Impact of Continuously Improved Air Quality on Mental Health: A Quasi-experimental Study Based on Longitudinal Observations for Chinese Adults

Long-term exposure to fine particulate matter (PM_2.5) has been linked with adverse mental health outcomes. However, questions remain regarding the nature of lagged effects over time and by extension potential benefits over time of continued reduction in pollution. Here, we aim to estimate the long-term association between exposure to PM_2.5 and depressive symptoms in China utilizing longitudinal models for prolonged exposures as well as a quasi-experimental design utilizing data from 23151 participants over 4 longitudinal waves that occurred in 124 cities in China between 2011 to 2018. Mixed-effects models as well as distributed lag nonlinear mixed models were fitted to assess the relationship between PM_2.5 and depressive symptoms. We also assessed the effect of the Clean Air Policy (CAP) based on a quasi-experimental difference-in-differences (DID) design. The overall average PM_2.5 concentrations generally declined with time from 59.40 to 39.35 μg/m³. A 10 μg/m³ increase in PM_2.5 concentration was associated with a 0.86% increase (95% confidence interval [CI]: 0.1, 1.64%) in depression score based on the first three waves of data. However, the associations were sensitive to secular trends. Flexible exposure–lag–response analysis indicated a potentially influential window for lag-years 0–6. Reduction in PM_2.5 led to 19.51% ([CI]: 11.57%, 26.73%) and 28.18%, ([CI]: 5.87%, 45.2%) lower depressive scores in waves 3 and 4, respectively, compared to no reduction or increase in exposures. Our analysis suggests an association between PM_2.5 and depressive symptoms with potential long-term effects of air pollution as well as potential for continued benefit of air pollution reduction over time.