Associations between green space and depressive symptoms in young adults: a longitudinal study

As the number of people living in urban areas rises, it is becoming increasingly important to quantify the relationship between exposure to green space and physical and mental health. Young adulthood in particular is a critical period with profound impacts on health, and often has implications for health outcomes later in life. Our study population consists of 779 Seattle area young adults (aged 18-23), followed for 30 months. Generalized linear mixed models were used to quantify the longitudinal associations between green space, measured by Normalized Difference Vegetation Index (NDVI), and self-reported depressive symptoms, measured by Patient Health Questionnaire 9 (PHQ-9), adjusting for covariates. Data from 3 time points were included in the models, with PHQ-9 collected at later times than NDVI to account for lag time: baseline NDVI with month 12 PHQ-9, month 12 NDVI with month 24 PHQ-9, and month 24 NDVI with month 30 PHQ-9. We found no significant associations between depressive symptoms and green space in our main models. In a sensitivity analysis, we did find that looking only at one time interval (baseline NDVI and month 12 PHQ-9), green space was inversely associated with depressive symptoms when comparing the lowest two quartiles of green space values (RR: 0.76, 95% CI: 0.61, 0.96). No other study has examined this age group making it difficult to draw firm conclusions. More research to further investigate the association between green space and depressive symptoms in the young adult population will further shed light on the role of green space in mental health.