Four years after overturning a major Environmental Protection Agency air pollution rule as inconsistent with the Clean Air Act, this week the D.C. Circuit Court of Appeals vacated the program that EPA had tailored to take its place, ruling that the replacement rule “exceeds the agency’s statutory authority” and imposes “impossible” burdens upon covered states. As a result, hundreds of power plants in 28 states are once again subject to the very rule the same court rejected in 2008.

The regulations in question implement the Clean Air Act’s “good neighbor” provisions, which prohibit states from significantly contributing to unsafe levels of air pollution, or interfering with Clean Air Act compliance, in downwind states. In 2005, EPA finalized the Clean Air Interstate Rule, establishing an emissions trading program for Eastern and Midwestern power plants aimed at reducing interstate air pollution transport.