A three-judge panel had upheld changes to the National Ambient Air Quality Standards (NAAQS) first proposed by the U.S. Environmental Protection Agency (EPA) four years ago.

Four years ago, EPA strengthened the NAAQS for ground-level ozone to 70 parts per billion (ppb) from 75 ppb. Ground-level ozone forms when nitrogen oxides and volatile organic compounds react in the air.

In 2016, 10 states and several industry organizations, including the American Petroleum Institute (API) and the Independent Petroleum Association of America (IPAA), filed petitions in federal appellate court challenging EPA's proposed changes in the case Murray Energy Corp. v. EPA; No. 15-1385. They argued that EPA's approach to background ozone levels caused by uncontrollable sources violated the Clean Air Act.

In an opinion issued Friday in response to that challenge and others, the U.S. Court of Appeals for the District of Columbia Circuit vacated the 2015 ozone standard's grandfathering provision, remanded to EPA for reconsideration the secondary standard included in the 2015 changes, and denied the other petitions.

Representatives of the oil and natural gas industry and from several states had argued that EPA's primary and secondary NAAQS were too protective, while petitioners from public health and environmental groups claimed they were not protective enough.