Seasonal and regional short-term effects of fine particles on hospital admissions in 202 US counties, 1999-2005

Abstract

The authors investigated whether short-term effects of fine particulate matter with an aerodynamic diameter < or =2.5 microm (PM(2.5)) on risk of cardiovascular and respiratory hospitalizations among the elderly varied by region and season in 202 US counties for 1999-2005. They fit 3 types of time-series models to provide evidence for 1) consistent particulate matter effects across the year, 2) different particulate matter effects by season, and 3) smoothly varying particulate matter effects throughout the year. The authors found statistically significant evidence of seasonal and regional variation in estimates of particulate matter effect. Respiratory disease effect estimates were highest in winter, with a 1.05% (95% posterior interval: 0.29, 1.82) increase in hospitalizations per 10-microg/m(3) increase in same-day PM(2.5). Cardiovascular diseases estimates were also highest in winter, with a 1.49% (95% confidence interval: 1.09, 1.89) increase in hospitalizations per 10-microg/m(3) increase in same-day PM(2.5), with associations also observed in other seasons. The strongest evidence of a relation between PM(2.5) and hospitalizations was in the Northeast for both respiratory and cardiovascular diseases. Heterogeneity of PM(2.5) effects on hospitalizations may reflect seasonal and regional differences in emissions and in particles' chemical constituents. Results can help guide development of hypotheses and further epidemiologic studies on potential heterogeneity in the toxicity of constituents of the particulate matter mixture.

Figure 1.

Percentage increase in total cardiovascular hospital admissions (1999–2005) per 10-μg/m³ increase in same-day (lag 0) particulate matter with an aerodynamic diameter ≤2.5 μm, by US region, for results from the harmonic model (curved lines) and seasonal interaction model (straight lines). Vertical lines mark the divisions among seasons as defined by the seasonal interaction model. For the harmonic model (curved lines), solid lines represent the central estimate and dashed lines the 95% posterior interval. For the seasonal interaction model (straight lines), horizontal dashed lines represent the central estimate and vertical dotted lines the 95% posterior interval. The number in parentheses (n) represents the number of US counties included in each region. Note that the y-axis scale is identical across regions. The 95% intervals for some regions are too large to fit on this scale (e.g., Northwest region for the harmonic model); a color version of this figure is available as supplemental Figure S3 (posted on the Journal's website (http://aje.oupjournals.org/)).

Figure 2.

Percentage increase in total respiratory hospital admissions (1999–2005) per 10-μg/m³ increase in same-day (lag 0) particulate matter with an aerodynamic diameter ≤2.5 μm, by US region, for results from the harmonic model (curved lines) and seasonal interaction model (straight lines). Vertical lines mark the divisions among seasons as defined by the seasonal interaction model. For the harmonic model (curved lines), solid lines represent the central estimate and dashed lines the 95% posterior interval. For the seasonal interaction model (straight lines), horizontal dashed lines represent the central estimate and vertical dotted lines the 95% posterior interval. The number in parentheses (n) represents the number of US counties included in each region. Note that the y-axis scale is identical across regions. The 95% intervals for some regions are too large to fit on this scale (e.g., Northwest region for the harmonic model); a color version of this figure is available as supplemental Figure S4 (posted on the Journal’s website (http://aje.oupjournals.org/)).

Figure 3.

Comparison of concentrations of chemical components of particulate matter with an aerodynamic diameter ≤2.5 μm (PM_2.5) (μg/m³), by US region and season, 1999–2005. A) Concentrations of specific components that were ≥20% higher in the Northeast than in other regions; B) concentrations of specific components that were ≥20% higher in winter than in other seasons. Each part uses 2 y-axis scales. Components to the left of the vertical line use the left-side y-axis scale; components to the right of the vertical line use the right-side y-axis scale. SO₄⁼, sulfate; Se, selenium; NH₄⁺, ammonium; NO₃⁻, nitrate; Ni, nickel; Zn, zinc; Pb, lead; EC, elemental carbon; Br, bromine; Cl, chlorine.