Reduced air pollution in 51 U.S. cities over a two-decade period ending in 2000 added an average of five months to the life expectancy of residents, according to researchers at Brigham Young University and the Harvard School of Public Health.

The study, which measured changes in air pollution from approximately 1980 to 2000, found that residents were living 2.72 years longer by the end of that period. It determined that an average of 15 per cent of that increase was due to improved air quality.

In arriving at that conclusion, researchers applied advanced statistical models to account for other factors that could affect average life spans, such as changes in population, income, education, migration, demographics and cigarette smoking.

In cities that had previously been the most polluted and cleaned up the most, the cleaner air added approximately 10 months to the average resident's life.

Other studies have shown that these gains are likely coming from reductions in the cardiovascular and cardiopulmonary disease that typically accompany air pollution.

"Such a significant increase in life expectancy attributable to reducing air pollution is remarkable," said C. Arden Pope III, a BYU epidemiologist and lead author of the study, which is published in the Jan. 22 issue of the New England Journal of Medicine.

"We find that we're getting a substantial return on our investments in improving our air quality," he said. "Not only are we getting cleaner air that improves our environment, but it is improving our public health."

The latest study evaluated the impact of decreases in particulate air pollution known as PM2.5 — tiny pollutants smaller than 2.5 microns in diameter, or smaller than 4/100ths of the width of a human hair.

The analysis found that for every decrease of 10 micrograms per cubic metre of PM2.5 in a city, its residents' average life expectancy increased by more than seven months.

During the two-decade period, the average PM2.5 levels in the 51 cities studied dropped from 21 to 14 micrograms per cubic metre. In industrial cities such as Pittsburgh and Buffalo, the decrease was closer to 14 micrograms per cubic metre.

Researchers also observed gains in life expectancy even in cities that initially had relatively clean air but had further improvements in air quality, suggesting there are continuing benefits to ongoing efforts to reduce air pollution.

The study was funded by the U.S. Centers for Disease Control and Prevention, the Association of Schools of Public Health, the Environmental Protection Agency, the National Institute of Environmental Health Sciences and the Mary Lou Fulton Professorship at BYU.