Carbon monoxide (CO) is regarded as a leading cause of morbidity and mortality. It endangers not only patients, but also health care professionals, especially emergency medical services (EMS) personnel because CO exposure is often unknown at the time EMS is called.Our objective was to report a case of unrecognized CO exposure during the treatment of a patient that finally led to the hospitalization of 11 EMS personnel.A 71-year-old man was found unconscious in the basement of his house. EMS was called and, due to ST-segment elevations on electrocardiogram, the patient was treated for acute coronary syndrome. Unknown to EMS personnel, ongoing CO exposure was the cause of the patient's symptoms. EMS staff finally had to be evacuated by firefighters, and a total of 12 persons, including the initial patient, had to be hospitalized.In the prehospital setting, hazardous environments always have to be considered as potential causes of a patient's altered status. Together with the correct use of modern equipment, such as permanently switched-on CO detectors, this can help avoid harm to both patients and staff.