The follow-up study found that an average of three percent fewer people died for every reduction of one �g/m3 in the average levels of PM2.5 fine particulate matter, defined as having a diameter of 2.5 microns or less -- narrower than the width of a human hair. This decreased death rate is approximate to saving 75,000 people per year in the U.S., said lead author Francine Laden, HSPH Assistant Professor of Environmental Epidemiology.

The largest drops in mortality rates were in cities with the greatest reduction in fine particulate air pollution. The findings remained valid after controlling for the general increase in adult life expectancy in the U.S. during both the original and follow-up study periods (1979 to 1989 and 1990 to 1998).

Particulate matter is a complex mixture of extremely small particles and liquid droplets that can be directly emitted, as in smoke from a fire, or it can form in the atmosphere from reactions of gases such as sulfur dioxide, according to the Environmental Protection Agency (EPA).

The original Harvard Six Cities Study evaluated the effects of pollution on adults in the 1970s and 1980s. The results found a strong, positive correlation between levels of air pollution and mortality. The study led to a revision of existing air quality standards by the EPA.

The follow-up study population consisted of nearly 8,100 white participants residing in Watertown, Massachusetts; Kingston and Harriman, Tennessee; St. Louis, Missouri; Steubenville, Ohio; Portage, Wyocena, and Pardeeville, Wisconsin; and Topeka, Kansas.

The annual mean concentration of fine particulate matter declined during the study period by seven micrograms �g/m3 of air per decade in Steubenville, five micrograms in St. Louis, three micrograms in Watertown, two micrograms in Harriman, one microgram in Portage, and less than a microgram in Topeka.

Recently, the EPA's external science advisors recommended that the agency back new air quality standards that would reduce by one to two �g/m3 of air the acceptable standard of average levels of PM2.5. The EPA proposed lowering the level of the 24-hour fine particle standard but keeping unchanged the annual standard, set in 1997.

"Our study supports the science advisors' position," said Laden. "When cities make those reductions, the results save lives."

A public comment period on the proposals will end on April 17, and the EPA is expected to make a final ruling in September. For more information about the proposals, visit here.

In addition to Laden, the study's authors are Joel Schwartz, HSPH Professor of Environmental Epidemiology; Frank Speizer, Edward H. Kass Professor of Medicine at Harvard Medical School and HSPH Professor of Environmental Science; and Douglas Dockery, chair of the HSPH Department of Environmental Health and lead author on the original Six Cities study.

For more information about the original study, see Harvard Public Health Review.

An editorial on the follow up has been published in the same issue of the American Journal of Respiratory and Critical Care Medicine.

The EPA and the National Institute of Environmental Health Sciences funded this follow up.

Related Links
American Journal of Respiratory and Critical Care Medicine
National Institute of Environmental Health Sciences
EPA