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).