A gas present in air, produced in combustion, fermentation and animal metabolism. Carbon dioxide is produced when any form of carbon or almost any carbon compound is burned in an excess of oxygen. In animal respiration carbon dioxide combines with haemoglobin in red blood cells, is carried to the lungs and is breathed out. Green vegetation uses chlorophyll to combine carbon dioxide with hydrogen to form carbohydrates (photosynthesis).

Typical products include fizzy drinks, carton fruit juice, wine.