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 is one of the byproducts of gasoline consumption. Prolonged inhalation of carbon monoxide causes asphyxia in most humanoid lifeforms. (VOY: "The 37's") Borg drones, however, are immune to the effects of carbon monoxide. (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")