Biofuels are a proven alternative to oil for transportation. Thirty percent of American fuel is already enriched with ethanol (it improves performance and reduces tailpipe emissions), and biodiesel is the fastest growing alternative fuel. The four branches of the U.S. military run their vehicles on 20% biodiesel blends (B20) at installations around the country, and the Navy is brewing its own biodiesel from mess hall cooking oil. Concerns over rising oil prices, national security, greenhouse gas emissions, and other forms of air pollution are also bolstering interest in biofuels.