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BACKGROUND: Population-based studies have estimated health risks of short-term exposure to fine particles using mass of PM(2.5) (particulate matter <= 2.5 mu m in aerodynamic diameter) as the indicator. Evidence regarding the toxicity of the chemical components of the PM(2.5) mixture is limited. OBJECTIVE: In this study we investigated the association between hospital admission for cardiovascular disease (CVD) and respiratory disease and the chemical components of PM(2.5) in the United States. METHODS: We used a national database comprising daily data for 2000-2006 on emergency hospital admissions for cardiovascular and respiratory outcomes, ambient levels of major PM(2.5) chemical components [sulfate, nitrate, silicon, elemental carbon (EC), organic carbon matter (OCM), and sodium and ammonium ions], and weather. Using Bayesian hierarchical statistical models, we estimated the associations between daily levels of PM(2.5) components and risk of hospital admissions in 119 U.S. urban communities for 12 million Medicare enrollees (>= 65 years of age). RESULTS: In multiple-pollutant models that adjust for the levels of other pollutants, an interquartile range (IQR) increase in EC was associated with a 0.80% [95% posterior interval (PI), 0.34-1.27%] increase in risk of same-day cardiovascular admissions, and an IQR increase in OCM was associated with a 1.01% (95% PI, 0.04-1.98%) increase in risk of respiratory admissions on the same day. Other components were not associated with cardiovascular or respiratory hospital admissions in multiple-pollutant models. CONCLUSIONS: Ambient levels of EC and OCM, which are generated primarily from vehicle emissions, diesel, and wood burning, were associated with the largest risks of emergency hospitalization across the major chemical constituents Of PM(2.5).