Page:Michigan v. EPA.pdf/27

 Cite as: 576 U. S. ____ (2015)

5

KAGAN, J., dissenting

reduce their emissions of hazardous air pollutants to acceptable levels. See ante, at 2. That prospect counseled a “wait and see” approach, under which EPA would give the Act’s acid rain provisions a chance to achieve that side benefit before imposing any further regulation. Accordingly, Congress instructed EPA to “perform a study of the hazards to public health reasonably anticipated” to result from power plants’ emissions after the 1990 amendments had taken effect. §7412(n)(1)(A). And Congress provided that EPA “shall regulate” those emissions only if the Agency “finds such regulation is appropriate and necessary after considering the results of the [public health] study.” Ibid. Upon making such a finding, however, EPA is to regulate power plants as it does every other stationary source: first, by categorizing plants and setting floor standards for the different groups; then by deciding whether to regulate beyond the floors; and finally, by conducting the cost-benefit analysis required by Executive Order. EPA completed the mandated health study in 1998, and the results gave much cause for concern. The Agency concluded that implementation of the acid rain provisions had failed to curb power plants’ emissions of hazardous air pollutants. Indeed, EPA found, coal plants were on track to increase those emissions by as much as 30% over the next decade. See 1 EPA, Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units—Final Report to Congress, p. ES–25 (1998). And EPA determined, focusing especially on mercury, that the substances released from power plants cause substantial health harms. Noting that those plants are “the largest [non-natural] source of mercury emissions,” id., §1.2.5.1, at 1–7, EPA found that children of mothers exposed to high doses of mercury during pregnancy “have exhibited a variety of developmental neurological abnormalities,” including delayed walking and talking, altered muscles,