FILE - This Nov. 9, 2009, file photo, shows the coal-fired San Juan Generating Station near Farmington, N.M. (Susan Montoya Bryan/AP)

ONE CAN understand why critics of the Obama administration are angry about the so-called mercury rule, an Environmental Protection Agency clean-air regulation that they will ask the Supreme Court on Wednesday to quash. The EPA did not conduct a cost-benefit analysis when it decided to regulate hazardous air pollutants, such as mercury, from power plants. When the agency did consider costs, the balance ended up looking good only because the rule would coincidentally reduce another type of pollution.

But this minor regulatory sleight-of-hand doesn’t mean the rule is misguided. It will save lives at an acceptable cost. Certainly there is no basis for Supreme Court intervention.

The EPA’s opponents, including the state of Michigan, have offered the court a seemingly unassailable argument. The Clean Air Act obligates the EPA to determine if regulating power plant emissions of “hazardous air pollutants,” a specific class of dangerous compounds, is “appropriate and necessary.” But the agency didn’t consider costs when it decided whether to submit power plants to hazardous air pollution regulation; it got around to conducting a cost-benefit analysis only after it decided to regulate, while it figured out how strong regulations should be. How could the agency have reasonably determined that regulation is “appropriate” without considering costs? Wouldn’t Congress have wanted the EPA to exercise a little common sense?

The government argues that it did. First, the EPA determined whether hazardous air pollutants emitted from power plants posed a public health threat. Because mercury spewed from coal-fired power plants lands on surrounding terrain, from there entering the food chain, the agency determined there was grounds for concern. Then it tailored regulations to address the issue, including cost estimates as a major consideration.

It turns out that requiring power plants to reduce hazardous air pollutants also cuts emissions of harmful particulate matter, which causes respiratory, heart and other health problems. All told, the EPA figured the regulations would cost about $9.6 billion a year but produce $37 billion to $90 billion in public health benefits annually. In other terms, the rule would prevent up to 11,000 premature deaths per year.

It’s true that these are only estimates. But the balance looks lopsided in the government’s favor. It’s also true that a separate EPA rule would reduce particulate emissions to a degree. But that doesn’t mean the mercury rule is useless; the agency tried to avoid double-counting when calculating the benefits each rule would have.

To prevail, those challenging the EPA must convince the court that the agency was wholly unreasonable in the way it interpreted the Clean Air Act’s command to regulate hazardous air pollution when “appropriate.” That’s a high legal hurdle, and the challengers aren’t anywhere close to clearing it.