Carbon monoxide alarm showing dangerous levels. Carbon monoxide (CO) is a colourless odourless gas that can lead to carbon monoxide poisoning, a type of asphyxiation. Household gas boilers and hobs that are working incorrectly may produce high levels of CO. This unit has detected a dangerous level of 208 parts per million (PPM) CO, and displays a red light whilst emitting a high-pitched alarm tone. Symptoms of CO poisoning include headache, dizziness, nausea, chest tightness, fatigue, lack of concentration and memory loss. Exposure to high concentrations can cause convulsions, unconsciousness and death.