The purpose of CO2 in cell culture is to maintain a stable physiological pH through the CO2-bicarbonate based buffer system. The atmospheric CO2 can dissolve into cell culture medium, and a small portion of it will react with water to form carbonic acid, which in turn interacts with the bicarbonate ion. The balance of dissolved CO2 and bicarbonate thereby controls the pH of the medium. For most cell culture experiments, 4-10% CO2 in air is commonly used.