Do trees in urban areas benefit human health? More than 100 million Americans live in large cities, yet little is known about the health benefits of the trees they live with. I provide causal evidence on the elasticity of air pollution and mortality to urban forest stocks from the exogenous introduction of the emerald ash borer insect to the United States. Trees primarily benefit urban health by reducing pollution, with losses in tree cover of up to 5.1% being associated with increases in mean PM2.5 of up to 5.8%. Unsurprisingly, mortality from related conditions like chronic respiratory disease, cardiovascular disease, and neonatal disorders increased by up to 3.5%, 2.5% and 2.1% respectively. These contributed to excess deaths of up to 1.4%; at the implied elasticity of 0.27, my results imply urban forests reduced all-cause mortality by 21.5%, or 197000 deaths total, in 2014.