Liquefied petroleum gas (also called LPG, GPL LP Gas, or liquid propane gas) is a  hydrocarbon gases used as a fuel in heating appliances and vehicles. It is increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer. When specifically used as a vehicle fuel it is often referred to as autogas.flammable Varieties of LPG bought and sold include mixes that are primarily propane (C₃H₈), primarily butane (C₄H₁₀) and, most commonly, mixes including both propane and butane, depending on the season — in winter more propane, in summer more butane^{[citation needed]}. Propylene and butylenes are usually also present in small concentration. A powerful odorant, ethanethiol, is added so that leaks can be detected easily. The international standard is EN 589. In the United States, thiophene or amyl mercaptan are also approved odorants. mixture of       LPG is synthesised by refining petroleum or "wet" natural gas, and is usually derived from fossil fuel sources, being manufactured during the refining of crude oil, or extracted from oil or gas streams as they emerge from the ground. It was first produced in 1910 by Dr. Walter Snelling, and the first commercial products appeared in 1912. It currently provides about 3% of the energy consumed, and burns cleanly with no soot and very few sulfur emissions, posing no ground or water pollution hazards. LPG has a typical specific calorific value of 46.1 MJ/kg compared with 42.5 MJ/kg for fuel-oil and 43.5 MJ/kg for premium grade petrol (gasoline).^[1] However, its energy density per volume unit of 26 MJ/l is lower than either that of petrol or fuel-oil LPG evaporates quickly at normal temperatures and pressures and is supplied in pressurised steel cylinders. They are typically filled to between 80% and 85% of their capacity to allow for thermal expansion of the contained liquid. The ratio between the volumes of the vaporized gas and the liquefied gas varies depending on composition, pressure, and temperature, but is typically around 250:1. The pressure at which LPG becomes liquid, called its vapour pressure, likewise varies depending on composition and temperature; for example, it is approximately 220 kilopascals (2.2 bar) for pure butane at 20 °C (68 °F), and approximately 2.2 megapascals (22 bar) (319 psi) for pure propane at 55 °C (131 °F). LPG is heavier than air, and thus will flow along floors and tend to settle in low spots, such as basements. This can cause ignition or suffocation hazards if not dealt with. Large amounts of LPG can be stored in bulk cylinders and can be buried underground