What should I do when the carbon monoxide alarm sounds?

Silence the alarm. Call your emergency services, fire department, or 111 and tell them your carbon monoxide alarm has triggered. Move everyone immediately to fresh air-outdoors or by an open door or window. Do a head count to check that all persons are accounted for. Do not re-enter the premises or move away from the open door or window until the emergency services responder has arrived, the premises have been aired out, and your carbon monoxide alarm remains in its normal condition.