Urban air pollution is reportedly responsible for nearly 1 million annual deaths worldwide. The burden of disease from air pollutant exposure may be greatest in growing urban populations in the developing world, but risk assessment is often impeded by lack of exposure information at the local level in many urban settings. Dionisio et al. (p. 607) conducted 1 week of morning and afternoon mobile and stationary air pollution measurements in four study neighborhoods of varying socioeconomic status (SES) in Accra, Ghana. Particulate matter ? 2.5 or ? 10