We study the international spillover effects of air pollution by developing a framework that integrates recent advances in atmospheric science into econometric estimation with 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 these estimates, we show that a recent Chinese environmental policy “war on pollution” generated a substantial international spillover benefit. Finally, we examine China’s strategic pollution reductions and provide their implications for the potential Coasian bargaining.