Varying effects of greenness in the spring and summer on the development of allergic rhinitis up to 27 years of age: The Espoo Cohort Study.

Background: Previous inconsistent evidence on effects of green space on the development of allergic rhinitis could be explained by the season of exposure. We explored whether the season and timing of exposure to green space play a role in the development of allergic rhinitis during the first 27 years of life. Methods: In a longitudinal study of 2568 participants from the Espoo Cohort Study, green space was assessed using the mean Normalized Difference Vegetation Index (NDVI) within 300 m of the participant’s residence during pregnancy and the first two years after birth during spring and summer seasons. We applied Cox proportional hazards regression to estimate adjusted hazard ratios (aHRs) and 95% confidence intervals (95% CI) for the associations between cumulative exposure to NDVI (1-unit increase) and allergic rhinitis. Results: Early-life exposure to abundant vegetation during the spring was associated with an increased risk of allergic rhinitis at 12 years of age [aHR (95% CI) = 1.726 (1.078; 2.765)] and 27 years of age [1.703 (1.139; 2.545)]. However, abundant vegetation during the summer was associated with a decreased risk of allergic rhinitis at 12 years of age [0.754 (0.585; 0.972)] and 27 years of age [0.801 (0.649; 0.989)]. Perinatal exposure to green spaces had no effect on allergic rhinitis. Conclusions: Green space has opposite effects on the development of allergic rhinitis in the spring and summer: early-life exposure to green spaces during the spring increases the risk of developing allergic rhinitis, whereas exposure to greenness in the summer decreases this risk.