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Improved understanding of the sources of air pollution that are most harmful could aid in developingÌýmore effective measures for protecting human health. The Denver Aerosol Sources and Health (DASH)Ìýstudy was designed to identify the sources of ambient fine particulate matter (PM2.5) that are mostÌýresponsible for the adverse health effects of short-term exposure to PM2.5. Daily 24-h PM2.5 samplingÌýbegan in July 2002 at a residential monitoring site in Denver, Colorado, using both Teflon and quartz filterÌýsamplers. Sampling continued through 2008. Chemical speciation was carried out forÌýmass, inorganic ionic compounds (sulfate, nitrate and ammonium), and carbonaceous components,Ìýincluding elemental carbon, organic carbon, temperature-resolved organic carbon fractions and a largeÌýarray of organic compounds. In addition, water-soluble metals were measured daily for 12 months inÌý2003. A receptor-based source apportionment approach utilizing positive matrix factorization (PMF) was used to identify PM2.5 source contributions for each 24-h period. The health studyÌýwill investigate associations between daily source contributions and an array of health endpoints,Ìýincluding daily mortality and hospitalizations and measures of asthma control in asthmatic children.ÌýFindings from the DASH study, in addition to being of interest to policymakers, by identifying harmfulÌýPM2.5 sources may provide insights into mechanisms of PM effect.