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.
Varieties of LPG bought and sold include mixes that are primarily 
propane (C
3H
8), primarily 
butane (C
4H
10) and, most commonly, mixes including both propane and 
butane, depending on the season — in winter more propane, in summer more butane
[citation needed]. In the 
United States, primarily only two grades of LPG are sold, commercial propane and HD-5. These specifications are published by the Gas Processors Association (GPA) and the American Society of Testing and Materials (ASTM). Propane/butane blends are also listed in these specifications. 
Propylene, 
butylenes and various other hydrocarbons are usually also present in small concentrations. HD-5 limits the amount of propylene that can be placed in LPG, and is utilized as an autogas specification. A powerful 
odorant, 
ethanethiol, is added so that leaks can be detected easily. The international standard is 
EN 589. In the United States, 
tetrahydrothiophene (thiophane) or 
amyl mercaptan are also approved odorants, although neither is currently being utilized.