Project summary Physical activity is an important protective factor against dementia, a condition increasingly affecting older adults worldwide. Currently, only 26% of men and 19% of women in the U.S. are sufficiently active. Public health initiatives aimed at encouraging physical activity have the potential of decreasing population-level dementia risk and related social costs. However, cumulating evidence is highly suggestive that exposure to high levels of air pollution is associated with a greater risk of dementia. Given these countervailing effects, the neuroprotective benefits induced by physical activity could be inhibited if people exercise in polluted air. The lack of knowledge on the interactive effects of physical activity and air pollution prevents the formulation of most effective dementia prevention strategies. This proposed study aims to determine if physical activity interacts with neighborhood air pollution to impact dementia risk. In addition, we will examine if certain types of air pollutants have a greater moderating effect on the relationship between certain types of physical activity and risk of dementia subtypes. This project will use a large, population-based dataset on adults ages between 40 and 69 years from UK Biobank to longitudinally examine the independent and interactive effects of physical activity and air pollution on incident dementia over ten years. The primary objective of this two year project is to test the central hypothesis that exposure to neighborhood air pollution inhibits the protective effects of physical activity on risk of incident dementia.