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.