The short-term daily standard, intended to control acute exposure to the minute particles, was cut nearly in half. But the annual standard, which affects chronic exposure, remains at its original 1997 level.
A large volume of research has implicated the soot particles — which are less than one-thirtieth the diameter of a human hair and can penetrate deep into the lungs and the circulatory system — in tens of thousands of deaths annually from both respiratory and coronary disease. Scientists say they are among the deadliest contaminants to which the public is regularly exposed and for which the E.P.A. sets exposure levels.
Stephen L. Johnson, a career scientist at the agency and the third administrator appointed during President Bush’s six years in office, said in a conference call Thursday that the annual standard would remain at its current level while research continued. No change was made now, Mr. Johnson said, “due to insufficient evidence’’ linking health problems to long-term exposure.
“Wherever the science gave us a clear picture, we took clear action,” Mr. Johnson said, adding, “All Americans deserve to breathe clean air, and through these more protective standards that is exactly what we are delivering today.”
The E.P.A. sets standards under the 36-year-old Clean Air Act and is required to review the standards every five years, taking into account the latest science. There have been no new standards since the first ones were established in 1997. The agency was under a court-ordered deadline to finalize new rules by Sept. 27.
The process of setting the new standards for soot, which comes from sources that include power plants, heavy industry and vehicle tailpipes and tires, drew intense scrutiny from medical and environmental, as well as industry, interests. Neither side was pleased with the final results.
In a rare display of solidarity, all but 2 of the 22 members of the agency’s Clean Air Scientific Advisory Council had urged that the long-term standard be lowered to a range of 12 to 14 micrograms per cubic meter, from 15.
The American Medical Association, which seldom intervenes in such cases, agreed with the scientists in a letter to the agency in April. The letter also recommended that the daily, or acute exposure standard, be further reduced. The action on Thursday reduced the daily standard to 35 micrograms per cubic meter from 65, though the A.M.A. favored a level of 25 micrograms.
The Edison Electric Institute, which represents investor-owned power plants that generate about 60 percent of the country’s electricity, said the agency was going too far in nearly halving the daily standard.
Dan Reidinger, an Edison spokesman, said power plant emissions associated with fine particles had been cut by 40 percent since 1980.
“E.P.A. persists in overemphasizing studies that suggest a possible benefit to tightening the air quality standard, ’’ Mr. Reidinger said, “while downplaying those suggesting that doing so may not provide the health benefits E.P.A. is seeking to achieve.”
The reactions from medical and environmental groups were sharper. Joel Schwartz, an epidemiologist at the Harvard School of Public Health who has written many peer-reviewed papers on the health effects of particulates, said lowering the standard to the level urged by the science advisers would prevent 3,000 premature deaths annually.
Frank O’Donnell, who heads Clean Air Watch, an environmental lobbying group in Washington, said, “Particle soot kills more people than any other form of air pollution, and this E.P.A. decision will allow particle soot to continue killing many thousands of Americans that would be spared if the air were cleaned up.”
In the teleconference, Mr. Johnson, the E.P.A. administrator, and two top agency officials, William Wehrum, the acting assistant administrator who heads the Office of Air and Radiation, and Dr. George Gray, an assistant administrator who heads the Office of Research and Development, stressed that the science advisers were not unanimous in their recommendations.
In an interview later, Mr. Johnson said, “The bottom line is these air standards are more protective today than they were yesterday.”
Asked about the health-related benefits of the tighter standard for daily exposure, he cited the agency’s statistical estimates that the new rule would avoid an estimated “2,500 premature deaths in people with heart or lung disease; 2,600 chronic bronchitis cases; 5,000 nonfatal heart attacks,” among other improvements. He said he could not provide estimates for the benefits associated with toughening the annual standard to 14 micrograms from 15.
Ron Wyzga, a scientist and biostatistician with the Electric Power Research Institute, an industry group, said a review of health studies showed that “there’s no magic number saying what a standard should or shouldn’t be, so it’s a matter of judgment.
“And different people,’’ Mr. Wyzga said, “have different judgments.”