U.S. census unit population exposures to ambient air pollutants

We converted 2001-2006 gridded data, generated by the U.S. Environmental Protection Agency (EPA) for CDC's (Centers for Disease Control and Prevention) Environmental Public Health Tracking Network (EPHTN), to census block group (BG) based on spatial proximities between BG and its four nearest grids. We used a bottom-up (fine to coarse) strategy to generate population exposure estimates for larger CGUs by aggregating BG estimates weighted by population distribution.The BG daily estimates were comparable to monitoring data. On average, the estimates deviated by 2 μg/m3 (for PM2.5) and 3 ppb (for ozone) from their corresponding observed values. Population exposures to ambient PM2.5 and ozone varied greatly across the U.S. In 2006, estimates for daily potential population exposure to ambient PM2.5 in west coast states, the northwest and a few areas in the east and estimates for daily potential population exposure to ambient ozone in most of California and a few areas in the east/southeast exceeded the National Ambient Air Quality Standards (NAAQS) for at least 7 days.These estimates may be useful in assessing health impacts through linkage studies and in communicating with the public and policy makers for potential intervention.Air pollution monitoring data has customarily been compiled and maintained by the EPA and/or state and local agencies. These data have been used in several studies that found ambient air pollutants associated with mortality [1-4] and morbidity [5-9]. However, air monitoring sites are typically sparsely located in very limited geographic areas - only 20% of U.S. counties have at least one monitoring station for PM2.5 - and the temporal resolution and type of pollutants measured vary by station (e.g., PM2.5 data is only available about every 3-6 days). Thus, studies based on monitoring data were usually limited to high population density areas such as cities or urban/suburban centers, where most monitoring stations are located.To expand geographic