Carbon monoxide detector. This wall-mounted household device is designed to detect excessive levels of the toxic and deadly gas carbon monoxide (CO) and emit an audible alarm to warn the occupants of the house. Carbon monoxide can be produced by malfunctioning domestic boilers. It kills people by affecting oxygen uptake in the blood. The reading shown here is 19 parts per million (ppm) of CO. 35 ppm of CO produces headaches and dizziness after six hours of exposure. 800 ppm of CO causes unconciousness within hours, with higher levels causing death. This is a Nighthawk CO detector, which uses an electrochemical sensor.