C02 is a colourless, odourless incombustible gas composed of a mixture of carbon and oxygen.

a heavy odorless colorless gas formed during respiration and by the decomposition of organic substances; absorbed from the air by plants in photosynthesis

The original gas used to propel paintballs, still the most common used today.

carbon monoxide

the gas used to draw beer from a keg.

Compressed gas used to power markers. Originally used because CO2 turns into a liquid at 'room temperature' at around 850psi, so bulk storage was reasonably cheap, and small 12g cartridges were readily available and could accommodate a number of shots. Has since been replaced by HPA tanks at the higher levels of the sport.

Carbon Dioxide