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Introduction
Epidemiologic studies have indicated acute cardiovascular effects of fine particulate matter (PM2.5; particulate matter with aerodynamic diameter ≤2.5 μm) (Brook 2008; Dominici et al. 2006; Pope and Dockery 2006; Stafoggia et al. 2013). Because PM2.5 is a complex mixture of various chemical species, there is an ongoing effort to identify its health-relevant components. Nationwide multisite studies in the United States have examined whether the associations between PM2.5 and cardiovascular morbidity and mortality are modified by PM2.5 chemical composition (Bell et al. 2009; Franklin et al. 2008; Zanobetti et al. 2009). Other time-series studies have estimated associations between cardiovascular morbidity and mortality and individual PM2.5 components directly (Atkinson et al. 2015; Basagaña et al. 2015; Bell et al. 2014; Ito et&nfbsp;al. 2011; Levy et al. 2012; Lippmann et al. 2013; Ostro et al. 2006; Peng et al. 2009; Sarnat et al. 2015; Suh et al. 2011). Although the specific components that are associated with health outcomes vary across studies, there is growing evidence on the acute cardiovascular effects of metals/metalloids and carbonaceous components of PM2.5 (Kelly and Fussell 2012; Lippmann 2014; Rohr and Wyzga 2012).
Metals/metalloids exist in PM2.5 in different forms, with some forms being more water soluble and thus more biologically accessible than others (Allen et al. 2001; Birmili et al. 2006; Fang et al. 2015a; Heal et al. 2005). However, most ambient air pollution monitoring networks only measure these components in total elemental concentrations, and not in water-soluble concentrations. As a result, few epidemiologic studies have estimated health associations with PM2.5 water-soluble metals in their attempts to identify health-relevant components of PM2.5 (Heal et al. 2009; Huang et al. 2003).
To advance our understanding of acute cardiovascular effects of PM2.5 and its components, we conducted a time-series study in Atlanta, Georgia, to estimate the associations between daily counts of emergency department (ED) visits for cardiovascular diseases and daily concentrations of PM2.5 components, including a suite of PM2.5 water-soluble metals/metalloids that are not routinely measured at the ambient level. This analysis utilized up to 15 y of data on ambient air pollution and ED visits obtained as part of our ongoing Study of Particles and Health in Atlanta (SOPHIA) (Metzger et al. 2004; Sarnat et al. 2008; Tolbert et al. 2000; Ye et...