Correction: An earlier version of this story incorrectly reported what critics of EPA regulations say would be the benefits of reduced emisions of hazardous air pollutants. The story has been corrected.

The Colstrip Steam Electric station, a coal burning power plant, in Colstrip, Mont. The justices appear split on the Obama administration’s plan for the EPA to cut emissions of hazardous pollutants. (Matt Brown/AP)

The Obama administration’s plan to limit emissions of mercury and other hazardous pollutants from coal- and oil-fired power plants encountered a wall of skepticism from the Supreme Court’s most conservative justices Wednesday.

The justices questioned whether the Environmental Protection Agency should have taken into account costs when it first decided to regulate the emissions. The agency said it was supposed to consider only health consequences and whether technology was available to correct the problem. It says costs were to be considered only when implementing the standards the plants were supposed to achieve.

The court’s four consistently liberal justices all voiced support for the EPA’s reading of the law. The outcome, then, would seem to hinge on Justice Anthony M. Kennedy, who gave both sides reason for optimism and concern.

It has taken decades to achieve the EPA’s goal of regulating mercury and other hazardous air pollutants, which are tied to birth defects and developmental problems in children as well as respiratory illnesses. The EPA’s rules are scheduled to take effect next month and be fully in place by 2016.

But they have been challenged by industry groups and 21 Republican-led states in which the older plants mostly are operating.

The issue comes down to what Congress meant when it ordered the EPA to study whether it was “appropriate and necessary” to regulate the pollutants from power plants but was silent on whether that study should include the costs of regulation.

Michigan Solicitor General Aaron D. Lindstrom told the court that costs traditionally were a part of such decisions.

In “the context of the question that the agency has to answer . . . costs are part of the relevant materials,” Lindstrom said.

But the liberal justices said that if Congress had meant for costs to be considered in the first part of the agency’s actions, it would have said so.

“My understanding of what EPA said is that it’s necessary because of public health harms and that it’s appropriate because there are technologies that can redress or remedy those public health harms,” Justice Elena Kagan said.

Justice Antonin Scalia interjected that it does not make sense for the EPA not to look at costs simply because a statute doesn’t require it.

“I would think it’s classic arbitrary and capricious agency action for an agency to command something that is outrageously expensive and in which the expense vastly exceeds whatever public benefit can be achieved,” he said.

Before Lindstrom could respond, Justice Sonia Sotomayor did. Congress wanted a study of the health hazards, she said. “Why in the world would one assume that Congress was thinking about cost?” she asked.

Kennedy at times seemed sympathetic to the administration’s reading of the law. But when U.S. Solicitor General Donald B. Verrilli Jr. was at the microphone, he joined Chief Justice John G. Roberts Jr. in questioning the EPA’s actions.

“Could this agency reasonably have considered costs at stage one?” Kennedy asked.

Verrilli acknowledged that the EPA could have but had decided that that was not what Congress had intended with its mandate. Roberts said: “Since you could have issued a regulation allowing the consideration of costs as appropriate, you’re saying that the agency deliberately tied its hands and said, ‘We’re not going to consider something. We’re going to issue a rule saying we can’t consider something that we could consider otherwise.’ ”

The costs and benefits are a matter of vigorous dispute. The states and industries opposing the regulations say that the annual costs of compliance under the rule would be $9.6 billion but that the benefits of reduced emissions of hazardous air pollutants are only $4 million to $6 million.

The EPA and environmental groups estimate the savings to be much more, from $37 billion up. Mercury can be especially dangerous to pregnant or breast-feeding mothers and young children, and some of the savings are calculated as coming from preventing as many as 11,000 deaths and more than a half-million lost days of work.

In addition, other environmental requirements mean that much of the capital expense of complying with the regulations has already been made, the government contends.

Sotomayor said the court should follow its usual practice and defer to the agency when a command from Congress is unclear.

“All we have to find is a plausible reading to uphold the EPA’s interpretation,” she said.

The combined cases are Michigan v. EPA; Utility Air Regulatory Group v. EPA; and National Mining Association v. EPA.