The U.S. Environmental Protection Agency on Friday proposed that allowable levels of soot in the nation's air be reduced to protect public health.
Soot, one of the deadliest forms of air pollution, has fine particles that can travel deep into the lungs and enter the bloodstream. It has been blamed for tens of thousands of premature deaths in the United States every year.
Environmental and health groups praised the decision, but some industry groups said it could stifle economic recovery.
"I can't think of any proposed rule that is going to have as positive a public-health benefit as this one," said Joseph O. Minott, executive director of the Clean Air Council, a Philadelphia nonprofit.
"The evidence about the health impacts of fine particulates is really overwhelming," he said.
Some have questioned whether this is the right time for such action based on economic factors, he said, "but the fact is we're already paying a high price in terms of health costs. This shifts the burden a little bit, and the benefits will be overwhelming."
Major sources of fine particles include fuel combustion, diesel engines, some industrial processes and wood-burning stoves.
Evan Tracey, spokesman for the American Coalition for Clean Coal Electricity, called the proposal "another example of how the agency is ignoring the harm its aggressive regulatory agenda is causing to the U.S. economy."
He said a number of states have not finished taking all the steps they need to meet the current standard for fine particles, so "EPA should focus its efforts on helping these states . . . not move the goalposts again."
A spokesman for the American Petroleum Institute said that the scientific analysis for the new standard was faulty and that final adoption could increase costs and undermine energy production.
But Douglas L. Biden, president of the Electric Power Generation Association, a Pennsylvania industry group whose members include coal-fired power plants, said he doubted his organization would oppose the standard.
He said that equipment installed on power plants to reduce other emissions - including sulfur dioxide, nitrogen oxide and mercury - also reduces particulates.
"So I don't see this necessarily having a tremendous impact on the electric power sector," Biden said.
Unlike other EPA regulations, which put limits on emissions, the soot regulations set a threshold for pollution levels in the nation's air. How to meet that standard is a matter for states and other entities to determine.
However, the EPA said that based on rules already in place, 99 percent of U.S. counties, including this region, would meet the new standard by 2020 without taking any further action.
Indeed, based on 2011 data, the air in most counties in this area is already clean enough to meet the proposed standard. Only Chester and Delaware counties fall short.
Meanwhile, some of the region's largest sources of soot pollution have cut back or ceased.
A 2010 EPA report identified PSEG's coal-fired Mercer power plant near Trenton, Exelon's Eddystone plant and Sunoco's refineries in South Philadelphia and Marcus Hook as among the area's worst fine-particle polluters.
However, Mercer has installed control equipment that is expected to reduce emissions by 99 percent. Eddystone's two coal-fired units were retired in 2011 and 2012. The Marcus Hook refinery is off-line.
The fate of the South Philadelphia refinery is uncertain. It may be idled in August if Sunoco is unable to reach an agreement with the Carlyle Group on a joint venture that would keep it running.
Federal law does not allow the EPA to factor in compliance costs when deciding what level of air pollution is too dangerous for public health. The agency nevertheless estimated a return of $30 to $86 in health benefits for every dollar spent on pollution control.
Health effects of fine-particle pollution include heart attacks, strokes, increased asthma attacks and other respiratory difficulties. Long-term exposure also has been linked to cancer, infant mortality and low birth weight.
Fine-particle pollution also contributes to haze and has been blamed for obscuring the vistas in national parks.
The rule would reduce the acceptable annual average level of soot pollution from the current 15 micrograms per cubic meter to between 12 and 13 micrograms per cubic meter. (A daily standard which allows some spikes well above the yearly average limit would remain the same.)
Some groups had pushed for a further reduction.
A report prepared last year by the American Lung Association, the Clean Air Task Force and Earthjustice estimated that if the annual standard were set at 11 micrograms per cubic meter and the daily limit were reduced, the Philadelphia region would see 1,550 fewer premature deaths a year - the fourth largest reduction in the nation.
The EPA is required to reassess ambient air quality standards every five years.
Friday's proposed rule came after a lengthy court battle that originated in 2006 when the Bush administration ignored the advice of science advisers who had recommended a tighter standard.
Paul Cort, the Earthjustice attorney who represented the American Lung Association and National Parks Conservation Association in one of the suits filed, called the proposal "long overdue."
"The fact that the EPA has been put back on track by the courts is an important first step in this process, but now the agency needs to set strong final standards to protect people from this deadly pollution."
The agency will hold public hearings in Sacramento, Calif., and Philadelphia, with the dates still to be determined. Final standards are expected to be issued by Dec. 14.