The US Environmental Protection Agency classifies air pollution releases that are due to accidents, malfunctions, or unanticipated facility start-ups and shutdowns as excess emissions, which violate the Clean Air Act. Despite this, states have historically granted emitting facilities exemptions, shielding facilities from enforcement and penalties. We outline recent research that documents the incidence, magnitude, environmental impacts, and health effects of these emissions to inform the considerable policy debate surrounding their regulation. The majority of prior research focuses on Texas because it is the only state that provides access to detailed data on excess emissions that can be easily used for research. This data limitation creates uncertainties about the incidence, magnitude, and impacts of these emissions outside of Texas. We argue that a requirement for detailed data reporting in all states would best enable policy makers to design an effective regulatory framework.