Invisible and odorless carbon monoxide gas kills over 5000 people each year. Clogged chimneys, malfunctioning stoves, water heaters, and space heaters, as well as auto, boat, and airplane exhausts, release carbon monoxide into the air that you breathe, Simply place the detector onto a wall, a door, or another surface and wait for it to turn in color if the spot turns gray or black- CO is present.