The Supreme Court has left intact a federal rule that targets mercury pollution—giving the Environmental Protection Agency time to fix legal problems and come out with a revision by April.
Twenty states wanted the court block the rule while the government decided how to account for its costs.
Chief Justice John Roberts turned down their request on Thursday.
The justices ruled last year that the EPA should have considered the costs and benefits before imposing limits on mercury and other air pollutants from coal- and oil-fired power plants.
But the justices let the rule stay in effect and sent it back to a federal appeals court to decide how a cost-benefits analysis should be conducted.
The appeals court also has declined to postpone the rule.
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