English: MICHIGAN ET AL. v. ENVIRONMENTAL PROTECTION AGENCY ET AL. CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR THE DISTRICT OF COLUMBIA CIRCUIT No. 14–46. Argued March 25, 2015—Decided June 29, 2015 The Clean Air Act directs the Environmental Protection Agency to regulate emissions of hazardous air pollutants from certain stationary sources (such as refineries and factories). 42 U. S. C. §7412. The Agency may regulate power plants under this program only if it concludes that “regulation is appropriate and necessary” after studying hazards to public health posed by power-plant emissions. §7412(n)(1)(A). Here, EPA found power-plant regulation “appropriate” because the plants’ emissions pose risks to public health and the environment and because controls capable of reducing these emissions were available. It found regulation “necessary” because the imposition of other Clean Air Act requirements did not eliminate those risks. The Agency refused to consider cost when making its decision. It estimated, however, that the cost of its regulations to power plants would be $9.6 billion a year, but the quantifiable benefits from the resulting reduction in hazardous-air-pollutant emissions would be $4 to $6 million a year. Petitioners (including 23 States) sought review of EPA’s rule in the D. C. Circuit, which upheld the Agency’s refusal to consider costs in its decision to regulate. Held: EPA interpreted §7412(n)(1)(A) unreasonably when it deemed cost irrelevant to the decision to regulate power plants. Pp. 5–15. (a) Agency action is unlawful if it does not rest “on a consideration