Oxygenated gasoline is a mixture of conventional gasoline and one or more combustible liquids which contain oxygen. At present, ethanol is the most common oxygenate in the United States. The government requires gasoline to be oxygenated during the winter in areas that have a carbon monoxide pollution problem (cold weather and atmospheric inversions worsen carbon monoxide pollution). Oxygenated gasoline helps engines run leaner, which helps some engines – particularly older engines – produce less carbon monoxide.