A 1993 Harvard study found people in America's dirtiest studied city died about two years sooner than those in its cleanest, tracking fine particulate matter called PM2.5. Industry attacked the study, an independent audit upheld it, and regulators tightened the standard step by step through 2024. A federal appeals court upheld that standard in June 2026.
Who's who
- Douglas Dockery+2 more episodesHarvard epidemiologist; first author of the 1993 Six Cities paper
- C. Arden Pope III+2 more episodesBYU economist; co-author and lead of the American Cancer Society studies
- Benjamin Ferris Jr.Harvard physician who launched the Six Cities study in 1974
- Frank SpeizerHarvard physician who co-designed the Six Cities study
- Daniel KrewskiEpidemiologist who led the independent reanalysis
- Carol BrownerEPA Administrator who set the first PM2.5 standard in 1997
- Thomas BlileyHouse committee chairman who demanded the raw study data
Where it happens
- Steubenville, Ohio+1 more episodeDirtiest of the six cities studied
- Portage, Wisconsin+1 more episodeCleanest of the six cities studied
- Utah Valley, UtahSteel mill strike showed hospital admissions fell when air cleared
- Harvard School of Public Health, BostonHome of the Six Cities project
Timeline
- 1974Harvard launches the Six Cities study, enrolling 8,111 adults.
- December 1993The New England Journal of Medicine publishes the Six Cities mortality findings.
- 1997EPA sets the first PM2.5 standards over fierce industry opposition.
- July 2000The independent Health Effects Institute reanalysis confirms the results.
- February 2001The Supreme Court rules EPA may not consider costs when setting air standards.
- June 2026A federal appeals court upholds the tightened 2024 standard.
Still debated
Whether risk keeps falling in a straight line down to the lowest concentrations is unsettled. A minority of researchers still argue residual confounding explains part of the link, though audits and many independent cohorts have upheld it.
