We study the international spillover effects of air pollution by developing a framework that integrates recent advances in atmospheric science with econometric estimation and microdata on mortality and health. Combining transboundary particle trajectory data with the universe of individual-level mortality and emergency room visit data in South Korea, we find that transboundary air pollution from China significantly increases mortality and morbidity in South Korea. Using our estimates, we quantify that a recent Chinese environmental regulation “war on pollution” had a substantial international spillover benefit. Finally, we examine China’s strategic pollution reductions and provide its implications for the potential Coasian bargaining.