Timing and duration of neighborhood greenness exposure and their associations with depressive Symptoms: A 12-year follow-up of 527,965 Korean adults

Background: Neighborhood greenness has been linked to better mental health outcomes. However, it remains unclear how temporal variations—such as the timing (e.g., season) and duration of greenness exposure—affect its association with depression. Methods: Data from 527,965 adults (mean age: 39.4) undergoing routine annual or biennial health checkups were analyzed to examine the association between greenness exposure and depressive symptoms, assessed by Center for Epidemiological Studies-Depression (CES-D) scores (elevated: ≥16; clinically significant ≥25). Neighborhood-level greenness was quantified using the Normalized Difference Vegetation Index (NDVI) at the zip-code level across short-term (past-month), long-term (annual average), and peak season (summer) periods. We used multivariable generalized estimating equations (GEE) and Cox proportional hazards models to assess associations accounting for variation in exposure timing and duration. Results: At baseline, the prevalence of depressive symptoms was 10.8 % (CES-D ≥16) and 3.3 % (CES-D ≥25). During a median follow-up of 5.6 years among 326,099 participants free of depression at baseline, 23,465 developed clinically significant depressive symptoms. Higher short-term greenness exposure was associated with lower odds of prevalent depressive symptoms (OR_{Q5 vs. Q1}:0.94, 95 % CI:0.90, 0.98, P-trend: 0.01), whereas long-term and peak season greenness showed no such associations. However, in individuals without baseline symptoms, higher long-term greenness was associated with reduced risk of incident depressive symptoms (HR_{Q5 vs. Q1}:0.93, 95 % CI:0.89, 0.97). Conclusions: Both recent and sustained exposure to neighborhood greenness may yield mental health benefits, with distinct effects depending on the timing of exposure. These findings support urban greening initiatives as promising, time-sensitive strategies for depression prevention in urbanizing populations.