– When writing environmental rules, one of the most important calculations involves weighing the financial costs against any gains in human life and health. The formulas are complex, but the bottom line is that reducing the emphasis on health makes it tougher to justify a rule.

Last week, the Trump administration took a crucial step toward de-emphasizing the life and health benefits in this calculus when the Environmental Protection Agency said it would rethink a major regulation that restricts mercury emissions by coal-burning power plants.

The 2011 mercury rule — based on decades of research showing that mercury damages the brain, lungs and fetal health — is among the costliest but most effective clean-air policies put forth by the EPA. Utilities estimate they have spent $18 billion installing clean-air technology, and mercury pollution has fallen by nearly 70 percent.

Modifying the rule could have an effect far beyond any immediate concerns about the release of toxic mercury into the air and water. In fact, the re-evaluation fits into a far-reaching administration strategy to loosen environmental rules affecting countless other industries for years to come by adjusting the factors used to judge the benefits to human health that the rule has brought.

“This goes way beyond just weakening the mercury rule,” said Alan Krupnick, an economist at Resources for the Future, a nonpartisan Washington research organization. “This is part of a change that would give the Trump administration a way to more easily justify loosening many other pollution regulations, such as rules on smog, and rules on climate-change pollution.”

For example, Krupnick and other experts said, tweaking the formulas could also make it easier for the EPA to justify its separate proposal last month to replace the Clean Power Plan, an Obama-era rule that was designed to cut global-warming emissions from power plants.

The electric utilities that operate the nation’s coal-fired plants, and thus are heavily affected by the mercury rule, are urging the administration to leave the rule alone. They have already spent billions of dollars to become compliant, the utilities say, so changes are of little benefit to them.

“It’s in our rearview mirror, so we want to stick with what we’ve done,” said Shannon Brushe, a spokeswoman for Duke Energy, one of the nation’s largest utilities. “We want to be able to plan our investments for the future, but if they change the rules, that becomes difficult.”

The EPA has said it is reviewing whether the rule is “appropriate and necessary.” That’s legal shorthand, based on a 2015 Supreme Court decision, for assessing whether the costs of compliance outweigh the benefits: The court found that the agency must assess industry’s compliance costs in determining whether a regulation is appropriate and necessary, but kept the rule in place. The EPA under the Trump administration asked a federal appeals court to delay the rule while it decides whether to continue to defend it in court.

Because of that, according to conservative groups that have welcomed the review, a loosening of the mercury rule would fit into a broader effort to fundamentally change the agency’s formulas for its cost-benefit reviews. This past June, Scott Pruitt, who was the EPA’s administrator at the time, called for a new way of calculating the costs and benefits of environmental regulations. “Many have complained that the previous administration inflated the benefits and underestimated the costs of its regulations through questionable cost-benefit analysis,” he said at the time. Under Andrew Wheeler, who became the EPA’s acting administrator when Pruitt stepped down in July, the agency is still considering an overhaul.

Nicolas Loris, an economist with the Heritage Foundation, a conservative think tank that has championed President Donald Trump’s environmental deregulatory agenda, said that rewriting the mercury rule by placing different values on the health costs of mercury pollution could help end what he called the EPA’s “egregious abuse” of the way it measures winners and losers. “This could be part of a broader approach,” he said.

Public health advocates say that the reformulation of the mercury rule, along with efforts to limit the type of scientific studies the EPA will allow to inform its understanding of how pollution affects human health, will weaken the agency’s ability to write new pollution standards in the future.

“All of this is leading up to a real sea change in the way this agency, which is supposed to be a public health and environment agency, thinks about the value of reducing air pollution from coal plants,” said Ann Weeks, senior counsel for the Clean Air Task Force, an environmental group.

The mercury rule, she said, “is where the rubber hits the road.”