Just how many months of life is clean air worth? Five to be precise, according to a new study published today in the New England Journal of Medicine.

"Who would have thought you could get almost half a year in increased life expectancy on average just from cleaning up our air somewhat?" says study co-author Arden Pope, an environmental economist at Brigham Young University in Provo, Utah. "That seems to me like a pretty good investment" in clean-air programs.

Pope and Harvard University colleagues compared improvements in air quality with increased life expectancy between 1980 and 2000. Their findings, based on air-monitoring and health data from 51 U.S. metro areas: five months of the nearly three additional years of life tacked on during that period stemmed from cleaner air.

The 1970 National Ambient Air Quality Standards set new limits on pollution and the Clean Air Act established procedures states had to follow to comply with them.

Researchers said that during the period studied, levels of soot (fine particulate matter that can get into the lungs and has been linked to cancer, asthma and heart disease) dropped in the 51 cities from a range of 10-to-30 micrograms per cubic meter to 5-to-20 micrograms per cubic meter. Pittsburgh, Buffalo and Gary, Ind., cleaned up the most, and Topeka, Kan., the least, according to the study. On average, the cities that improved the most had the greatest increases in life expectancy, and those with the dirtiest air  had the least improvement.

Pope acknowledges, however, that cleaner air isn't the only reason life expectancy rose. "Nobody believes that reductions in air pollution is the primary reason we had an increase in life expectancy. It's only one of the factors," he says. "Other factors really do come into play: changes in cigarette smoking are very, very important."

But the increased life expectancy from clean air held even when researchers controlled for smoking, income, education, race and whether a county was urban or suburban, Pope says. Previous research has found high rates of childhood asthma in poor, urban areas with heavy traffic, such as the South Bronx in New York City.

George Thurston, a professor of environmental medicine at NYU School of Medicine who has studied asthma and traffic pollution in the South Bronx, says the results "are plausible."

"Everyone benefits from clean air, some more than others," Thurston tells ScientificAmerican.com. "But one of the universal things about this is we all breathe same air. When it's a polluted day in the Bronx it's also polluted across the metro area because pollution is regional in nature."

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