Supercritical carbon dioxide is a fluid state where CO₂ is held at or above its critical temperature (31.10 °C or 304.25 K) and critical pressure (72.9 atm or 7.39 MPa). Carbon dioxide usually behaves as a gas in air at standard temperature and pressure or as a solid called dry ice when frozen. If the temperature and pressure are both increased to be at or above the critical point for carbon dioxide, it can adopt tunable properties midway between a gas and a liquid: notably a very low viscosity (as for gas) and at the same time a very high density (as for liquids). CO₂ offers also a low toxicity and environmental impact, while the relatively low process temperature and its stability protect most treated species from damage or denaturing. Thus, supercritical CO₂has become an important commercial and industrial solvent particularly for chemical extraction (for decaffeinating as an example), for dry cleaning (much more attractive than hydrocarbons or perchloroethylene), for separation...