Carbon monoxide (CO) is a colorless, odorless, and toxic gas, which is predominately produced by incomplete combustion of carbon-containing materials. Incomplete combustion occurs when insufficient oxygen is used in the fuel (hydrocarbon) burning process. Consequently, more carbon monoxide, in preference to carbon dioxide, is emitted. Some examples of this are the following: vehicle exhausts, fuel burning furnaces, coal burning power plants, small gasoline engines, portable gasoline-powered generators, power washers, fire places, charcoal grills, marine engines, forklifts, propane-powered heaters, gas water heaters, and kerosene heaters.
via CDC – Carbon Monoxide – NIOSH Workplace Safety and Health Topic.