Association between long-term PM2.5 exposure and depression among Chinese adults in the context of population aging: a quasi-experimental study

Background: Recent studies suggest an association between mental disorders such as depression and air pollution. However, few studies examined the association between air pollution reduction and mental health improvement. Since 2013, China has carried out a series of clean air actions that have rapidly improved air quality, and provided a quasi-experimental scenario to examine the association. Method: Based on nationwide surveys of Chinese adults from 2011 to 2015, we evaluated the association between long-term PM2.5 exposure and a widely-used depression score (C-ESD score), using a mixed-effects model with multivariate adjustment. The association between PM2.5 reduction and the score change was further explored using a difference-in-difference analysis of the temporal contrast between 2011 (before the actions) and 2015 (after the actions). To increase interpretability of the association, the estimated impact of PM2.5 levels was compared to that of aging, a well-known risk factor for depression. Results: A 10 ug/m3 increase in PM2.5 concentration was associated with a 3.63% (95% confidence interval [CI]: 2.00~5.27%) increase in the C-ESD score (a higher score indicates larger probability of depression). Aging of 1 year was associated with a score increase of 0.76% (0.45~1.07%), equivalent to the effect of a 2.1 ug/m3 (95% CI: 1.1~4.2 ug/m3) increase in PM2.5 concentration. Difference-in-difference analysis confirmed the significant association between PM2.5 reduction and the score reduction. We also found improved air quality during 2011-2015 offset the negative impact from 5-years9 aging. Conclusions: This study added the epidemiological evidence on the association between depression and long-term exposure to PM2.5. Our findings also suggested the mental health benefits from China9s recent efforts to reduce air pollution.