Exposure to residential greenness and subsequent risk of common mental disorders among individuals with COVID-19 in South Korea: A nationwide cohort study

Background Residential greenness is considered protective for mental health, but its impact on clinically diagnosed psychiatric disorders following COVID-19 remains unclear. This study investigated the association between residential greenness and risk of common mental disorders after COVID-19, and whether this relationship differed by population subgroup. Methods We conducted a nationwide cohort study of 3,097,179 adults aged ≥19 years with confirmed COVID-19 in South Korea between 2018 and 2023 using the K-COV-N cohort, which integrates administrative and clinical data from the National Health Insurance Service and COVID-19 data from the Korea Disease Control and Prevention Agency. Residential greenness was assessed using the normalized difference vegetation index (NDVI) and categorized as low (0.2– < 0.4), moderate (0.4– < 0.6), or high (≥0.6). The primary outcome was first diagnosis of anxiety disorder, depression, insomnia, or bipolar disorder ≥30 days post-infection. Weighted Cox proportional hazards models with inverse probability of treatment weighting were used to estimate adjusted hazard ratios (aHRs). Results Compared to low NDVI, moderate NDVI was associated with reduced risk of any common mental disorder (aHR, 0.97; 95% CI, 0.96–0.98), with the strongest effect for bipolar disorder (0.84; 0.79–0.88). High NDVI showed no overall benefit (1.00; 0.99–1.01) and was associated with increased anxiety risk (1.04; 1.03–1.06), especially in older adults and those with comorbidities. Younger individuals experienced the greatest benefit from moderate greenness. Conclusion Moderate residential greenness was linked to lower risk of mental disorders following COVID-19, while high greenness conferred limited benefit and potential risk. Greening strategies should be paired with strengthened health and social infrastructure to reduce disparities.