Validation of the Harvard Six Cities Study of Particulate Air Pollution and Mortality

To the Editor: In a 16-year prospective cohort study of six cities in the northeastern and midwestern United States beginning in the 1970s, Dockery et al.1 reported that long-term exposure to ambient fine particles (with an aerodynamic diameter of 2.5 μm) was positively associated with both overall mortality and mortality from cardiopulmonary causes. Because the Harvard Six Cities Study played a pivotal and controversial role2–6 in the establishment of the current U.S. objective for ambient air quality with respect to fine particles,7 the Environmental Protection Agency, industry, and nongovernmental organizations called for an independent reanalysis of this study to . . .