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.
Incomplete fuel combustion used in transport –
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.
Natural gases in coal mines
Photo-dissociation of CO in upper atmosphere
Formation of CO during electrical storms
Endogenous CO production by land animals
From vegetation during seed germination
From marine brown algae or kelp
Marine hydrozoans: e.g. jellyfish