The rule presents a dilemma for Pennsylvania's aging fleet of coal-fired plants.
About half of the state's 38 plants lack the technology to meet the rule, according to the EPA. Their owners would face difficult decisions - whether to spend millions to install pollution-control equipment, convert to cleaner natural gas, or close.
Unlike Pennsylvania, New Jersey enacted a law in 2004 to cut mercury and other emissions. That state's five plants have installed equipment to meet state regulations - and presumably the proposed federal rule - or are doing so.
Jackson said the proposed rule would level the playing field and provide "regulatory certainty" for the industry. Though just over half of the nation's power plants have installed controls, the rest have been putting off the investment until they know the rules, she said.
Industry officials will likely launch a vigorous campaign to have the proposed rule weakened.
The Electric Reliability Coordinating Council said the proposed rule was one of the most expensive in EPA history and could cost the industry $100 billion.
Council director Scott Segal termed the proposed rule "an extraordinary threat to the power sector" because about half the nation's electricity comes from coal.
The council released a four-page analysis of the rule, rebutting its stated health and jobs benefits.
The proposed rule - which the EPA will make final after a public-comment period - would reduce mercury emissions by 91 percent. Plants would have up to four years to comply.
Jackson said the cost of the rule on an annualized basis would be $10 billion to $12 billion, but "the benefits are 10 times that."
For every dollar spent to reduce pollution from power plants, the EPA estimates returns of $13 in health and economic benefits.
Jackson said the impact on residential utility bills would be "quite small" - along the order of $3 to $4 a month.
She spoke at an event in Washington that included a class of second graders from the nearby Amidon-Bowen Elementary School.
"Kids, today it's about you and millions of other children across the country, and the opportunity for all of you to grow up healthier and stronger," American Lung Association president Charles Connor said.
Power plants are the largest source of airborne mercury emissions, the EPA said.
When the mercury falls back to earth and into waterways, it becomes methylmercury, which is more toxic. It migrates up the food chain into fish that people eat.
Exposure can cause developmental delays in young children, reducing IQs. It can cause birth defects.
Other toxic metals the rule would address - arsenic, chromium, nickel - can cause cancer.
The rule also would limit emissions of acid gases, which cause lung damage and contribute to other respiratory ailments.
In recent years, Pennsylvania has remained among the nation's highest-emitting states for mercury.
In 2009, the last year for which data are available, the Keystone plant in Armstrong County and Conemaugh in Indiana County were the nation's 15th- and 22d-largest emitters of mercury.
The plants' chief operating officer, Dave Benson, said the owners had already spent $1 billion to cut emissions and would meet any new rule.
Shawn Garvin, administrator for the regional EPA office that covers Pennsylvania, said the new rule offered plants without new technology a chance to catch up.
Douglas L. Biden, president of the Electric Power Generation Association, a Pennsylvania industry group, said the plants generating 70 percent to 75 percent of Pennsylvania's coal-fired power likely meet the proposed standards now.
But many of the smaller, older plants - the oldest dates to 1949 - will face difficult decisions.
"I'm sure some companies can already see the handwriting on the wall, and they know some plants are going to go," Biden said.
He said the industry would ask for maximum flexibility in meeting new standards.
The time frame "forces the industry to go to market all at the same time. There are a finite number of competent engineering and construction contractors who can do this work," which drives up the price, he said.
Charles McPhedran, law staff chair for Citizens for Pennsylvania's Future, an environmental advocacy group that tried to get state mercury legislation passed, called the EPA's proposal a good step that would save lives.
Then again, "it's just a proposal," he said. "You can bet there's going to be a strong effort by industry to water this down. People who care about this issue need to be engaged."
William O'Sullivan, director of the New Jersey Department of Environmental Protection's division of air quality, said officials there welcomed the new rule because it would reduce pollution blowing downwind from Pennsylvania.
His state has taken legal action, seeking emission reductions at several Pennsylvania plants.
However, he was still evaluating which method the EPA is requiring for controlling hydrochloric acid and whether it would do the job.
"This EPA rule will help, but it's not likely to solve all the problems," he said.
The EPA estimates the rule would prevent 17,000 premature deaths, 11,000 heart attacks, and 120,000 cases of childhood asthma. It also would prevent 12,000 emergency-room visits and 850,000 sick days from work, the agency said.
"This is a clock that first started ticking in 1990," Jackson said.
In 1990, a bipartisan Congress amended the Clean Air Act, opening the door for the EPA to regulate toxic emissions from power plants.
A period of analysis, proposals and comment ensued. In 2005, under the Bush administration, the EPA issued a mercury rule that was later struck down in court.
"Some coal-burning power companies, and, of course, their lawyers and lobbyists, have used every trick in the book to delay complying for all these 20 years," the Lung Association's Connor said.
His message to the coal industry is: "Start now to save lives tomorrow. I can assure you that no one will complain if the air gets cleaner, faster."
Contact staff writer Sandy Bauers at 215-854-5147 or email@example.com.
Visit her blog at http://go.philly.com/greenspace