The Environmental Protection Agency has proposed new national air quality standards that would significantly reduce levels of fine-particle soot.

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The rule, to be announced on Friday, would reduce the range of fine particulates allowed in the atmosphere by roughly 17 percent, according to an agency official who spoke on condition of anonymity, and Paul G. Billings, a vice president of the American Lung Association, who said he had been briefed on the standard by Obama administration officials.

A Thursday deadline had been imposed by a federal judge in a legal battle waged by 11 states and health and conservation groups.

Still, the industry and many Congressional Republicans are likely to oppose that level — 12 to 13 micrograms per cubic foot of air, down from 15, calculated on an annual basis — as burdensome and potentially damaging to the economy.

The agency is required to issue a final rule by mid-December after holding hearings to seek public comment. Under the Clean Air Act , the E.P.A. is required to consider revising its soot standards every five years, and it last did so in 2006. Yet as it approached the five-year deadline last October, it said it wanted to delay issuing the rule until the summer of 2013 because it needed more time to sift through the latest scientific research.

Eleven states, including New York and California , and the American Lung Association and the National Parks Conservation Association, challenged the delay in court, arguing that it violated the Clean Air Act.

On June 2, Judge Robert L. Wilkins of the United States District Court for the District of Columbia ordered the E.P.A. to sign a proposed rule by June 7, a deadline that was later extended to Thursday under an agreement between the agency and the plaintiffs.

Fine-particle soot, which can settle deep into the lungs and the circulatory system, is among the deadliest contaminants to which the American public is regularly exposed and causes thousands of premature deaths annually from heart and lung complications.

The E.P.A.’s proposal comes as President Obama faces both a re-election battle focusing on his handling of the economy and fierce opposition from Congressional Republicans on any new environmental regulation. In one concession, the administration backed off last September from efforts to tighten ozone regulations — a delay that has also been challenged by states and public health advocates in federal court.

The proposed soot rule is certain to be debated until a final particulate level for the new standard is set after the November elections.

Public health advocates praised the proposed rule as an effective one, if overdue, that could help prevent tens of thousands of premature deaths each year from asthma , heart attacks and lung disease.

“The American Lung Association is pleased that they have proposed a much tighter particle pollution standard that will prevent thousands of premature deaths and tens of thousands of asthma attacks each year,” Mr. Billings said. “We will examine the proposal and work through the public comment process to urge the most protective standard.”

He said his group hoped to persuade the agency to further reduce the allowable range of soot, to 11 micrograms per cubic meter.

But Jeffrey R. Holmstead, who led the E.P.A.’s Air Office under the administration of President George W. Bush and is now a partner at Bracewell & Giuliani, a lobbying firm with clients in energy and manufacturing, predicted that such an increase would result in a vast array of restrictions for those sectors. “It may not sound like much, lowering the standard from 15 to 13, but it will mean a lot more regulations in many parts of the country,” he said. “I suspect this action will attract a lot of attention from the Hill.”

Disputes over new standards for soot, which includes tiny particles less than one-30th of the diameter of a human hair that come largely from combustive activities like running cars and industrial power plants, have dragged on for years.

Enforcement of the “annual” maximum ceiling for fine particulates is based on an average measured on a rolling basis over a year. That standard has not been updated since 1997.

In 2006, the Bush administration lowered its so-called 24-hour soot ceiling by nearly half, to 35 micrograms per cubic meter. But it said that the science on health consequences did not justify lowering the annual ceiling.