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lead. These pollutants are emitted from numerous and diverse sources, and at certain concentrations and length of exposure they are anticipated to endanger public health or welfare. The NAAQS are threshold concentrations based on a detailed review of the scientific information contained in criteria documents prepared by EPA and peer reviewed. Pollution concentrations below the NAAQS are intended to expected have no adverse effects for humans and the environment. For each criteria pollutant, NAAQs comprise: a primary standard, which is intended to protect the public health with a margin of safety, and a secondary standard, which is intended to protect the public welfare as measured by the effects of the pollutant on vegetation, materials, and visibility.

Primary and secondary NAAQS for ozone, which were originally called NAAQS for oxidants, were established by EPA in 1971. Photochemical oxidants, a group of chemically related pollutants, are defined as those compounds giving a positive response using the iodide oxidation technique. In 1979 EPA revised the NAAQS for oxidants to the current NAAQS for ozone only. Both the primary and secondary NAAQS for ozone are now defined as a daily maximum 1-hour average concentration of 0.12 parts per million (ppm), or 120 parts per billion (ppb), not to be exceeded on average more than once each year. The average number of days exceeding the standard is calculated for a 3-year period. Based on recent health effects studies, the ozone NAAQS requires the shortest averaging time of any of the criteria pollutants' NAAQS. An area, whose boundaries are designated by EPA, is considered to exceed this threshold and is classified as being in ''nonattainment'' if a violation occurs anywhere within the area. For ozone, areas have consisted of Metropolitan Statistical Areas (MSAs), Consolidated Metropolitan Statistical Areas (CMSAs), and counties. The Clean Air Act requires that a SIP be developed for areas in nonattainment to reduce precursor emissions enough to bring air quality into compliance with the NAAQS. SIPs must be adopted by local and state governments and then approved by EPA. Once a SIP is fully approved, it is legally binding under both state and federal law.

In the Clean Air Act amendments of 1970, Congress set 1975 as the deadline for meeting the NAAQS. By 1977, 2 years after this deadline, many areas were still in violation of the ozone NAAQS. The 1977 amendments to the Clean Air Act delayed compliance with the ozone and carbon monoxide NAAQS until 1982, and areas that demonstrated they could not meet the 1982 deadline were given extensions until 1987. In 1990, 3 years after the final deadline, more than 133 million Americans were living in the 96 areas that were not in attainment of the ozone NAAQS the year before (EPA, 1990b).

The 1990 amendments classify nonattainment areas according to degree of



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