In 1998, faced with mounting evidence that many eastern and midwestern states could not comply with national air quality standards for ozone (O₃) pollution due, in part, to emissions originating in upwind states,¹ the U.S. Environmental Protection Agency (EPA) issued a highly controversial rule, the "NO_x SIP [State Implementation Plan] Call," requiring 23 states to reduce their emissions.² For the first time, EPA had initiated an attempt under the federal Clean Air Act (CAA)³ to reduce long-range interstate O₃ pollution.

Unprecedented in nature and affecting nearly half the nation, the NO_x SIP Call was challenged by 8 of the 23 upwind states along with dozens of industries.⁴ They made two key arguments: first, that EPA had improperly identified which upwind states' emissions were responsible for interstate O₃ pollution; and, second, that EPA had violated states' rights under the cooperative federalism mandated by the CAA.⁵ On March 3, 2000, however, in Michigan v. U.S. Environmental Protection Agency,⁶ a divided panel of the U.S. Court of Appeals for the District of Columbia Circuit upheld the rule in all major respects. To explain the importance of this complex case, which affirmed EPA's authority to regulate interstate air pollution, this Article provides an overview of the NO_x SIP Call and discusses the two most important issues addressed by the Michigan court.⁷