This article is a commentary on the following three studies: Villeneuve PJ, Goldberg MS, Krewski D, Burnett RT, Chen Y. Fine particulate air pollution and all-cause mortality within the Harvard Six-Cities Study: variations in risk by period of exposure. Ann Epidemiol. 2002 Nov;12(8):568-76. Krewski D, Burnett RT, Goldberg MS, Hoover BK, Siemiatycki J, Jerrett M, Abrahamowicz M, White WH. Overview of the reanalysis of the Harvard Six Cities Study and American Cancer Society Study of Particulate Air Pollution and Mortality. J Toxicol Environ Health A. 2003 Aug 22–Oct 10;66(16–19):1507–51. Laden F, Schwartz J, Speizer FE, Dockery DW. Reduction in fine particulate air pollution and mortality: Extended follow-up of the Harvard Six Cities study. Am J Respir Crit Care Med. 2006 Mar 15;173(6):667-72. The North American cohort study "Harvard Six Cities Study" (HSCS) was the first to demonstrate an association between long-term exposure to fine particulate matter (PM2.5) and all-cause mortality. Its findings were confirmed by a larger cohort study conducted by the American Cancer Society. Initially, the six-city study focused on changes in symptoms and lung function related to air pollution. Because vital status data were obtained for each person included in the study, it was possible to conduct an analysis to determine whether mortality rates varied according to pollution levels. In the 16-year "Harvard Six Cities Study" cohort study, a significant association was observed between air pollution on the one hand, and all-cause mortality and mortality from cardiopulmonary diseases on the other. A 26% increase in all-cause mortality was observed when comparing the least polluted city to the most polluted city. Environmental agencies in the United States have relied on these two studies to establish regulatory air quality standards. On this occasion, the Health Effects Institute (HEI) was asked to conduct a reanalysis of the data from these cohorts to address the main methodological criticisms raised by the scientific community. This reanalysis and supplementary analysis are presented in the first two articles analyzed here. Subsequently, the cohort follow-up was extended, which is the subject of the third article analyzed. (Reduction in fine particulate air pollution and mortality). (Excerpt from the article)