They function similar to the way a smoke detector does. When they sense CO, they set off an alarm to alert the occupants. The symptoms can mimic other medical conditions so the situation may need to be further evaluated. The fire department and the gas company have survey meters that can measure the amount of CO in an atmosphere, measured in parts per million. Low level poisonings can produce headaches, fatigue, nausea, dizzy spells and confusion.
Higher levels can include unconsciousness and death.
Carbon monoxide enters your body by breathing it in through your lungs. It displaces the oxygen in your system and can suffocate you from the inside, with the possibility of brain damage or death. The flames will be burning clean and blue in color. If the efficient burning process is interrupted somehow, the flames will no longer look clean and blue. They will have a large amount of yellow flames in the burner.