The longitudinal relationship between exposure to air pollution and depression in older adults

Background: Air pollution, especially PM_2.5 (particulate matter with a diameter of below 2.5 μm), has been recognized as a key environmental factor that affects mental health, but few studies have focused on its influence on older adults, who are a vulnerable group. 
Objective: This study focused on the influence of PM_2.5 on health-related behaviors, such as physical activities and social contact, to assess their role as mediators of depression among older adults in China. 
Methods: We used data (N = 24 623) from the CHARLS (China Health and Retirement Longitudinal Study) of 2011-2015. CES-D 10 (Center for Epidemiology Studies of Depression scale) was used to measure depression. Using multilevel linear models, we examined the relationships between the variables, with different times nested within the same individual and individuals nested within the cities. 
Results: Before mediators were added, depression symptoms among older adults increased with annual concentration of PM_2.5 (Coeff = 0.57, SE = 0.11). However, after the mediators were added, the coefficient of the annual concentration of PM_2.5 decreased (Coeff = 0.37, SE = 0.10). While both physical activities (Sobel test Z score = 2.37, P value =.02) and social contact (Z score = 7.33, P value =.00) mediated the relationship between PM_2.5 and depression, the mediating effects decreased with increasing PM_2.5. 
Conclusions: Exposure to PM_2.5, therefore, increases depressive symptoms in older Chinese adults by decreasing their physical activities and social contact. Also, the positive effects of physical activities and social contact on depression decreased with increasing PM_2.5 concentrations.