Frequently asked questions about MTBE, methyl tertiary butyl ether.
When was MTBE first used in gasoline?
MTBE has been used as a gasoline blending component since the 1970s. Originally, it was used to help raise the octane of gasoline and its concentration in gasoline was low (only about 2-3%). Now, it is also used to raise the oxygen content of gasoline ("oxygenated gasoline") and its concentration in gasoline is much higher (generally from 11-15%).
Putting oxygen in gasoline helps cars, particularly older cars, produce less carbon monoxide.
Oxygenated gasoline has been required in the winter in many cities to help reduce carbon monoxide pollution. This program has been expanded and "reformulated gasoline" is now required in the summer in many areas to reduce air pollution. The most widely used gasoline oxygenate is MTBE. Ethanol and some other oxygenates like ETBE and methanol are used in some areas.
Where is MTBE-gasoline used?
The Clean Air Act requires use of reformulated gasoline in those parts of the country with the worst ozone air pollution problems. Other areas have elected to use it.
Since January 1, 1995 reformulated gasoline has been used state-wide in Connecticut, Delaware, Massachusetts, New Jersey, Rhode Island, and the District of Columbia. It has also been required in portions of California, Illinois, Indiana, Kentucky, Maine, Maryland, New Hampshire, New York, Pennsylvania, Texas, Virginia, and Wisconsin. This includes the greater metropolitan areas.
Because of market forces, gasoline containing MTBE has been used in many areas where it was not required by law. Therefore, it may have been present (or is present) in virtually any state at any time in the last decade.
Why is MTBE in gasoline?
The 1990 Clean Air Act (Act) required the Environmental Protection Agency (EPA) to issue regulations that would require gasoline to be "reformulated" so as to result in significant reductions in vehicle emissions of ozone-forming and toxic air pollutants. The regulations were subsequently developed through negotiations with industry, federal and state governments, and environmental and consumer groups. The resulting gasoline, called reformulated gasoline (RFG) is cleaner burning and provides the same automotive engine performance characteristics as conventional gasoline.
The primary goal of the reformulated gasoline program is to reduce vehicle emissions that contribute to the formation of smog, a noxious pollutant that is harmful to human health and the environment, and reduce toxic emissions from vehicles such as benzene, a known human carcinogen. RFG is required to be used in nine major metropolitan areas of the United States with the worst ozone air pollution problems. In addition, many other areas with ozone levels exceeding the public health standard have recognized RFG as being a cost- effective measure for protecting public health and have voluntarily chosen to use RFG.
What are the health effects of MTBE?
MTBE is a potential human carcinogen. Although tests on rats have demonstrated that MTBE can cause cancer in animals, no studies have yet been completed to determine if MTBE causes cancer in humans. Preliminary data suggests that if MTBE does cause cancer in humans, the dosage required is much higher that the levels at which MTBE can be tasted or smelled in drinking water.
Non-cancer effects of exposure to (or ingestion/inhalation of) MTBE include: headaches, eye irritation, nose and throat irritation, cough, nausea, dizziness, and disorientation.
MTBE in drinking-water sources is of concern because it is a possible human carcinogen and it has low taste and odor thresholds which can make a water supply nonpotable even at low concentrations. Although there is no established drinking- water regulation, USEPA has issued a drinking-water advisory of 20 to 40 micrograms per liter (µg/L) on the basis of taste and odor thresholds. This advisory concentration is intended to provide a large margin of safety for noncancer effects and is in the range of margins typically provided for potential carcinogenic effects
What is the difference between Oxyfuel and RFG?
The Oxyfuel and RFG Programs were initiated by the U.S. EPA in 1992 and 1995, respectively, to meet requirements of the 1990 Clean Air Act Amendments.
The Oxyfuel Program requires the use of gasoline with 2.7- percent oxygen (by weight) in areas with high levels of carbon monoxide during the fall and winter. When MTBE is used to meet this requirement, it is used at a concentration of 15 percent (by volume) in gasoline. Because ethanol has a higher oxygen content, it can meet this requirement with a concentration of 7.3 percent (by volume).
The RFG Program requires 2.0-percent oxygen (by weight) throughout the year in the most polluted metropolitan areas. MTBE meets this level with an 11- percent (by volume) concentration, and ethanol can be used with a 5.4-percent (by volume) concentration.
United States Environmental Protection Agency
Last updated October, 1999.
(Should MTBE be given the red light - or the green light? Comment in the MTBE: Double-Edged Sword? discussion.)