Biodiesel is registered with the Environmental Protection Agency as a pure fuel or as a fuel additive and is a legal fuel in commerce. It is typically produced through the reaction of a vegetable oil or animal fat with methanol in the presence of a catalyst to yield glycerin and biodiesel (chemically called methyl esters). It is an alternative fuel that can be used by itself or blended with petroleum diesel for use in diesel engines. Its use can result in substantial reduction of unburned hydrocarbons, carbon monoxide, and particulate matter.