Second Trimester Sunlight and Asthma: Evidence from Two Independent Studies

One in twelve Americans suffers from asthma and its annual costs are estimated to exceed $50 billion. Simultaneously, the root causes of the disease remain unknown. A recent hypothesis speculates that maternal vitamin D levels during pregnancy affect the probability the fetus later develops asthma. In two large-scale studies, we test this hypothesis using a natural experiment afforded by historical variation in sunlight, a major source of vitamin D. Specifically, holding the birth location and month fixed, we see how exogenous within-location variation in sunlight across birth years affects the probability of asthma onset. We show that this measurement of sunlight correlates with actual exposure, and consistent with pre-existing results from the fetal development literature, we find substantial and highly significant evidence in both datasets that increased sunlight during the second trimester lowers the subsequent probability of asthma. Our results suggest policies designed to augment vitamin D levels in pregnant women, the large majority of whom are vitamin D insufficient, could be very cost-effective.