Invisible enemy: The health impact of ozone

China has exerted great efforts in the past decades to fight against particulate matters and has achieved initial results, but ozone pollution has increased significantly. Using a unique diagnostic data of a peninsula city in China, this study evaluates the health effect of ozone pollution on emergency room admission and medical expenditure. We present strong evidence that ozone has a significant negative impact on health. Specifically, one standard deviation increase in ozone pollution will result in an additional 3.30, 2.25, and 4.94 weekly emergency room admissions in respiratory, ophthalmology, and internal medicine. Our analysis of medical expenditure implies that one standard deviation increase of ozone pollution generates a 5.89 million yuan increase of annual medical expenditure for a representative city. We further show that land wind, sunny weather, and dense roads significantly moderate the adverse health effects of ozone. Our findings offer a timely reminder for policymakers to pay attention to “invisible” ozone pollution while managing particulate pollution.