Ethanol is an oxygenate, a chemical that adds oxygen to fuel to make it burn more cleanly.
The federal government has required oxygenates in gasoline sold in smoggy areas, including most of California's major urban areas, to protect air quality. Ethanol, made from corn starch, is popular in the East and Midwest. But in California, the oxygenate of choice is MTBE, or methyl tertiary butyl ether, a suspected carcinogen with unknown health effects in humans. MTBE accounts for about 11 percent of California's gasoline, and it has contaminated thousands of wells and groundwater supplies across the state. Gov. Gray Davis and the Legislature ordered MTBE banned by the end of 2002.