A carbon dioxide tank was a physical storage device on board 22nd century Starfleet vessels which contained a gas capable of rendering Humans unconscious. The tanks could be connected to a ship's central ventilation by rerouting the transfer lines via the atmospheric recycling manifold. In 2152, Doctor Phlox was forced to use the manifold to access Enterprise NX-01's carbon dioxide tanks and distribute the gas throughout the ship, in order to evict a series of non-corporeal lifeforms called "Wisps" from the bodies of crewmembers who had been taken over by the aliens. (ENT: "The Crossing")