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.