Role of science and judgment in setting national ambient air quality standards: how low is low enough?

The Clean Air Act (CAA) requires listing as criteria air pollutants those pollutants that arise from multiple sources and are found across the United States. The original list included carbon monoxide, nitrogen oxides, sulfur oxides, particulate matter, photochemical oxidants (later regulated as ozone), and hydrocarbons. Later, the listing of hydrocarbons was revoked and lead was listed. The CAA requires the EPA Administrator to set National Ambient Air Quality Standards (NAAQS) for these pollutants using the "latest scientific knowledge" at levels that, in the judgment of the Administrator, are "requisite to protect public health" while "allowing an adequate margin of safety" without considering the cost of implementing the NAAQS. The NAAQS are set using scientific knowledge to inform the Administrator's policy judgments on each NAAQS. Recently, there has been increasing tension and debate over the role of scientific knowledge versus policy judgment in the setting of NAAQS. This paper reviews key elements of this debate drawing on the opinion of Supreme Court Justice Stephen Breyer, in Whitman v. American Trucking Associations, to resolve the conundrum posed by the CAA language. I conclude that scientists should carefully distinguish between their interpretations of scientific knowledge on specific pollutants and their personal preferences as to a given policy outcome (i.e., specific level and form of the NAAQS), recognizing that these are policy judgments as to acceptable levels of risk if the science does not identify a threshold level below which there are no identifiable health risks. These policy judgments are exclusively delegated by the CAA to the EPA Administrator who needs to articulate the basis for their policy judgments on the level and form of the NAAQS and associated level of acceptable risk.