Carbon dioxide (CO2) is an odorless, colorless, nonflammable gas composed of one carbon atom and two oxygen atoms. It is expelled by animals and consumed by plants.
Three subterranean pockets of carbon dioxide were located on Penthara IV. When the planet was struck by a type C asteroid and was threatened to freeze over because of a large cloud cover, the USS Enterprise-D paser drilled large holes into the pockets in order to release the gas, thereby creating a greenhouse effect and preventing the world from freezing over. (TNG: "A Matter of Time")
The alien lifeform, later discovered to be Odo infected with volcanic gas, was determined to require a higher concentration of carbon dioxide than Deep Space 9's atmosphere provided. (DS9: "The Alternate")
The planet surveyed in the Gamma Quadrant by Jake and Nog had an atmosphere composed of 77% nitrogen, 21% oxygen and 2% carbon dioxide. (DS9: "The Jem'Hadar")
A dosage of 15 cc's of triox, given every four hours to a human, can be used to compensate for excess carbon dioxide in a planet's atmosphere. (DS9: "The Sound of Her Voice")