Carbon monoxide is a product of combustion of organic matter under conditions of restricted oxygen supply, which prevents complete oxidation to carbon dioxide (CO2). Sources of carbon monoxide include cigarette smoke, house fires, faulty furnaces, heaters, wood-burning stoves,[43] internal combustion vehicle exhaust, electrical generators, propane-fueled equipment such as portable stoves, and gasoline-powered tools such as leaf blowers, lawn mowers, high-pressure washers, concrete cutting saws, power trowels, and welders.[18][33][44][45][46][47][48] Exposure typically occurs when equipment is used in buildings or semi-enclosed spaces.[18]

Riding in pickup trucks has led to poisoning in children.[49] Idling automobiles with the exhaust pipe blocked by snow has led to the poisoning of car occupants.[50] Any perforation between the exhaust manifold and shroud can result in exhaust gases reaching the cabin. Generators and propulsion engines on boats, especially houseboats, has resulted in fatal carbon monoxide exposures.[51][52]

Poisoning may also occur following the use of a self-contained underwater breathing apparatus (SCUBA) due to faulty diving air compressors.[53]

In caves carbon monoxide can build up in enclosed chambers due to the presence of decomposing organic matter.[54] In coal mines incomplete combustion may occur during explosions resulting in the production of afterdamp. The gas is up to 3% CO and may be fatal after just a single breath.[42] Following an explosion in a colliery adjacent, interconnected, mines may become dangerous due to the afterdamp leaking from mine to mine. Such an incident followed the Trimdon Grange explosion which killed men in the Kelloe mine.[55]

Another source of poisoning is exposure to the organic solvent dichloromethane, found in some paint strippers,[56] as the metabolism of dichloromethane produces carbon monoxide.[57][58][59]

Carbon Monoxide is the scariest shit out there. There was a big uproar in my old community because, in the process of digging a new well on their property, 2 pinnacle community members died. The first guy went down the well to check it out and grab some tools, he fell unconscious. Then his Father went to go get him and the same fate befell the father. It was quite the tragedy. I only remember the funeral.