Propane is produced as a by-product of petroleum refining and the processing of natural gas. Propane and butane are extracted from natural gas to keep these liquids from condensing in gas pipelines. When oil refineries distill gasoline and heating oil, the secondary product of propane is generated.
Propane is normally an odorless, colorless gas. When used as a fuel, a foul smell is added to propane to help aid in the detection of gas leaks. Propane gas becomes liquid at temperatures below -44 degrees Fahrenheit, or when it is stored under sufficient pressure. Propane vapor is heavier than air, so it tends to collect at the lowest available point. If vented well enough, the propane vapor dissipates fairly quickly. If not adequately vented, propane vapor can build up to dangerous levels that may inadvertently ignite.