BackgroundExposure to natural environments may affect respiratory health. This study examined the association of exposure to green and blue spaces with lung function in children, and assessed the mediation effect of air pollution and physical activity.MethodsThe study used data from the Generation XXI, a population-based birth cohort from the Porto Metropolitan Area (Portugal). Residential Normalised Difference Vegetation Index (NDVI) at different buffers (100, 250 and 500 m), the accessibility to urban green spaces (UGS) within 400 and 800 m and the minimum distance to the nearest UGS and to the nearest blue spaces were assessed at birth, 4, 7 and 10 years of age. Three life-course measures were calculated: averaged exposure, early-life exposure (birth) and exposure trend over time (change in exposure). Forced vital capacity (FVC), forced expiratory volume in 1 s (FEV₁) and forced expiratory flow between 25% and 75% of FVC (FEF_25–75%) at 10 years were used as outcomes. To assess associations, linear regression models and path analysis were used.ResultsThis study included 3278 children. The adjusted models showed that increasing the NDVI exposure over time within 100 m of the child's residence was associated with higher values of FEV₁(L) and FEF_25–75%(L·s⁻¹) (β 0.01, 95% CI 0.0002–0.03 and β 0.02, 95% CI 0.001–0.05, respectively). No significant associations were observed for the remaining measures of exposure, and no mediation effect was found for pollution or physical activity.ConclusionIncreasing exposure to greenness at close proximity from residences was associated with improved lung function. While the mechanism remains unknown, this study brings evidence that city greening may improve children's respiratory health.