The fourth ingredient listed on cans of Barbasol original shaving cream is isobutane. Isobutane, also known as methylpropane, is a gas that is used as a propellant.
Isobutane's purpose in Barbasol is simply to allow the shaving cream to spray from the can. Isobutane is commonly used in aerosol products for this purpose and is also widely used as a gas for refrigeration systems. It became widely used when freon gases were found to negatively impact the ozone layer. Isobutane is recognized as generally safe for use by the FDA and the Cosmetic Ingredient Review. The gas evaporates very quickly once it leaves the can, but it should not be inhaled.