Both short- and long-term exposures to fine particulate matter (PM2.5) are associated with mortality. However, whether the associations exist below the new EPA standards (12 μg/m3 of annual average PM2.5, 35 μg/m3 daily) is unclear. In addition, it is not clear whether results of previous time series studies (fit in larger cities) and cohort studies (fit in convenience samples) are generalizable to the general population. The objective of the study was to estimate the effects of low-concentration PM2.5 on mortality.