Community-level spatial heterogeneity of chemical constituent levels of fine particulates and implications for epidemiological research

Studies of the health impacts of airborne particulates' chemical constituents typically assume spatial homogeneity and estimate exposure from ambient monitors. However, factors such as local sources may cause spatially heterogeneous pollution levels. This work examines the degree to which constituent levels vary within communities and whether exposure misclassification is introduced by spatial homogeneity assumptions. Analysis considered PM2.5 elemental carbon (EC), organic carbon matter, ammonium, sulfate, nitrate, silicon, and sodium ion (Na+) for the United States, 1999-2007. Pearson correlations and coefficients of divergence were calculated and compared to distances among monitors. Linear modeling related correlations to distance between monitors, long-term constituent levels, and population density. Spatial heterogeneity was present for all constituents, yet lower for ammonium, sulfate, and nitrate. Lower correlations were associated with higher distance between monitors, especially for nitrate and sulfate, and with lower long-term levels, especially for sulfate and Na+. Analysis of colocated monitors revealed measurement error for all constituents, especially EC and Na+. Exposure misclassification may be introduced into epidemiological studies of PM2.5 constituents due to spatial variability, and is affected by constituent type and level. When assessing health effects of PM constituents, new methods are needed for estimating exposure and accounting for exposure error induced by spatial variability. Journal of Exposure Science and Environmental Epidemiology (2011) 21, 372-384; doi: 10.1038/jes.2010.24; published online 28 July 2010