Predicting chronic fine and coarse particulate exposures using spatiotemporal models for the northeastern and midwestern United States

Background: Chronic epidemiologic studies of particulate matter (PM) are limited by the lack of monitoring data, relying instead on citywide ambient concentrations to estimate exposures. This method ignores within-city spatial gradients and restricts studies to areas with nearby monitoring data. This lack of data is particularly restrictive for fine particles (PM with aerodynamic diameter < 2.5 μm; PM_2.5) and coarse particles (PM with aerodynamic diameter 2.5-10 μm; PM_10-2.5), for which monitoring is limited before 1999. To address these limitations, we developed spatiotemporal models to predict monthly outdoor PM_2.5 and PM_10-2.5 concentrations for the northeastern and midwestern United States. Methods: For PM_2.5, we developed models for two periods: 1988-1998 and 1999-2002. Both models included smooth spatial and regression terms of geographic information system-based and meteorologic predictors. To compensate for sparse monitoring data, the pre-1999 model also included predicted PM₁₀ (PM with aerodynamic diameter < 10 μm) and extinction coefficients (km^-1). PM_10-2.5 levels were estimated as the difference in monthly predicted PM₁₀ and PM_2.5, with predicted PM₁₀ from our previously developed PM₁₀ model. Results: Predictive performance for PM_2.5 was strong (cross-validation R² = 0.77 and 0.69 for post-1999 and pre-1999 PM_2.5 models, respectively) with high precision (2.2 and 2.7 μg/m³, respectively). Models performed well irrespective of population density and season. Predictive performance for PM_10-2.5 was weaker (cross-validation R² = 0.39) with lower precision (5.5 μg/m³). PM_10-2.5 levels exhibited greater local spatial variability than PM₁₀ or PM_2.5, suggesting that PM_2.5 measurements at ambient monitoring sites are more representative for surrounding populations than for PM₁₀ and especially PM_10-2.5. Conclusions: We provide semiempirical models to predict spatially and temporally resolved long-term average outdoor concentrations of PM_2.5 and PM_10-2.5 for estimating exposures of populations living in the northeastern and midwestern United States.