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National Ambient Air Quality Standards for Ozone
The 8-Hour Standard
In July 1997, U.S. EPA revised the National Ambient Air Quality Standards (NAAQS) for ozone to protect against longer exposure periods. The threshold value for both the primary and secondary 8-hour standard is 0.08 parts per million (ppm), measured as maximum daily 8-hour average concentrations. To attain the ozone NAAQS, the 3-year average of the annual 4th-highest daily maximum 8-hour ozone concentration must be less than or equal to 0.08 PPM
The standards were challenged by several business and state groups who claimed that EPA misinterpreted the Clean Air Act to give itself unlimited discretion to set air standards. On February 27, 2001, the US Supreme Court unanimously upheld the constitutionality of the Clean Air Act as EPA had interpreted it in setting those health-protective air quality standards. The Supreme Court also reaffirmed EPA's long-standing interpretation that it must set these standards based solely on public health considerations without consideration of costs. On March 26, 2002, the D.C. Court rejected all remaining challenges to EPA's 1997 NAAQS for ozone. Now, EPA, in partnership with state and local governments, is moving forward to implement the 8-hour standard.