What is carbon dioxide? Carbon dioxide (CO2) is a colourless, odourless gas. CO2 is produced during fermentation of alcoholic beverages and used in tank counter-pressure and dispensing systems. At higher levels, CO2 can become toxic or can suffocate workers by displacing the oxygen from the air they breathe. Because it is heavier than air, CO2 can pool in low-lying areas. This means that workers can quickly move into an area of high CO2 concentration with little warning.