THE SUPREME Court considered two cases this term in which somewhat unclear wording in the federal Clean Air Act left sensible environmental rules open to challenge. On Tuesday, in the first case, the justices rightly sided with the sensible rules.

In Environmental Protection Agency v. EME Homer City Generation L.P., a 6-to-2 majority upheld EPA rules designed to stop groups of states from contributing harmful ozone and particulate air pollution to downwind neighbors. The agency’s “Good Neighbor” provision comes directly from the Clean Air Act, which obliges states to limit emissions that “contribute significantly” to pollution problems elsewhere. The EPA has been trying to apply that language for two decades. Its latest attempt, dating to 2011, divvies up required emissions reductions among states based on the amount each pollutes and on the cost each would have to pay to cut cross-border emissions. No state, then, would incur unnecessary and excessive costs.

The EPA, in other words, sensibly figured that a polluter’s emissions are “significant” to regulators inasmuch as they are solvable. That approach, Justice Ruth Bader Ginsburg wrote in the majority opinion, “is an efficient and equitable solution.” It is efficient, she explained, because the government can get its emissions reductions “at a much lower overall cost” than from alternatives. It is equitable because the “EPA’s rule subjects to stricter regulation those states that have done less in the past to control their pollution.” A proposed alternative, Justice Ginsburg explained, relies on “a method that works neither mathematically nor in practical application.”

Critics have contended that the text of the law requires the EPA to regulate irrationally, without weighing costs. That would be a more reasonable conclusion if the statute were clearer. But, Justice Ginsburg wrote, nothing in the text requires a different result, which gives the EPA some legal flexibility.

A legitimate exercise of federal power is preventing states from dealing direct harm — in this case, air pollution — to others in search of advantage — in this case, economic development. The EPA found a reasonable way for the agency to exercise the authority Congress delegated it.

The court is still considering another case of awkward wording in the Clean Air Act, on the EPA’s efforts to cut back on greenhouse gas emissions that contribute to climate change.Yet Congress issued the Clean Air Act so that the government would have ongoing, comprehensive and full authority to address serious air pollution problems. As with EME Homer, the court must keep that in mind.