Airborne Fine Particles and Risk of Hospital Admissions for Understudied Populations: Effects by Urbanicity and Short-Term Cumulative Exposures in 708 U.S. Counties

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 μm (PM(2.5)) 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 28 million Medicare beneficiaries in 708 counties. Daily PM(2.5) exposures were estimated using the Community Multiscale Air Quality (CMAQ) downscaler. We used time-series analysis of hospitalization rates and PM(2.5) to evaluate associations between PM(2.5) levels and hospitalization risk in single-pollutant models. RESULTS: We observed an association between cardiovascular hospitalizations and same-day PM(2.5) 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-μg/m(3) increment in PM(2.5) 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-μg/m(3) increase in PM(2.5), 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 PM(2.5) 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. CITATION: Bravo MA, Ebisu K, Dominici F, Wang Y, Peng RD, Bell ML. 2017. Airborne fine particles and risk of hospital admissions for understudied populations: effects by urbanicity and short-term cumulative exposures in 708 U.S. counties. Environ Health Perspect 125:594–601; http://dx.doi.org/10.1289/EHP257