Refrigerants are substances with low vaporization temperatures that are used to create the cold conditions necessary for chilling foods and beverages. Examples include ammonia, older chlorofluorocarbon types such as Freons, ice, and solid carbon dioxide, known as dry ice. In a mechanical system the refrigerant circulates through evaporator, compressor, condenser, and expansion valve, alternately evaporating and condensing to carry heat out of the storage space. Choice of refrigerant balances cooling performance against safety, environmental impact, and regulation, since ammonia is toxic and older CFCs damage the ozone layer.