higher confidence) risk estimates generated for the urban study areas. And finally, we have included two national-scale analyses that help assess the degree to which the 15 urban study areas represent the national distribution of PM$_(2.5)-related risk. These include: (a) a representativeness analysis providing a comparison of the 15 urban study areas against national distributions for key PM risk-related attributes and (b) a national-scale assessment of long-term mortality related to PM$_(2.5) exposure (reported at the county-level using recent air quality data for the continental U.S.). The latter analysis not only provides an estimate of the national impact of long-term PM$_(2.5) exposure on mortality, it also allows us to compare our urban study areas against the national PM$_(2.5) risk distribution.