Setting Air Quality Standards for PM_2.5 : A Role for Subjective Uncertainty in NAAQS Quantitative Risk Assessments?

The U.S. Clean Air Act (CAA) requires the Administrator of the U.S. Environmental Protection Agency (EPA) to set and periodically review national ambient air quality standards (NAAQS) for criteria pollutants. Because NAAQS must be set without balancing health risks against cost, Administrators look for where health risk tapers off. For some pollutants, however, no evidence exists of such a diminishment. The Administrator must instead evaluate how the strength of evidence for the scientific validity of risk estimates weakens for exposure levels below the central mass of observations indicating a pollutant-health risk relationship. Such an evaluation requires judgments about uncertainties that are inherently subjective. The risk assessments the Agency prepares during NAAQS reviews provide a natural platform for quantitatively characterizing these subjective uncertainty judgments, but the Agency is no longer making use of this opportunity. This article describes EPA's early development of methods to quantitatively characterize subjective uncertainty in NAAQS risk assessments, then traces the progressive elimination of such uncertainty analysis in the risk assessments for the three past NAAQS reviews for fine particulate matter (PM_2.5 ), even while judgments about this uncertainty were becoming increasingly central to Administrators' NAAQS decisions. As a result, the risk assessments now lack relevance to NAAQS decision making. To reestablish a meaningful decision-support role for NAAQS risk assessments, this article suggests alterations to the process of preparing them. Taking no position on the scientific or legal appropriateness of past NAAQS decisions, the suggested process is intended to better synthesize the scientific evidence to better inform (without constraining) the Administrator's policy decisions.