LPG is a mixture of hydrocarbon gases containing propane, butane, iso-butane and to a lesser extent propylene or butylene. It can be converted to the liquid state under conditions of moderate pressure at ambient temperature. It is manufactured during the refining of crude oil, or extracted from oil or gas streams as they emerge from the ground. At normal temperatures and pressures, LPG will evaporate. Because of this, it is supplied in pressurized steel bottles. In order to allow for thermal expansion of the contained liquid, these bottles are not filled completely. It is heavier than air, and thus will flow along floors and tend to settle in low spots, such as basements. It is used as a fuel in heating appliances and vehicles, and increasingly replacing chlorofluorocarbons as an aerosol propellant and a refrigerant to reduce damage to the ozone layer.