Ambient air pollution and pregnancy outcomes—a study of functional form and policy implications

We utilize a new data set on ambient air pollution and births from the Salt Lake Valley to study how intensive and cumulative exposure to PM^2.5 in the first trimester affect two important pregnancy outcomes—gestational age at birth and the risk of preterm birth. For identification, we use variation in cumulative exposure for siblings from the same mother in subsequent pregnancies, which can be substantial due to the large seasonal and annual variations in the valley. Controlling for other air pollutants and individual confounders, we find strong evidence of reduced gestational age and increased probability of preterm birth resulting from PM^2.5 exposure and estimate that the marginal effects are larger as cumulative exposure increases. We find weak evidence of an increased marginal effect of intensive exposure vs. total exposure. As cumulative exposure plays a larger role than intensive exposure, this indicates that policies which decrease average pollution levels can be more effective than policies targeted at peak pollution from a pregnancy perspective.