"Implementation of revised ozone standards by the U.S. Environmental Protection Agency (EPA) is in limbo. EPA finalized the revised standards--formally known as National Ambient Air Quality Standards (NAAQS) for ground-level ozone--in October 2015. Under a statutory deadline, the agency was to have identified areas that exceed the standards by October 1, 2017. The agency did not do so. Missing such a deadline is not unusual: EPA has frequently missed such deadlines under both Republican and Democratic Administrations. But the agency has displayed a notable degree of uncertainty as it has delayed implementation of the 2015 standards over the past year. In April 2017, EPA announced that it would reconsider the standards but gave no schedule for doing so. In June, it announced that it would delay the designation of nonattainment areas for a year, only to reverse course two months later. In November, EPA released a list of 2,646 counties that it was designating as 'attainment/unclassifiable,' but it took no action regarding 493 counties that include those most likely to be in nonattainment of the standards. Most recently, a court has imposed an April 2018 deadline for completing the designation process."
CRS Report for Congress, R43092