The Clean Air Act treats electric utilities differently from other sources of hazardous air pollutants. Other sources are required to limit their emissions if they exceed quantitative thresholds. 42 U.S.C. § 7412(c)(1) & (d)(1). By contrast, before EPA regulates hazardous air pollutants from electric utilities, it must first conduct a study of the hazards to public health resulting from those emissions even after imposition of all the other requirements of the Clean Air Act, and then decide whether it is "appropriate and necessary" to regulate such residual emissions under § 7412 after considering the results of the study. 42 U.S.C. § 7412(n)(1)(A).
The question for the Court is:
Whether EPA's interpretation of "appropriate" in 42 U.S.C. § 7412(n)(1)(A) is unreasonable because it refused to consider a key factor (costs) when determining whether it is appropriate to regulate hazardous air pollutants emitted by electric utilities.
THE SUPREME COURT GRANTED CERT LIMITED TO THE FOLLOWING: Whether the Environmental Protection Agency unreasonably refused to consider costs in determining whether it is appropriate to regulate hazardous air pollutants emitted by electric utilities.