WASHINGTON—The Environmental Protection Agency (EPA) tightened the nation’s soot standards by 20 percent Friday, a move that will force communities across the country to improve air quality by the end of the decade while making it harder for some industries to expand operations without strict pollution controls.

The new rule limits soot, or fine particulate matter, which stems from activities ranging from burning wood to vehicle emissions, and which causes disease by entering the lungs and bloodstream. Fine particulate matter measuring less than 2.5 micrometers in diameter, or one-30th the width of a human hair, ranks as the country’s most widespread deadly pollutant.

The new rule is a result of a 2009 court ruling that said the EPA standards for the amount of soot permissible in the air, based on an annual average, ignored the advice of scientific advisers by maintaining the standard established in 1997 and must be rewritten. That limit was 15 micrograms per cubic meter of air.

The EPA has decided to cut the level to 12 micrograms per cubic meter.

The soot rule “is among the most critical standards that EPA could set” because it triggers a series of measures local governments must take to comply or risk facing federal penalties, said Clean Air Watch President Frank O’Donnell.

The rule is significant because once areas are found in violation, it becomes harder for new pollution sources, such as industrial facilities and power plants, to get operating permits. While the federal government offers several incentives to cut soot — such as money to phase out dirty diesel school buses — funding for these initiatives is in short supply.

The agency will determine which areas are out of attainment in 2014, and those communities will then have six years to come into compliance. According to people familiar with the rule, 66 of 3,033 counties will be found in violation of the new standard, though EPA projects just seven will be out of compliance by 2020.

Air concentrations of soot can vary widely. In Pittsburgh’s Liberty-Clairton neighborhood, the annual average is 15 micrograms per cubic meter; in more bucolic Harrisonburg, Va., near Shenandoah National Park, it averages 10.2. An estimated 17.3 million Americans are living in areas that don’t meet the soot standard of 15.

The EPA issued a draft rule in June, and faced a court-ordered deadline of Dec. 21 to finalize it. The agency picked the more stringent standard of the choices laid out in the draft rule. The agency soon is likely to finalize another air-pollution rule, the first carbon standard for new power plants.

The agency sent the final rule over to the Office of Management and Budget (OMB) Tuesday, and business groups have been scrambling to alter it. Several trade groups — including the National Association of Manufacturers (NAM), the American Chemistry Council and the Rubber Manufacturers Association — got less than a day’s notice to weigh in with the OMB on Wednesday, where they argued that the EPA could protect public health and minimize economic harm with a more lenient standard of between 13 and 15 micrograms per cubic meter.

Ross Eisenberg, NAM’s vice president for energy and resources policy, said the stricter rule will raise the price of doing business in the United States.

“It is impossible to ignore that, against the backdrop of a still-fragile economy and a looming fiscal crisis, EPA is heaping another new set of costs and burdens on manufacturers,” he said.

“EPA seems wedded to the notion that it must push its policies as hard as it possibly can, with the goal being to enact the strictest possible standard that will survive legal scrutiny. That’s not EPA’s job.”

Brooke Suter, who heads the Clean Air Task Force’s National Diesel Clean-up Campaign, noted that the EPA’s scientific-advisory panel said a standard that would protect public health would range between 11 and 13 micrograms per cubic meter, and that the Clean Air Act requires the EPA to base the rule on scientific rather than economic considerations. Her group has estimated a soot limit of 12 micrograms would save 15,000 lives a year by the time the rule takes full effect in 2020.

“When it comes down to this, we have to do this,” Suter said. “This is the law.”

It is difficult to pin down the new standard’s economic impact, partly because some of the required reductions will be achieved under other EPA rules limiting mercury and sulfur dioxide. But one of the main rules that would have cut sulfur-dioxide emissions in the eastern half of the country, the Cross State Air Pollution Rule, was struck down in federal court last summer. Also, some members of Congress are threatening to block an upcoming rule curbing cruise-ship pollution.

“EPA has said that the rule will not cost anything because the costs are effectively ‘baked in’ through compliance with a laundry list of other regulations. ... We don’t think it’s quite that clean,” Eisenberg said.

Brad Muller, vice president of marketing for Charlotte Pipe and Foundry, said his company couldn’t get an air permit to build a new facility in rural North Carolina because county officials feared it would push them out of compliance.

“We’re not going to make this investment if we can’t get an air permit,” said Muller, who estimated his firm would have hired 1,985 employees and generated 815 temporary construction jobs if they had built the new plant.