The 1990 amendments to the federal Clean Air Act required that "reformulated gasoline" be used in nine geographic areas with the worst smog pollution, to reduce harmful levels of ozone in air. At that time, reformulated gasoline (RFG) had to contain two percent oxygen by weight. Refiners met the oxygen requirement by adding ethers or alcohols that contain oxygen to gasoline. The two most commonly used additives were MTBE (methyl tertiary butyl ether, which was used in about 87 percent of RFG) and ethanol (which has been used primarily in the Midwest where it is produced from corn).

The Clean Air Act amendments also allowed states without severe smog pollution to require the use of RFG to help meet the emission reduction targets established in their State Implementation Plans. Massachusetts voluntarily joined the federal RFG program in 1992.  

The use of RFG has significantly reduced levels of smog and toxic air pollutants such as benzene. However, MTBE is highly soluble. When it gets into groundwater (typically through a leaking underground tank used to store gasoline), it persists longer and travels farther and faster through soil than other fuel constituents.  As a result, it has been detected in drinking water wells, requiring costly cleanups.  

To avoid groundwater contamination, more than a dozen states have banned the use of MTBE, which means that RFG will be made with ethanol, the only practical alternative today. Also, the U.S. Energy Policy Act of 2005 required the U.S. Environmental Protection Agency (EPA) to remove the two percent oxygen standard. The Act also added a national renewable fuel standard, which will increase the use of ethanol in RFG and non-RFG. These changes mean that RFG blended with ethanol is being used in the Northeast.

U.S. Environmental Protection Agency (EPA): Reformulated Gasoline (RFG)