A Nonattainment Area is a region in which the level of a Criteria Air Pollutant is higher than allowed by the federal NAAQS. For ground-level ozone, the one-hour standard is 120 parts per billion. EPA puts a city or metropolitan area on its "nonattainment" list when ozone exceeds, or violates, the standard more than once a year averaged over a three-year period.

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Secondary NAAQS ■■■■■■
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Criteria pollutant ■■■■■■
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Criteria ■■■■■■
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Background concentrations: ■■■■■
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Designated Pollutant ■■■■■
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