As discussed above, infants of smoking mothers could be either more or less affected than other infants.
We investigate this issue in Table 4, which shows estimates for mothers who smoked during both pregnancies.
The point estimates in Table 4 are generally much larger than those in Table 2, suggesting the same level of pollution exposure is more harmful to the infants of smokers.
Although the effects of CO are no longer statistically significant in the model for birth weight, the point estimate of − 39.2 in the model with mother fixed effects is twice as large as the Table 2 coefficient.
The coefficient on CO in the models of low birth weight is 0.044 compared to 0.008 in Table 2.
For gestation, the Table 4 coefficient on CO is − 43 compared to − 074 in Table 2.
These estimates indicate that the harmful effects from CO are two to six times greater for smoking mothers than for non-smoking mothers, depending on the outcome.
Similarly, the impact of ozone is four to six times larger for smoking mothers.
Furthermore, we now also find that PM10 in the second and third trimesters has a statistically significant impact on birth weight, while PM10 in the first and second trimesters are both estimated to increase the incidence of low birth weight. PM10 in the second trimester is also estimated to reduce gestation significantly.