Carbon monoxide

Carbon monoxide is a highly poisonous gas; its molecular structure contains one carbon atom and one oxygen atom.

Borg drones are apparently immune to carbon monoxide, as it is one of the compounds found in the preferred Borg atmosphere, along with fluorine and methane. 

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.