from http://www.eia.doe.gov/kids/energyexplained/sources/non-renewable/propane.html
Propane Is a Clean-Burning Fossil Fuel
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Propane is a nonrenewable fossil fuel, like the natural gas and oil it is produced from. Like natural gas (methane), propane is colorless and odorless. Although propane is nontoxic and odorless, foul-smelling mercaptan is added to it to make gas leaks easy to detect.
Propane is a clean burning fossil fuel, which is why it is often chosen to fuel indoor equipment such as fork lifts. Its clean burning properties and its portability also make it popular as an alternative transportation fuel.
Propane-fueled engines produce much fewer emissions of carbon monoxide and hydrocarbons compared to gasoline engines. Like all fossil fuels, propane emits water vapor and carbon dioxide, a greenhouse gas.