What is MTBE?
MTBE (methyl tertiary-butyl ether) is a flammable, colorless liquid that dissolves easily in water. It is part of a group of chemicals known as fuel oxygenates. Oxygenates do not occur naturally in gasoline; they are added to increase gasoline's oxygen content. MTBE and other oxygenates make gasoline burn better, which helps prevent engine knocking and decreases harmful carbon monoxide and other emissions from vehicles, thus reducing air pollution.
MTBE was first used in gasoline at low levels in the United States in 1979 to replace lead as an octane enhancer. Since 1992, MTBE has been used at higher concentrations in some gas to fulfill the oxygenate requirements set by Congress in the 1990 Clean Air Act Amendments (CAA). The CAA required the use of oxygenated gasoline in areas with unhealthy levels of air pollution.
Starting in 1995, the CAA required reformulated gasoline (RFG) year-round in cities with the worst ground-level ozone (smog). RFG is gasoline that is specially blended to have fewer polluting compounds than conventional gasoline. As part of the CAA, RFG had to have an oxygen content of at least 2% by weight.
Some RFG contained MTBE to add the oxygen, but the CAA did not specifically require MTBE to be used as the oxygenate in gasoline. Gas refiners could choose to use other oxygenates, such as ethanol. MTBE was the main oxygenate in RFG in some areas outside of the Midwest, mainly for economic reasons and for its blending characteristics. Unlike ethanol, MTBE could be shipped through existing pipelines, and its volatility is lower, making it easier to meet the emission standards.
Problems with MTBE
Despite its potential benefits to air quality, MTBE has some properties that can cause problems. MTBE is much more soluble in water than most other components of gasoline. It can migrate faster and farther in the ground than other gasoline components. This makes it more likely to contaminate public water systems and private drinking water wells if gasoline is spilled on the ground or leaks out of underground storage tanks. MTBE also does not biodegrade (break down) easily. As a result, it is harder to clean up once contamination occurs.
In the late 1990s, it was discovered that many community drinking water supplies in areas that used a lot of MTBE had detectable levels of MTBE. Since then, many areas have begun to phase out MTBE use in gasoline because of groundwater contamination. Even fairly small amounts of MTBE in water can give it an unpleasant taste and odor. Although it is not clear what effects MTBE in drinking water might have on health, many states have passed laws limiting or banning the use of MTBE in gasoline. Today MTBE is commonly replaced with ethanol.
As part of the Energy Policy Act of 2005, Congress voted to remove the oxygen content requirement for reformulated gasoline. (This went into effect in 2006.) Since then, a number of companies have removed MTBE from their gasoline because of state laws and because of concerns over possible legal liability. Government tax subsidies for using ethanol have also contributed to the declining use of MTBE.
How are people exposed to MTBE?
People could be exposed to MTBE by:
- Breathing in air while pumping gasoline containing MTBE or pouring it into engines such as in lawn mowers
- Breathing exhaust fumes while in a vehicle or near a road
- Drinking, swimming, or showering in water contaminated with MTBE
As the use of MTBE in gasoline is being phased out, drinking water contamination is the most likely source of exposure for most people. MTBE can contaminate drinking water that comes from ground sources, such as wells, and water from surface sources, such as lakes or reservoirs. MTBE enters the water through leaking underground gas storage tanks and pipelines, as well as from gasoline spilled on the ground. Smaller amounts of MTBE may have also entered water sources from boat engines, especially older ones.
MTBE does not build up in the body. It is broken down and exhaled or excreted, usually within a couple of days.
Does MTBE cause cancer?
Researchers use 2 main types of studies to try to determine if a substance or exposure causes cancer. (Substances that cause cancer or help cancer grow are called carcinogens.)
In studies done in the lab, animals are exposed to a substance (often in very large doses) to see if it causes tumors or other health problems. Researchers may also expose normal cells in a lab dish to the substance to see if it causes the types of changes that are seen in cancer cells. It's not always clear if the results from these types of studies will apply to humans, but lab studies are the best way to detect the potential for a substance to cause cancer in humans before widespread exposure occurs.
Another type of study looks at cancer rates in different groups of people. Such a study might compare the cancer rate in a group exposed to a substance versus the rate in a group not exposed to it, or compare it to what the expected cancer rate would be in the general population. But studies in people can sometimes be hard to interpret, because there may be other factors affecting the results that are hard to account for.
