Background: Associations between airborne particulate matter (PM) and health outcomes have been documented worldwide; however, there is scarce evidence relating health effects to different particle sizes. Aims: To explore the association between size-fractionated particles and daily mortality in Shenyang, China. Methods: Data on cause-specific mortality, particle number concentrations (PNCs) of particle size distributions from 0.25 μm to 10 μm, and mass concentrations of PM10 (PM≤10 μm), PM2.5 (PM ≤ 2.5 μm), PM10-2.5 (2.5 μm ≤ PM ≤ 10μm), nitrogen dioxide (NO2), and sulfur dioxide (SO2) were collected from December 1, 2006 to November 30, 2008. Quasi-Poisson regression generalized additive models (GAM) were used to explore associations between PM sizes and mortality, with natural spline smoothing functions to adjust for time-varying covariates and long-term and seasonal trends. Single- and two-pollutant models and different lag structure models were also used in our analysis. Results: PM fractions with size distributions less than 0.50 μm were found to be consistently significantly associated with increased total and cardiovascular mortality, but not with respiratory mortality. The excess risk per interquartile range increase in PNC grew with decreasing particle size and was much larger on warm days than on cool days. These associations were generally stable after co-adjustment of PM mass and gaseous pollutants. Conclusions: Our analyses suggest that PM less than 0.5 μm in size is most responsible for the adverse health effects of particulate air pollution, and that adverse health effects increase with decreasing PM size. Additionally, the PM-related health effects are much severe in warm days than in cool days in the city.
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