The high court said the EPA was allowed under the Clean Air Act to implement federal plans in states that had not adequately addressed pollution that blows downwind. The court also ruled that the EPA also was authorized to consider how costly controls on pollution are and did not have to require states to reduce pollution by the exact amount they contribute to downwind states.
The court, agreeing with EPA, found such a requirement to be impossible in practice.
"The realities of interstate air pollution, however, are not so simple," Ginsburg said in the opinion. "Most upwind states contribute to pollution to multiple downwind states in varying amounts."