Ethanol is added to gasoline to enhance the octane rating of gasoline. Gasoline with a higher octane rating resists detonation, so it burns rather than exploding. Ethanol has a higher octane number than gasoline, providing premium blending properties. Lower-octane gasoline is blended with 10% ethanol to attain the standard 87 octane. Ethanol is also a cleaner fuel than gasoline, and it helps reduce emissions when mixed with gasoline. Ethanol is available as E10 (10% ethanol, 90% gasoline), which reduces air pollution. Ethanol is also available as E85 (or flex fuel), which can be used in flexible fuel vehicles, designed to operate on any blend of gasoline and ethanol up to 83% . The energy content of ethanol is about 33% less than pure gasoline, so the impact of fuel ethanol on vehicle fuel economy varies depending on the amount of denaturant that is added to the ethanol.