The built environment affects public health in many ways (Frumkin et al. 2004), depending on the interplay between factors such as community design, travel patterns, physical activity, transportation safety, and air and water pollution. This study investigated interactions between a) walkability, a measure of how conducive the built environment is to walking and that predicts physical activity and active transportation (Frank et al. 2005; Owen et al. 2004; Sallis et al. 2004), and b) exposure to outdoor air pollution, which is associated with a wide array of negative health impacts.