A direct link between death rates and small airborne particles 2.5 microns in diameter or less -- dubbed PM2.5 -- has been noted in numerous epidemiologic studies, but it was unclear if improvements in particle exposure would actually lead to better survival, according to a report by Laden and her colleagues.

As they explain in the American Journal of Respiratory and Critical Care Medicine, in an earlier analysis of data from the Harvard Six Cities study, long-term exposure to ambient PM2.5 was associated with increased mortality.

Laden's team analyzed data for 8 additional years of follow-up, during a period when air pollution was declining in many of the cities studied. The urban areas included in the study were Watertown, Massachusetts; Kingston and Harriman, Tennessee; St. Louis, Missouri; Steubenville, Ohio; Portage, Wyocena and Pardeeville, Wisconsin; and Topeka, Kansas.

Consistent with previous findings, the overall mortality in those cities rose steadily with each increase in PM2.5 of 10 microgram per cubic meter. As PM2.5 levels fell during follow-up, so did overall mortality.

The results suggest that increases in mortality related to PM2.5 are "at least in part reversible," the researchers conclude