Long-term exposure to residential greenness and decreased risk of depression and anxiety

Residential greenness is considered a unique and potentially modifiable exposure construct to reduce physiological stress and improve human health. Here this study aims to investigate the longitudinal relationships of residential greenness with incident depression and anxiety and to explore and compare the pathways in which greenery may influence mental health. After excluding participants with depression or anxiety at baseline, a family history of severe depression, loss to follow-up or missing information on greenness exposure, we analyzed data of 409,556 participants from the UK Biobank. Residential greenness was assessed utilizing the normalized difference vegetation index (NDVI) within a buffer region of 300 m, 500 m, 1,000 m and 1,500 m. Incident depression and anxiety cases were identified by linking to records on the death register, hospital admissions, primary care and self-report. Time-varying Cox proportional hazards models were used to analyze the associations between greenness and incident depression and anxiety. During a median follow-up of 11.9 years, 14,309 (3.5%, 306.9/100,000 person-years) and 16,692 (4.1%, 358.0/100,000 person-years) patients were diagnosed with depression and anxiety, respectively. The hazard ratios and 95% confidence intervals (CIs) for depression and anxiety were 0.84 (95% CI, 0.82–0.85 and P < 0.001) and 0.86 (95% CI, 0.84–0.87 and P < 0.001) in the highest quartile compared with the lowest quartile of NDVI 300 m, respectively. Similar trends were shown for NDVI 500 m, 1,000 m and 1,500 m. Air pollution (particulate matter with an aerodynamic diameter of 2.5 μm or less (PM2.5), NO2, NOx, SO2 and O3) played a major mediator role in the associations. For example, the association between NDVI 300 m and depression was 52.9% (95% CI, 31.6–73.1%), 28.4% (95% CI, 13.4–50.3%), 30.9% (95% CI, 17.8–48.1%), 2.4% (95% CI, 1.4–4.1%) and 27.7% (95% CI, 19.4–37.9%) mediated by the reduced PM2.5, NO2, NOx, SO2 and O3, respectively. This national study highlights that long-term exposure to residential greenness was linked to a decreased risk of incident depression and anxiety. Reduced air pollution was a significant mediator linking green environments to depression and anxiety.