Physical activity may be influenced by walking opportunities, which can differ substantially across neighborhoods. Air pollution similarly varies by location. It remains unclear whether more walking occurs in highly polluted neighborhoods and whether socially disadvantaged populations are less likely to experience walkable neighborhoods with clean air. We studied the spatial distributions and associations of neighborhood level air pollution, walkability, and social factors with self-reported walking. Overall walking was reported between 2010-2012 by 3,661 Multi-Ethnic Study of Atherosclerosis...