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BACKGROUND: Evidence of health risks associated with ambient airborne fine particles in nonurban populations is extremely limited.

OBJECTIVE: We estimated the risk of hospitalization associated with short-term exposures to particulate matter with an aerodynamic diameter < 2.5 mu m (PM2.5) in urban and nonurban counties with population >= 50,000.

METHODS: We utilized a database of daily cardiovascular-and respiratory-related hospitalization rates constructed from Medicare National Claims History files (2002-2006), including 28million Medicare beneficiaries in 708 counties. Daily PM2.5 exposures were estimated using the Community Multiscale Air Quality (CMAQ) downscaler. We used time-series analysis of hospitalization rates and PM2.5 to evaluate associations between PM2.5 levels and hospitalization risk in single-pollutant models.

RESULTS: We observed an association between cardiovascular hospitalizations and same-day PM2.5 with higher risk in urban counties: 0.35% [95% posterior interval (PI): -0.71%, 1.41%] and 0.98% (95% PI: 0.73%, 1.23%) increases in hospitalization risk per 10-mu g/m(3) increment in PM2.5 were observed in the least-urban and most-urban counties, respectively. The largest association for respiratory hospitalizations, a 2.57% (95% PI: 0.87%, 4.30%) increase per 10-mu g/m(3) increase in PM2.5, was observed in the least-urban counties; in the most-urban counties, a 1.13% (0.73%, 1.54%) increase was observed. Effect estimates for cardiovascular hospitalizations were highest for smaller lag times, whereas effect estimates for respiratory hospitalizations increased as more days of exposure were included.

CONCLUSION: In nonurban counties with population >= 50,000, exposure to PM2.5 is associated with increased risk for respiratory hospitalizations; in urban counties, exposure is associated with increased risk of cardiovascular hospitalizations. Effect estimates based on a single day of exposure may underestimate true effects for respiratory hospitalizations.