To summarize: third trimester CO has statistically significant,negative effects on infant health in all of our specifications, with the estimated effect gradually increasing as we control more thoroughly for potential confounders.
In contrast, the estimated effects of PM10 and ozone are inconsistent across specifications, with none statistically significant at the 95% level in the models that control for mother fixed effects.
The estimates in Table 2 imply that a one unit increase in the mean level of CO during the last trimester (where the mean is 1.64 and standard deviation is 0.79) would reduce average birth weight by 16.65 g (from a base of 3236 g)—a reduction of about a half a percent.
The proportional effects are greater for low birth weight where a one unit change in mean CO would lead to an increase in low birth weight of 0.0083 (from a base of 0.106)—an 8% increase in the incidence of low birth weight.
The greater effect for low birth weight than for mean birth weight suggests that infants at risk of low birth weight are most likely to be affected by pollution, an observation that we explore further below by examining infants with various risk factors.
Additionally, a one unit change in mean CO is estimated to reduce gestation by 0.074 week (from a base of 38.55 weeks)—a reduction in mean gestation of 0.2%.