Fig. 2 provides odds ratios for childhood asthma by exposure
quartiles of air pollutants. The results were similar to those obtained
by the continuously increasing model. It is clear that the
upper quartiles of exposure for SO2 and NO2 increased the asthma
risk with respect to the lowest quartile of exposure both in utero
and during the first year; the risks were not significant at low
exposure levels but became significant at high exposure levels. The
ORs of PM10 for asthmatic risk were not significant at all exposure
quartiles in utero, but during the first year the risk of PM10 may be
significant at high exposure level although it was not significant
from the view of the whole exposure domain (Table 3).
To consider the impact of children’s migration, we analyzed the
exposure level and its asthmatic risk for the 1671 (67.1%) children
who did not move during pregnancy and the first year of life
(please see Tables S1 and S2 in the Supplementary materials) since
the moving patterns of the children were not known. The air
pollutant exposure levels for the stayed children were nearly the
same as those for the whole 2490 children. The odds ratios of
asthma for the stayed children were slightly changed due to different
number of children but the tendencies were consistent with
those for the whole children. Therefore, the influence of the children’s
migration in our present work was very limited.