Association for Public Policy Analysis & Management

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. We test this using a natural experiment afforded by historical variation in sunlight (a major source of vitamin D), conducting two large-scale studies using microdata from the National Health Interview Survey and emergency room administrative discharge records from the Health Care Utilization Project.  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 prevalence and incidence of emergencies. Our results are robust to including placebo sunlight levels before and after gestation.  The findings 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 and yield a substantial surplus.