Carbon monoxide (CO) is a highly poisonous gas comprised of one carbon atom and one oxygen atom, and is produced by the incomplete combustion of hydrocarbons and/or due to the lack of oxygen. If there is sufficient oxygen, carbon dioxide forms instead of carbon monoxide.
Carbon monoxide was one of the byproducts of gasoline consumption. Prolonged inhalation of carbon monoxide caused asphyxia in most humanoid lifeforms. (VOY: "The 37's") Borg drones, however, were immune to the effects of carbon monoxide. (Star Trek: First Contact)
In an alternate timeline where the Borg assimilated Earth after stopping Zefram Cochrane's warp flight, the planet had an atmosphere of methane, carbon monoxide, and fluorine in 2373. (Star Trek: First Contact)
In 2374, when Tom Paris cited working on his car in the holodeck as the reason why he wasn't keeping up with his medical studies, The Doctor remarked that the "lethal levels of carbon monoxide" produced by the internal combustion engine might be just the thing to bring him to sickbay. (VOY: "Vis à Vis")
In 2376, B'Elanna Torres detected elevated levels of carbon monoxide and ammonium on Kelemane's planet during a series of scans, indicating evolutionary progress by the planet's inhabitants. (VOY: "Blink of an Eye")