Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor. At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.5 times that of air. Dry ice is commonly used as a cooling agent, and it is relatively inexpensive. Moist carbon dioxide is generally corrosive by its formation of carbonic acid.