Table 1 suggests that the models that do not adequately control for characteristics of the mother’s location and for her own characteristics can be misleading.
For example, although urban mothers are typically exposed to higher levels of pollution, they are also wealthier and more educated in our data and may have better access to health care.
Failure to control for these factors could yield estimated coefficients that are biased down and possibly even wrong-signed.
Few of the pollution measures in columns (1), (4),and (7) are statistically significant, and when they are, they are as likely to suggest positive effects on birth weight and gestation as negative ones.