One challenge in assessing the health effects of human exposure to air pollution in epidemiologic studies is the lack of widespread historical air pollutant monitoring data to characterize past exposure levels. Davis et al. (p. 614) developed an alternative model of exposure based on the hypothesis that economic activity predicts exposure conditions for air pollutants, especially at the local level. Statewide unemployment and county-level trucking industry characteristics were used to estimate historical coefficient of haze (COH), a marker of vehicle-related PM predominantly from diesel exhaust. The authors conclude that economic activity data can serve as a surrogate marker of changes in exposure levels over time in the absence of direct monitoring data, thus providing an alternative method for reconstructing historical exposures that may greatly aid epidemiologic research on chronic health effects of urban air pollution.