The committee notes that reduction of uncertainty, the overall goal of its research agenda, is never complete. EPA by the mandate of the Clean Air Act with regard to the NAAQS is inevitably making decisions in a background of scientific uncertainty. As it seeks to satisfy the requirements of the Clean Air Act to set standards to protect public health with an “adequate margin of safety,” its decisionmaking inherently involves PM concentrations at which effects do not occur with known probability or certainty. Additionally, the health effects of PM and their mitigation are a topic of evident complexity. Often, resolution of one uncertainty only highlights other uncertainties that still cloud decisionmaking. Furthermore, as the methods of scientific inquiry deepen, uncertainties that have seemingly been resolved may again become a focus of research as new methods facilitate resolution at a more profound level of inquiry.

Although the committee addressed a broad range of research topics in evaluating scientific process, it did not choose to judge one of its initial recommendations, the establishment of centers for research on PM. The EPA Science Advisory Board (SAB) completed an evaluation of the centers in 2002 (EPASAB 2002).