Sunlight in Utero and Allergic and Asthmatic Emergencies

American asthma and allergy rates have risen substantially over the past generation, as individuals have spent more time indoors and as vitamin D levels (which sunlight exposure promotes) have plummeted. Using a within birth-month-county estimator, I find that cohorts with a relatively sunny second trimester in utero had lower per capita rates of asthma emergencies by 0.6 percentage points (6%), consistent with the medical literature that links second trimester maternal vitamin D levels to lung development. This result is driven by the male per capita rate, and the effect is counterintuitively greater in counties with higher median incomes and for white individuals. Given the high cost of emergency department visits and the low cost of vitamin D supplements, this finding motivates a cost effective solution. Results on allergy emergency rates are inconclusive. Asthma results are robust to include relative sunlight levels before conception and after birth, whereas allergy results are not.