Residential greenness during pregnancy and early life in the spring and summer seasons and the risk of asthma
Distribution of the mean values of the neighborhood levels and cumulative levels of NDVI are presented in Table 2.
The risk of asthma by the age of 6 years increased monotonously by an increase in residential greenness during pregnancy with an aHR of 3.72 (95% CI: 1.11, 12.47) per 1 unit-month increase in cumNDVI (Table 3 and Table S1). This association was found to be linear between the quartiles of cumNDVI exposure: children whose mothers were exposed to the highest quartile of cumNDVI during pregnancy in the spring had a significantly increased risk of asthma between 0 to 6 years of age compared to children whose mothers were exposed to the third, second and first quartiles of cumNDVI, the aHR being 2.31 (95% CI: 1.20, 4.45) for the highest NDVI quartile contrasted with the lowest quartile. Similar pattern was present for asthma up to 12 and 27 years of age, although the confidence intervals were broader and included unity (Table 3). The risk of asthma by the age of 6 years was also related to greenness in the summer during pregnancy (aHR=1.41; 95% CI: 0.85, 2.32), although the effect estimate was weaker than the corresponding effect estimate for the spring and the 95% CI was broader including unity.
There was no consistent evidence of an association between NDVI in early childhood and the risk of asthma (Table 3 and Table S1).
In the sensitivity analysis, we calculated season-specific effects estimates for cumNDVI exposure, defining March-May as the spring and June-September as the summer season. The adjusted HR of asthma by the age of 6 years (aHR=2.51; 95% CI: 1.14, 5.54) (Table S2) was somewhat weaker for the spring exposure compared to the main analysis shown in Table 3 (aHR=3.72).