Decisions in July 1997 by the Administrator of the Environmental Protection Agency (EPA) to revise the national ambient air quality standards (NAAQS) for ozone and particulate matter (PM) refocused attention on the criteria and the process by which these decisions are made. The new standards have been attacked both as overly stringent and as inadequately protective of health; as ignoring costs and as giving costs too much deference; as going beyond what is scientifically conclusive; and as failing to be sufficiently precautionary. These issues were the subject of numerous oversight hearings as well as litigation, which culminated in a Supreme Court ruling February 27, 2001. The court upheld those NAAQS setting procedures at question, in particular definitively rejecting the consideration of costs in setting NAAQS. However, the court's ruling also raised questions concerning the implementation of the EPA's new ozone standard. With the court's decision, continuing controversy over the PM and ozone standards, along with other concerns about the Clean Air Act (CAA), the expiration in 1998 of the authorizations for appropriations in the statute, and the Bush Administration's air quality initiative that would amend the Act, it is possible that the Congress may take up amendments to the CAA. If so, the NAAQS decisionmaking process may command attention, especially with respect to how scientific evidence is used – and possibly reopening the question of whether costs should be considered in setting NAAQS. Because of discrepancies in views on what transpired—or should have transpired—in EPA's development of the 1997 ozone and PM standards, this report provides background on the processes and procedures for setting and revising NAAQS. It lays out the steps of the decision process and identifies the statutory criteria established by the Act for NAAQS, to aid the reader's understanding of the Act's policy for NAAQS. It does not evaluate the decisionmaking process nor assess EPA's decisions on the ozone and PM NAAQS; nor does it address the several other categories of pollutants (e.g., hazardous air pollutants) or other standards-setting activities under the Act. The process by which EPA sets and revises NAAQS evolved over many years. Initially, federal air quality law focused on supporting state programs, through the conduct of research (a central, federal research program was more efficient than 50 state research efforts) and through technical and financial support (regulation was seen as a state and local matter). By the late 1960s, the federal research effort, located in the National Center for Air Pollution Control, Public Health Service, Department of Health, Education, and Welfare (HEW), included the preparation of "criteria"—a summary of scientific knowledge concerning selected air pollutants—and "guidelines"—a summary of control technologies. These criteria and guidelines were to support state programs to control air pollution. As environmental awareness heightened in the late 1960s, the federal role enlarged. With the Clean Air Act Amendments of 1970, the federal clean air program was moved to the newly created EPA and reconstituted, with the federal government becoming responsible for establishing "national ambient air quality standards" for air pollutants endangering public health or welfare and resulting from numerous or diverse mobile or stationary sources. The states remained primarily responsible for developing implementation plans to attain and maintain compliance with those national standards. Major amendments in 1977 and 1990 refined the NAAQS setting and implementation process. The 1977 amendments formally established the Clean Air Scientific Advisory Committee (CASAC) and the 5-year review process. The 1990 amendments increased federal guidance to states in implementing existing NAAQS. Thus the present process for setting and revising NAAQS consists of the statutory steps incorporated in the CAA over a series of amendments. In addition, several other steps of varying degrees of import have been added by the EPA, by executive orders, and by subsequent regulatory reform enactments by the Congress. The steps are depicted in Appendix A ; for an example chronology of the steps, see Appendix B . The documentary language spelling out those steps, along with explanatory notes, follows. The pollutants for which NAAQS are set are often called "criteria pollutants." This term reflects the evolution of the process. Before there were NAAQS, the Air Quality Act of 1967 (a predecessor of the CAA) required the Federal government to list air pollutants and to prepare "criteria"—a report summarizing scientific evidence concerning their health effects. The states then set standards. Thus the air pollutants for which NAAQS were later set were originally air pollutants for which the Office of Air Quality in HEW prepared "criteria." The current language specifying the listing of "criteria" air pollutants, as modified by the 1970 and 1977 