The Environmental Protection Agency (EPA) has issued a final rule addressing which areas of the United States are in attainment of the 2015 ozone National Ambient Air Quality Standard (NAAQS). The agency found that 2,646 of the over 3,100 counties in the United States are in attainment of the ozone NAAQS. The EPA will continue to review the designation of the remaining counties.

In June 2017 the EPA announced a plan to extend the deadline for making attainment designations by one year, but in August the agency withdrew that proposal and instead determined to issue attainment designations on a case-by-case basis. Section 107(d)(1)(B) of the Clean Air Act requires the EPA to issue area designations within two years of promulgating a NAAQS, but allows the agency to extend the period for an additional year under certain circumstances. Environmental non-government organisations have criticised the EPA's approach, asserting that the statute does not give the EPA the option of addressing only some of the counties and threatened to challenge the EPA's action.

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