A mixture of short hydrocarbons with most of the volume being propane and butane. LPG is considered an alternative fuel that burns cleaner than gasoline because of the very short hydrocarbon chains that tend to not produce a high volume of CO₂ (when compared to the energy harvested) or NO_x radicals. LPG is now being found in the air in the unburned state most likely from leaking pipes and tanks; in this physical state LPG is now being seen a contributor to urban air pollution.