Statistical analysis
We assessed the association between exposure to green spaces, measured as the cumulative exposure to NDVI during pregnancy and during early life, and the risk of asthma applying Cox’s proportional hazard regression models. The analysis was based on the time to onset of asthma, and crude and adjusted hazard ratios (HR) with 95% confidence intervals (CI) were analyzed as the measure of effect. We applied three overlapping age periods (baseline age to the ages of 6, 12 and 27 years) to investigate the influence of greenness on the risk of asthma over time. We fitted the exposure in the models as a continuous (for a 1 NDVI-month change in exposure) and as a categorical (4 quartiles) variable. Multivariate analyses were adjusted for all potential confounders, and NDVI in the spring and summer as well as in pregnancy and early life were mutually included in the model.