The Environmental Protection Agency announced Friday that it will set new limits for the airborne microscopic particles known as soot, one of the most deadly forms of air pollution.

The widely watched decision, which was expected to signal how the Obama administration will approach environmental issues in its second term, should curtail the amount of soot released from diesel exhaust, coal-fired power plants, refineries and other emitters by requiring costly pollution controls.

The minute particles of soot – smaller than the diameter of a strand of human hair – are considered particularly dangerous for children because they lodge deep in their lungs. Soot also is the primary cause of haze, and it’s considered a short-lived climate pollutant that can contribute to global warming.

Exposure to soot costs tens of thousands of premature deaths annually, EPA Administrator Lisa Jackson said in announcing the new limits. The EPA estimates that the health benefits of the revised standard will range from $4 billion to more than $9 billion per year. The agency estimates that it will cost $53 million to $350 million to implement.

“Families from around the country will benefit from the simple fact of being able to breathe cleaner air,” said Jackson, who noted that clean air isn’t just an abstract concept to her. Her own two sons have battled asthma, she said.

Currently, 66 U.S. counties are out of compliance with the existing air-quality rules on soot. The agency projects that with the exception of seven counties in California that have persistent air-quality problems, most U.S. communities should be able to comply with the new limits by 2020.

Clean-air advocates say that lowering the limit will reduce thousands of premature deaths, heart attacks and cases of asthma. They applauded monitoring requirements near highways in urban areas, which will keep tabs on emissions to help determine the next round of air pollution standards. People who live along highways are at particular risk for exposure.

“By setting a more protective standard, the EPA is stating that we as a nation must protect the health of the public by cleaning up even more of this lethal pollutant,” said Dr. Norman Edelman, the chief medical officer for the American Lung Association. “Reducing particle pollution will prevent heart attacks and asthma attacks, and will keep children out of the emergency room and hospitals. It will save lives.”

Some industries, led by the American Petroleum Institute, fought unsuccessfully to delay implementation of the rules, saying they’d be expensive and would have "doubtful benefit" on health. Industry officials said they were especially concerned about the effect of the new limits when they were coupled with pending greenhouse-gas regulations for refineries and forthcoming rules on ozone.

“The collective impacts of all of these and other potential new regulations at a time when 12 million Americans are still unemployed would be a blow to our economy as it struggles to recover and put Americans back to work,” said Howard Feldman, the director of regulatory and scientific affairs at the petroleum institute. “These rules could significantly slow business development and job creation. It makes no sense to risk this when the necessity of many of these regulations is ambiguous at best.”

The new soot standard was lowered from its 1997 level of 15 micrograms per cubic meter to 12 micrograms per cubic meter. The standards define how much soot pollution is unhealthy for humans to breathe, and they set clean air limits to ensure healthy air quality. The EPA made no changes to the 24-hour fine particle standard or the coarse particle standard – the daily averages of soot levels in the air over the course of a year – although the lung association and other groups pressed the agency to consider evidence that both standards need strengthening.

The EPA will identify counties in states that are likely to be out of compliance with the new rules. States then must develop their own rules to meet the new standards and curtail soot-producing emissions. They have several years to do so.

There’s been a long history of regulatory foot-dragging on the issue. The new limits come only after Earthjustice, an environmental legal group, went to court on behalf of the American Lung Association and the National Parks Conservation Association to force the EPA to apply standards recommended by its own scientific advisory committee. The Clean Air Act requires the EPA to review air-quality standards every five years and update them as needed. The agency updated one national limit in 2006, but it didn’t strengthen the standard for soot 2.5 micrometers in diameter or less.

In the week before the new rules were made public, clean air advocates and industry representatives gathered at the White House to hear details of the rules. The EPA had sought a lower limit and asked for public comment on setting it at 11 micrograms per cubic meter.

The agency wrangled with the White House Office of Management and Budget, which sought a limit of 13 micrograms per cubic meter. Ultimately, the agency settled on 12.