New federal rules announced Wednesday will for the first time limit mercury and other toxic air pollutants from coal-fired power plants - and push many of them in the Carolinas toward retirement.

The new standards, which utilities call the most expensive ever forced on them, have been anticipated for two decades. Power plants release half of all mercury and acid gases in the United States and more than 25 percent of toxic metals such as arsenic, the Environmental Protection Agency says.

Those releases contribute to mercury so widespread in North Carolina waters that a statewide advisory on eating contaminated largemouth bass is in place.

Despite that, mercury emissions from plants in the state have dropped dramatically in the past decade. That will position Duke Energy and Progress Energy, which have spent a combined $7 billion in pollution controls over the past decade, to more easily meet the new mandate.

One way the utilities will do that is to retire older coal plants rather than invest in upgrades.

By 2015, Duke will shutter 18 units in the Carolinas and 34 throughout its five-state territory. Progress will retire 11 coal units, 30 percent of its North Carolina coal fleet, by 2014.

Some of those plants will be replaced by new ones, including models fueled by cleaner-burning natural gas. But the biggest reason for the retirements is the expense of complying with a wave of new or upcoming federal air, water and coal ash standards.

"It's a combination of all of them," Duke spokeswoman Rita Sipe said. "As we look at modernizing our fleet, we consider all that." Duke expects to spend another $5 billion to $6 billion on pollution upgrades over the next 10 years.

Duke and Progress, still digesting the 1,117-page EPA rule, say they don't know what further upgrades their plants will need or what costs could be passed to customers.

Duke and the N.C. Division of Air Quality, in earlier comments to EPA, had urged the agency to stretch its three-year compliance deadline to four or more years.

"By the EPA's own estimate, this rule is the most expensive in its history, and it has the potential to have significant impacts on consumers," Progress CEO Bill Johnson said in a statement. He said his company will work with state regulators to comply in the cheapest way for customers.

EPA has estimated that the rule will cost utilities about $11 billion a year, increasing typical residential bills $3 or $4 a month.

In return, the agency says, it expects the rule to prevent up to 11,000 premature deaths and 4,700 heart attacks a year - $9 in public health benefits for every dollar utilities spend.

Apart from mercury, the new standards limit releases of acidic gases and toxic metals such as nickel, some of which are released as tiny particles that can cause lung and heart problems.

Advocacy groups ranked the rule's importance with that of taking lead, another powerful toxin, out of gasoline in the 1970s. But some industry groups predicted the new rule will kill jobs and threaten reliable electric service by utilities struggling to upgrade plants in a short time.

The Southern Environmental Law Center, which has an office in Chapel Hill, represented the American Nurses Association and Physicians for Social Responsibility in a court case that led to an order directing EPA to issue the rule. EPA had been authorized to address such pollutants in the 1990 Clean Air Act amendments.

"These standards are a huge victory for families and children as they will slash levels of mercury and other toxic air pollution that disproportionately harm kids," said John Suttles, a senior attorney at the center.

The Edison Electric Institute, a utility trade group, took a dimmer view.

"We believe the (Obama) administration is underestimating the complexity of implementing this rule in such a short period of time, which can create reliability challenges and even higher costs to customers," it said.

Pollution controls in place

More than half the nation's coal-fired plants already have pollution-catching technology that will help them meet the new mandate, EPA says. Among them are many of the Duke and Progress plants.

North Carolina's 2002 Clean Smokestacks Act forced the utilities to sharply reduce emissions of compounds that form smog and tiny, harmful particles. The pollution controls that were installed also were good at capturing mercury.

Between 2002 and 2010, mercury emissions from North Carolina coal plants dropped 71 percent and hydrogen chloride by 77 percent, the state air-quality division said. Those numbers, which include plants scheduled to close soon, are expected to drop further by 2016.

State air quality staff will analyze the EPA rules to determine whether further controls will be needed at the remaining power plants, the division said.

State air and water divisions are working to quantify how much mercury is released in-state and how much blows in from neighboring states, with a report to the state Environmental Management Commission expected in January.