Carbon monoxide is a colorless, odorless, poisonous gas that's produced as a by-product of incomplete combustion -- that is, it's present when things burn and give off smoke.

Dangerous levels of CO can occur when vehicles are left running in closed garages and when fuel-burning appliances malfunction. Dangerous levels of CO may be present when there are:

- Vehicles running in a garage
- Rust or water streaks on roof vents or chimneys
- Loose or missing furnace panels
- Sooting on appliances
- Loose or disconnected roof vents or chimney connections
- Debris or soot falling from chimney, fireplace, or appliances
- Loose masonry on chimney
- Moisture inside of windows or on the walls of furnace rooms
- Improperly adjusted gas burners
- Gas-burning space heaters used as the primary source of heat in the room
- Blocked chimney flues for wood-burning fireplaces or stoves
- Occupants with symptoms of CO poisoning
- A CO detector that's sounding
- Charcoal or gas grills operated indoors

To avoid the risk of illness, make sure that all gas-burning appliances are serviced by professionals annually, that chimney flues are cleaned regularly, that gas-burning space heaters are well ventilated, and that grills are never used indoors.

And be sure there's an operating CO detector near each frequently occupied space in your home.