WEDNESDAY, Jan. 21 (HealthDay News) -- A new study shows there is a direct relationship between the level of fine-particle pollutants in the air people breathe and life expectancy in cities across the United States.

Reducing the average level of fine-particle pollutants -- the most damaging kind -- by 10 micrograms per cubic meter of air adds about seven months of life expectancy, according to the study of 51 metropolitan areas from Portland, Wash., to Tampa Bay, Fla.

The research, reported in the Jan. 22 issue of the New England Journal of Medicine differs in one significant way from previous studies showing a link between fine-particle pollution and mortality, explained study author C. Arden Pope III, a professor of economics at Brigham Young University.

"They were either daily-time series, in which you follow people from day to day, or studies in which you enroll a cohort of individuals, follow them up, then see when they die and what they die of," Pope said. "From all of those studies, the evidence is fairly clear that fine-particle pollution increases the risk of dying."

"In this study, we took life expectancy in 51 metropolitan areas for which we had information on air pollution levels in the late '70s and early '80s, and again in the '90s and 2000, to see if the differential changes in air pollution were associated in changes in life expectancy. The results were comparable to what we would have expected from previous studies," Pope said.

Average U.S. life expectancy increased by about three years over the period of the study, and cleaner air was responsible for as much as 15 percent of the increase in some metropolitan areas, the report claimed.

One expert described the finding as a "finger in the eye" of former President George W. Bush.

Morton Lippmann, director of the New York University Center for Particulate Matter Health Effects Research Center, was a member of the U.S. Environmental Protection Agency's Clean Air Science Advisory Committee, which followed EPA staff recommendations in 2007 and voted 19 to 2 to set a limit of no more than 14 micrograms per cubic meter.

Instead, the EPA under Bush administration set the limit at 15, a level that Lippmann said "is just too high. From a public health perspective, in my mind, that was inexcusable."

Looking at the new report, "there is no surprise here," Lippmann said. "They found a slightly smaller estimate than those that have been around before."

Some scientific questions remain to be answered, he said. "What is it about these fine particles that make them dangerous?" he said. "What has been settled is that they undoubtedly do great harm. But which chemical entities within the particles that do the damage is unknown."

The study does help settle one basic issue, said study co-author Majid Ezzati, an associate professor of community health at the Harvard School of Public Health.

"We knew that air pollution was bad, but has lowering it been good over the long run?" he said. "The political spectrum has been divided on it. This study indicates that, yes, having lower air pollution has been good for the health of people in these cities."

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