called criteria document1 prepared by EPA scientists and consultants, and to set standards at levels that are “requisite to protect the public health with an adequate margin of safety.” The act does not include costs of implementation in the criteria for setting the NAAQS, and EPA—with consistent support from the courts—has interpreted that to mean that it may not consider costs in NAAQS decisions (American Trucking Associations, Inc. v. U.S. EPA, D.C. Cir 97-1440 and 97-144).

In essence, that has meant that each time a NAAQS is reviewed, the administrator must weigh the most recent evidence and continuing uncertainties and make a “public-health policy judgment” about whether the newest evidence provides enough certainty about the likelihood and public-health significance of effects above, at, and below the current standard to warrant a determination that the current standard is adequate to protect the public health with an adequate margin of safety or should be lowered or raised.

Once that determination is made for a particular pollutant or class of pollutants, EPA is expected to make decisions about four aspects of the standards:

  • The indicator (the pollutant to be monitored and assessed for attainment).

  • The level of the standard.

  • The averaging time (for example, 1 h, 8 h, 1 d, or 1 y).

  • The statistical form of the standard (for example, whether the standard will not be met if it is exceeded more than 1% of the time, on the third-highest day each year, or other similar measure).

Since the inception of NAAQS, EPA has determined that photochemical-oxidant air pollution, formed when specific chemicals in the air react with light and heat, is of sufficient public-health concern to merit establishment of a primary NAAQS. In implementing that determination, EPA has since 1979 identified ozone, a prominent member of the class of photochemical oxidants (such as nitrogen dioxide), as an indicator for setting the NAAQS and tracking whether areas of the country are in attainment of the standards. Over the last 38 y, as scientific understanding of ozone health effects has evolved, EPA has reviewed and updated, as needed, the primary NAAQS for ozone and other photochemical oxidants in 1971, 1979, 1993, and 1997 (see Table 2-1). As illustrated in Figure 2-1, under the 1997 NAAQS, ozone nonattainment has occurred largely in heavily populated areas east of the Mississippi River,


In December 2006, EPA indicated that after the current ozone NAAQS review process, it would no longer use the historical terminology of criteria document to summarize the science and staff paper to summarize staff risk assessment and recommendations to the administrator. The criteria document would be replaced by an integrated science assessment and the staff paper would be replaced by an advanced notice of proposed rulemaking (EPA 2007c).

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