Although it has been hypothesized that air pollution, particularly PM2.5 and PM10, causes depressed symptoms, their interactions with greenness have not yet been confirmed. This study examined the association between depression symptoms and air pollution, as well as the potential moderating effects of greenness.

A total of 7657 people from all around South Korea were examined using information from the Korean Longitudinal Study of Aging, for the years 2016, 2018 and 2020. Depressive symptoms were assessed using the CES-D 10 score (Center for Epidemiology Studies of Depression scale, Boston form), and annual air pollution levels (PM2.5, PM10) and greenness (NDVI, Landsat Normalized Difference Vegetation Index) at the district level (si-gun-gu) were considered for the association analysis. The investigation was primarily concerned with determining how the CES-D 10 score changed for each 10 ${\mu \text{g/}}{{\text{m}}^{\text{3}}}$ increase in PM2.5 and PM10 according to NDVI quantiles, respectively. The analysis used generalized estimating equation models that were adjusted with both minimal and complete variables. Subgroup analyses were conducted based on age groups (<65, ≥65 years old), sex and exercise status.

The impact of PM10 on depression in the fourth quantile of NDVI was substantially less in the fully adjusted linear mixed model (OR for depression with a 10 ${\mu\text{ g/}}{{\text{m}}^{\text{3}}}$ increment of PM10: 1.29, 95% CI: 1.06, 1.58) than in the first quantile (OR: 1.88, 95% CI: 1.58, 2.25). In a similar vein, the effect of PM2.5 on depression was considerably reduced in the fourth quantile of NDVI (OR for depression with a 10 ${\mu\text{ g/}}{{\text{m}}^{\text{3}}}$ increment of PM2.5: 1.78, 95% CI: 1.30, 2.44) compared to the first (OR: 3.75, 95% CI: 2.75, 5.10). Subgroup analysis results demonstrated beneficial effects of greenness in the relationship between particulate matter and depression.

This longitudinal panel study found that a higher quantile of NDVI was associated with a significantly reduced influence of air pollution (PM10, PM2.5) on depression among older individuals in South Korea.