To steer a ship, sailors cannot direct the wind, but they can adjust the sails. Likewise, the Environmental Protection Agency (EPA) cannot direct where air pollution drifts, but it can adjust the rules for combating interstate air pollution between states. The “good neighbor provision” of the Clean Air Act (CAA) does this by prohibiting upwind states from substantially interfering with the ability of a downwind state to meet National Ambient Air Quality Standards (NAAQS) requirements. In 2016, EPA promulgated the Cross-State Air Pollution Rule Update for the 2008 Ozone NAAQS (CSAPR Update) to regulate interstate air transport of nitrous oxides (NOx), a pollutant that forms ozone. Yet, the CSAPR Update provided no deadline for upwind state elimination of interstate air pollution, leaving open the potential of persistent downwind interference. Three years later, in Wisconsin v. Environmental Protection Agency, the D.C. Circuit reviewed the CSAPR Update and held that the deadline for upwind states to stop any substantial interference must align with the deadline for downwind states’ NAAQS compliance. Wisconsin has important implications for air quality regulation because aligning deadlines for upwind and downwind states improves EPA enforcement
of the good neighbor provision. When upwind and downwind compliance deadlines are aligned, the good neighbor provision becomes more effective because EPA can better balance administrative certainty and flexibility.