Reformulated gasoline (RFG) is gasoline blended to burn cleaner and reduce smog-forming and toxic pollutants in the air we breathe. The Clean Air Act requires that RFG be used in cities with the worst smog pollution contain oxygen to reduce harmful emissions of ozone.

The Act also specified that RFG contain oxygen - 2 percent by weight. MTBE (methyl tertiary butyl ether) and ethanol are the two most commonly used substances that add oxygen to gasoline. Oil companies decide which substance to use to meet the law's requirements. Read more about fuel additives.

A state may seek a waiver from the Congressional mandate, but only by providing clear evidence that the RFG requirement will interfere with the state's ability to meet the national health- based standards or the National Ambient Air Quality Standards (NAAQS) for clean air.