amendments to the CAA, is as follows: The Administrator shall ... publish, and shall from time to time thereafter revise, a list which includes each air pollutant—(A) emissions of which, in his judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare; [and] (B) the presence of which in the ambient air results from numerous or diverse mobile or stationary sources .... [CAA §108(a)(1)] With the Clean Air Act Amendments of 1970, the EPA became responsible for determining national ambient air quality standards for the "criteria pollutants"—those of such national scope in release or effect that national standards are appropriate. Also, EPA was to assist states in attaining these standards through grants and technical assistance, and could impose sanctions on states for failures to fulfill their obligations. At present, six pollutants are designated "criteria pollutants" for which NAAQS have been set: particulate matter (PM), ozone (O 3 , a key measure of smog), nitrogen dioxide (NO 2, , or, inclusively, nitrogen oxides, NOx), sulfur oxides (SOx, or, specifically, SO 2 ), carbon monoxide (CO), and lead (Pb). The Administrator ... shall publish ... regulations prescribing a national primary ambient air quality standard and a national secondary ambient air quality standard for each air pollutant for which air quality criteria have been issued .... [CAA §109(a)(1)(A)] A NAAQS is a uniform, national standard establishing the maximum permissible concentration of an air pollutant in the ambient air—the "portion of the atmosphere, external to buildings, to which the general public has access." The CAA directs the Administrator to set a NAAQS at a level that provides protection from adverse effects on the public health and welfare. The Act provides for "primary standards" to protect health with a margin of safety and for "secondary standards" to protect welfare. National primary ambient air quality standards ... shall be ambient air quality standards the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health. [CAA §109(b)(1)] An ambient air quality standard ... should be the maximum permissible ambient air level of an air pollution agent or class of such agents (related to a period of time) which will protect the health of any group of the population. [Senate Report No. 91-1196, p. 10] As stated in §109, a NAAQS defines the acceptable concentration of an air pollutant in the ambient air necessary to protect health. As discussed later, costs are not considered in setting primary standards. Areas—defined as air quality control regions—complying with the standard are designated "attainment areas" and areas in which the pollutant exceeds the standard are designated "nonattainment areas." An area can be in attainment for one air pollutant and out of attainment for another. Although a primary NAAQS has a federally enforceable deadline, a NAAQS does not itself establish what to do when concentrations exceed the standard (nor how to protect clean air in attainment areas). Other provisions of the CAA lay out the process of implementation (specifically, §110). For nonattainment areas, this includes monitoring ambient air quality to determine compliance; requiring states to prepare state implementation plans (SIPs) to bring areas not in compliance into compliance; and requiring states to implement their plans to achieve and maintain the NAAQS by a specified deadline (Title I, Part D of the Act). Federal measures to protect air quality include new source performance standards (§111), which are national standards on specified categories of new sources of selected air pollutants, and mobile source emission standards (§202). Also, the federal government continues to prepare "guidance documents" spelling out available control measures for the NAAQS pollutants. Finally, in attainment areas where the air is cleaner than NAAQS, the CAA establishes a program for new source permitting to "maintain" that clean air quality (Title I, Part C). ... [A]llowing an adequate margin of safety .... [CAA §109(b)(1)] In setting such [national ambient air quality] standards the [Administrator] should consider and incorporate not only the results of research summarized in air quality criteria documents, but also the need for margins of safety. Margins of safety are essential to any health-related environmental standards if a reasonable degree of protection is to be provided against hazards which research has not yet identified. [Senate Committee on Public Works, Report No. 91-1196 (1970), pp. 9-10] The phrase, "margin of safety," has been central to several debates. The phrase seems to imply that NAAQS are based on thresholds—that there is a concentration of a pollutant below which adverse health effects do not occur. In this view, the Administrator determines the "no effect" threshold for the pollutant from the scientific evidence in the "criteria document" and then adds a safety factor. From this perspective, the issue is how large the margin of safety should be.