Introduction: Previous studies have suggested that residential green space has protective health effects, but potential effects on asthma are contradictory.

Objectives: To investigate 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.

Methods: The study population included all the 2568 members of the Espoo Cohort Study. We calculated individual-level cumulative exposure to Normalized Difference Vegetation Index (NDVI) within 300 m of the participant?s residence during pregnancy and first two years of life in both spring and summer seasons. The incidence of asthma was assessed using information from the baseline and follow-up surveys and register data. Adjusted hazard ratios (HR) and their 95% confidence intervals (CI) were estimated using Cox proportional hazards models, where the exposure was fitted as a continuous variable (i.e. for a 1 NDVI-month change in exposure).

Results: Exposure to increased residential greenness in spring season during pregnancy was associated with an increased risk of asthma up to the 6 years of age with an aHR 3.72 (95% CI 1.11-12.47) per 1 increase in NDVI. Increased greenness in summer during pregnancy associated with asthma with an aHR 1.41 (95% CI 0.85-2.32) up to 6 years. Significance of the associations disappeared by the 12 and 27 years. No association was found between NDVI in the first two years of life and development of asthma.

Conclusions: Our findings provide evidence that exposure to greenness during pregnancy may increase the risk of developing asthma in the first 6 years of life.