Benefit of residential greenness on visual acuity among 3-year-old children through reduction of exposure to ambient air pollutants

A growing body of evidence highlights the health benefits of green space, but underlying mechanisms remain unclear. We hypothesized that the benefits of greenness on children's visual acuity may be mediated through reduced air pollution exposure. This study examined the mediating role of air pollutants in the association between greenness and visual acuity in 3-year-old children. We used data from the Korean Children's Environmental Health Study. Of 5458 cohort participants, 794 children were included. Residential greenness during pregnancy was measured using the Normalized Difference Vegetation Index (NDVI), and PM_2.5, PM₁₀, and NO₂ were estimated with kriging interpolation for the first year after birth. Visual acuity was assessed with Jin's chart and averaged for both eyes. Associations between NDVI, pollutants, and visual acuity were analyzed using generalized additive models, multivariable regression, and causal mediation analysis. A 1 % increase in z-score- standardized NDVI at 200 m was associated with decreases of 0.25 μg/m³ (95 % CI: -0.31, -0.18), 0.24 μg/m³ (95 % CI: -0.35, -0.22), and 0.55 ppm (95 % CI: -0.61, -0.50) in PM₁₀, PM_2.5, and NO₂. A 1 % NDVI increase at 200 m was also associated with a 0.023-point (95 % CI: 0.013, 0.032) increase in visual acuity score (1.0 equals 40/40 on Snellen chart). Smaller buffers showed stronger associations. At 200 m, decreases in PM₁₀, PM_2.5, and NO₂ mediated the NDVI-visual acuity relationship, with mediation proportions of 11.6 %, 9.8 %, and 55 %. In two-pollutant models, mediation by both PM₁₀ and PM_2.5 was no longer significant after adjustment for NO2. These findings suggest that nearby residential greenness may protect visual function in early childhood primarily through reduced exposure to NO₂.