Exposure to ambient air pollution may be associated with impaired mental health, including depression, anxiety, and even suicide rate. However, there are only a limited number of published studies investigating the associations between ambient air pollution and mental health and there exists considerable inconsistencies regarding the conclusions. This project aims to deepen our understanding of the links between exposure to fine particulate matter and depression in Chinese megacities, as well as to examine the social inequalities in relation to ambient air pollution.