For those wondering just how much effect cleaning up the air can have, researchers now have a much fuller picture.
Reductions in particulate air pollution during the 1980s and 1990s led to an average five-month increase in life expectancy in 51 U.S. metropolitan areas, with some of the initially more polluted cities such as Buffalo, N.Y., and Pittsburgh showing a 10-month increase, researchers said Wednesday.
The reductions in pollution accounted for about 15% of a nearly three-year increase in life expectancy during the two decades, said epidemiologist C. Arden Pope III of Brigham Young University, lead author of the study appearing today in the New England Journal of Medicine.
It is well known that particulate air pollution reduces life expectancy, said environmental epidemiologist Joel Schwartz of the Harvard School of Public Health, who was not involved in the study. But public policy makers "are interested in the question of, 'If I spend the money to reduce pollution, what really happens?' " he said.
Schwartz reported two years ago that a study in six cities revealed increased life expectancy was associated with reductions in particulate pollution. Pope and his colleagues expanded on that connection, finding that in a large fraction of the U.S. population "the more particulate pollution went down, the more life expectancy went up."
Their finding "greatly strengthens the foundation of the argument for air quality management," wrote environmental health scientist Daniel Krewski of the University of Ottawa in an editorial accompanying the report.
The particulates in question are called fine particulates because they are smaller than 2.5 microns in diameter, allowing them to burrow deep into the small air passages of the lung. They have repeatedly been shown to produce cardiovascular and pulmonary disease. Larger particulates, which cause visibility problems, have a much smaller effect on health.
The fine particulates are produced by cigarettes, gasoline and diesel engines, coal power plants, foundries and a variety of other urban sources.
Pope and his colleagues studied two sets of data collected in 214 counties, comprising 51 metropolitan areas, in 1980 and 2000, comparing reductions in particulate levels and increases in life expectancies. They used a variety of advanced statistical methods to try to eliminate effects linked to changes in population, income, education, migration and demographics.
They concluded that for every decrease of 10 micrograms per cubic meter of particulate pollution in a city, average life span increased a little more than seven months -- about the same amount seen in previous, smaller studies.
"We are getting a return on our investment to improve air quality," Pope said.
Overall, the average life span in the 51 areas increased 2.7 years over the two decades, with the major share of the increase attributed to reductions in smoking and changes in socioeconomic factors.
Los Angeles, and Southern California in general, had large increases in life expectancy during the period, even though pollution levels did not drop as much as in other cities. Pope attributed the increase in life span to a string of smoking bans begun in 1994.
Pope thinks there is room for further improvement. The average countrywide fine-particulate concentration in the early 1980s was about 20 micrograms per cubic meter, and that dropped to about 14 micrograms by 2000.
"It's reasonable to expect that we could reduce it by that much again, but then we reach a point of substantially diminishing marginal returns," he said.