Last month marked the 20th anniversary of the publication of Harvard School of Public Health’s groundbreaking Six Cities Study, which—by revealing a strong link between air pollution and mortality risk—paved the way for strengthened U.S. regulations on fine particulate matter. Douglas Dockery, lead author of the Six Cities Study and chair of HSPH’s Department of Environmental Health, answers three questions about the seminal study.

Q: What was the Six Cities study’s “aha” finding?

A: People in the dirtier cities were dying faster than people in the clean cities. We found that the mortality risk was strongly associated with fine particulate concentrations (particles smaller than 2.5 microns in diameter, or PM2.5). The differences we found in life expectancy—two to three years shorter—were remarkable. Those are big numbers in terms of population life expectancy. We were astonished that people in the clean cities were living that much longer, just because of where they lived. The “dirty” communities were all within air pollution standards at the time—they weren’t defined as being “unhealthy” by the Environmental Protection Agency (EPA)—but the Six Cities Study strongly suggested negative health effects in those communities.

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