Overcrowding in emergency departments is a recognized safety hazard for patients. This study of heart attack patients in California demonstrates that overcrowding can have population-level effects when it is severe enough to force hospitals to close themselves to incoming ambulances ("diversion"). Patients whose closest emergency department experienced significant ambulance diversion had decreased access to life-saving therapies and had an increased one-year mortality risk. These results highlight the importance of patient flow throughout the hospital in order to ensure emergency department capacity. A past AHRQ WebM&M commentary discussed overcrowding in emergency rooms and triage processes to manage patient volume.