Carbon Monoxide is a poisonous gas produced by the incomplete burning of various fuels, including coal, wood, oil, charcoal, kerosene, propane, and natural gas. Indoors, products that product CO include malfunctioning fuel-burning appliances such as furnaces such as furnaces, ranges, water heaters and room heaters; engine-powered equipment such as portable generators; fireplaces; and charcoal that is burned in homes and other enclosed areas.