Residential greenness during pregnancy and early life and development of asthma up to 27 years of age: The Espoo Cohort Study.

Previous studies have suggested that living close to green spaces has protective health effects, but potential effects on asthma are contradictory. We investigated the association between the amount of greenness in the residential area during pregnancy and early life and development of asthma in the first 27 years of life. The study population included all 2568 members of the Espoo Cohort Study, Finland. We calculated individual-level exposure to green space measured as cumulative Normalized Difference Vegetation Index (cumNDVI in unit-months) within 300 m of the participant's residence during pregnancy and the first two years of life in both spring and summer seasons. The onset of asthma was assessed using information from the baseline and follow-up surveys. Exposure to residential greenness in the spring season during pregnancy was associated with an increased risk of asthma up to 6 years of age, with an adjusted hazard ratio (aHR) of 3.72 (95% confidence interval (CI): 1.11, 12.47) per a unit increase in cumNDVI. Increased greenness in the summer during pregnancy associated with asthma up to 6 years, with an aHR of 1.41 (95% CI: 0.85, 2.32). The effect was found to be related to increased greenness particularly during the third trimester of pregnancy, with an aHR of 2.37 (95% CI: 1.36, 4.14) per unit increase of cumNDVI. These associations were weaker at the ages of 12 and 27 years. No association was found between NDVI in the first two years of life and the development of asthma. Our findings provide novel evidence that exposure to greenness during pregnancy increases the risk of developing asthma. The adverse effects were strongest for the prenatal greenness in the spring season and in the third trimester of pregnancy. Both the season and trimester of exposure to greenness are critical in the development of asthma.