Propane is a fossil fuel. When sold as fuel, it is commonly known as liquefied petroleum gas, which can be a mixture of propane along with small amounts of propylene, butane and butylenes. Ethanethiol is added as an odorant so that people can easily smell the gas in case of a leak. When it is burned, it produces primarily carbon dioxide and water, but may also emit carbon monoxide (CO), nitrogen oxide (NO) and particulates. Propane combustion is cleaner than gas combustion, but not as clean as natural gas.