The United Nations has identified improving the walkability and bikeability of cities as a key goal in efforts to reduce dependence on greenhouse gas–emitting automobiles and promote routine exercise for public health. However, increased walking and cycling can come with their own health risks. Time spent in the open air, in addition to other factors, increases exposure to airborne pollutants, for example. The most harmful category of pollutants to human health includes inhalable particles smaller than 2.5 micrometers, referred to as PM2.5. It has been estimated that more than 4 million premature deaths worldwide are attributable to these pollutants each year. To help reduce health risks from this pollution, Tong et al. developed a new approach for estimating ground-level PM2.5 concentrations from satellite observations and incorporated it into a mobile mapping application that identifies routes through Hong Kong to minimize pedestrians' exposure to PM2.5.