Carbon monoxide was a highly poisonous gas. Its molecular structure contained one carbon atom and one oxygen atom.
Carbon monoxide was a common component of the exhaust of a 20th century Earth automobile. Holodecks in 2368 were capable to simulating its smell. (TNG - Assimilation² comic: "Issue 2")
Borg drones were apparently immune to carbon monoxide, as it was one of the compounds found in the preferred Borg atmosphere, along with fluorine and methane. (TNG movie: Star Trek: First Contact)
In an alternate timeline where the Borg were able to assimilate entire star systems in the Alpha Quadrant, increasing levels of carbon monoxide were noted in the atmospheres of Alpha Centauri and Andor while the Borg adapted their worlds. (Star Trek novel: Engines of Destiny)