Copyright 2008 Air & Waste Management Association awma.org july 2008 em 29 The Clean Air Interstate Rule (CAIR) will further reduce regional emissions of sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ), thus reducing fine particulate matter (PM 2.5 ) and ground-level ozone pollution in the eastern United States. The U.S. Environmental Protection Agency (EPA) estimates that CAIR will provide the largest benefits of any Clean Air Act rule issued in the past 12 years. Regulations such as CAIR, however, come at a substantial economic cost. Moreover, understanding whether we have sufficiently protected the public is of critical con- cern. 1,2 Thus, determining whether regulatory actions actually reduce air pollution and improve public health and the environment is an important step in environ- mental policy implementation. This article presents an “accountability” framework for evaluating the impact of CAIR that consists of “met- rics” (i.e., predictions of changes associated with the promulgation of CAIR) and “indicators” (i.e., actual levels of the same or closely related parameters observed during the implementation of CAIR). The basic chal- lenge in this evaluation is that only the actual value of an indicator parameter will be observable, not the incremental change caused by CAIR. Thus, modeling is used to understand and supplement the observed parameter. The approach presented here addresses the regional-scale transport of pollutants and the indica- tors and techniques needed to discern a relatively small signal of change embedded in a highly confounded set of outcomes. The NO x State Implementation Plan (SIP) Call (implemented by the NO x Budget Trading Program [NBP]) is used as an example to discuss how some of em feature Valerie Garcia is with the U.S. Environmental Protection Agency (EPA), Office of Research Development, National Research Exposure Laboratory in Research Triangle Park, NC; Neal Fann and Phil Lorang are both with the EPA, Office of Air and Radiation, Office of Air Quality Planning and Standards in Research Triangle Park, NC; and Richard Haeuber is with the EPA, Office of Air and Radiation, Office of Atmospheric Programs, Washington, DC. E-mail: garcia.val@epa.gov. Assessing the Public Health Impact of Regional-Scale Air Quality Regulations by Valerie Garcia, Neal Fann, Richard Haeuber, and Phil Lorang