Researchers in the U.S. say reductions in fine particulate air pollution appears to have an affect on mortality.
The Harvard Six Cities study (1979 to 1990) revealed an association between levels of fine particulate matter pollution and mortality risk.
The new study, which was conducted from 1990 to 1998, reports on this later period of reduced air pollution concentration.
Although the earlier studies showed a direct link between death rates and air pollution it was never clear if improvements in particle exposure would actually lead to better survival.
According to a report by Dr. Francine Laden, from Harvard Medical School in Boston and her colleagues reductions appear to translate into a survival benefit on a population level.
Laden's team analyzed data during a period when air pollution was declining in many of the cities studied.
The study population consisted of 8,096 white participants living in Watertown, Massachusetts; Kingston and Harriman, Tennessee; St. Louis, Missouri; Steubenville, Ohio; Portage, Wyocena, and Pardeeville, Wisconsin; and Topeka, Kansas.
The average age of participants at the start of the original study was 50, with women comprising 55 percent of the cohort.
As was found in previous studies 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.
Laden says the drop in mortality was particularly noticed in deaths due to cardiovascular and respiratory disease but not from lung cancer, which is a disease with a longer latency period and less reversibility.
Investigators found that long-term exposure to ambient PM2.5 was associated with increased mortality.
The researchers conclude the results suggest that increases in mortality related to PM2.5 are "at least in part reversible.
The results appear in the second issue for March 2006 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.
What is PM2.5?