On April 30, 2021, the Environmental Protection Agency (EPA) published a final rule announcing its determination that, for 9 of the 21 states for which it had previously issued Federal Implementation Plans to address their good neighbor obligations, their projected ozone emissions do not significantly contribute to National Ambient Air Quality Standards (NAAQS) nonattainment or maintenance problems in other states. For the other 12 states, EPA determined that their ozone emissions do significantly contribute to downwind states’ nonattainment and/or maintenance problems, and issued new or amended FIPs for those states. The rule was issued in response to the D.C. Circuit Court of Appeals’ remand of a previous EPA rule establishing FIPs for 22 states to address their interstate pollution-transport obligations in Wisconsin v. EPA. The rule was proposed under the Trump Administration.