Carbon Monoxide Sources:

CO is produced when:

carbonaceous materials are burned with insufficient oxygen. Its lethal effects have been known for centuries; in ancient Rome, "coal gas" was used for executions and suicides.

Man-made sources of CO include:

--- Incomplete fuel combustion used in transport -
Automobile/Truck exhaust
Airplane exhaust
--- Smoking of cigarettes, cigars, pipes, etc.
--- Defective heating (furnace, water heater) systems
--- Defective cooking appliances
--- Industrial plant exhausts
--- Burning of solid waste
--- Detonation of explosives

Natural sources of CO also exist but they contribute very little to the overall atmospheric CO level.