In most cases neither type of study provides definitive evidence on its own, so researchers usually look at both lab-based and human studies if they are available.
Studies done in the lab
Several studies have looked for health effects in lab animals that inhaled or ingested MTBE, often at very high doses over their lifetimes. Some of these studies have found that exposure to high levels of MTBE caused tumors of the kidneys, liver, testicles, and some other organs. The levels of MTBE used in these studies were generally much higher than humans would normally tolerate, based on the taste and smell of MTBE. Whether these results would apply to humans is not known.
Studies in people
Though MTBE has been used as a fuel additive since 1979, there have been no long-term studies of the ability of MTBE to cause cancer in workers or other people exposed to high concentrations. People whose work might expose them to MTBE-containing gasoline are also exposed to a number of other chemicals in gasoline, which makes studying this issue difficult.
What expert agencies say
Several agencies (national and international) study different substances in the environment to determine if they can cause cancer. The American Cancer Society looks to these organizations to evaluate the risks based on evidence from laboratory, animal, and human research studies.
Based on the available evidence, some of these expert agencies have evaluated the cancer-causing potential of MTBE.
The National Toxicology Program (NTP) is formed from parts of several different US government agencies, including the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA). Exposures that are thought to be carcinogenic are included in the Report on Carcinogens, published every few years. The NTP has reviewed MTBE and voted not to include it in the lists of compounds known or reasonably anticipated to be human carcinogens.
The International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). Its major goal is to identify causes of cancer. The IARC has concluded that there is inadequate human evidence, and limited animal evidence, for the carcinogenicity of MTBE, leading to an overall evaluation of MTBE as "not classifiable as to its carcinogenicity to humans."
The Environmental Protection Agency (EPA) maintains the Integrated Risk Information System (IRIS), an electronic database that contains information on human health effects from exposure to various substances in the environment. The EPA is currently reviewing MTBE.
(For more information on the classification systems used by these agencies, see our document, Known and Probable Human Carcinogens.)
Does MTBE cause any other health problems?
Some people have complained of symptoms such as nausea, dizziness, lightheadedness, headaches, and nose and throat irritation after breathing vapors from gas containing MTBE. However, it's not clear if these symptoms are caused by MTBE or other components of gasoline. Several studies in people have not found any symptoms specifically related to MTBE exposure. At this time, the evidence linking these symptoms to MTBE is inconclusive.
It is not known if there are any health effects of long-term exposures to smaller amounts of MTBE, such as in drinking water. Studies in animals using exposures many times higher than the levels typical for humans have shown effects on the nervous system ranging from hyperactivity to seizures and unconsciousness. Other studies have shown kidney damage and effects on fetal development, in addition to some excess cancers as discussed above. It is not known if these results would apply to the lower levels of exposure people might have.
How is MTBE regulated?
There are no federal regulations that limit or ban the use of MTBE in gasoline, although many states have passed such laws in recent years.
The Environmental Protection Agency (EPA) is responsible for ensuring the safety of drinking water in the United States. The EPA requires all large public water systems and a sampling of small and medium public water systems to monitor and report the presence of MTBE in their water supplies.
However, the EPA has not set a maximum contaminant level of MTBE in drinking water at this time. The EPA has issued a non-binding advisory level of 20 to 40 parts per billion (ppb) in drinking water, to prevent bad odor and taste in the water. While this level is not based on safety, the EPA states that keeping MTBE below this level should provide protection against any health effects with a wide margin of safety, based on studies in animals.
Public water systems in the United States are not legally required to meet the EPA advisory levels, although many states have set their own limits on levels of MTBE allowed in their drinking water. Some of these are lower than the EPA advisory levels.
Can I find out about my exposure to MTBE?
It is not clear if exposure to MTBE causes long-term health effects. The main concern for many people is exposure in drinking water. The levels at which MTBE causes water to taste or smell unpleasant for most people are generally far below the levels found to cause health problems in lab animals, although the possible health effects of MTBE are still being studied.
If you are concerned, you can find out more about your possible exposure to MTBE and may be able to take steps to limit it.
If you get your drinking water from a public water system, you can contact the system directly and ask whether they monitor for MTBE and what levels, if any, have been detected. If you have a private well, your local health department might be able to tell you if MTBE has been found in water in your area. If you want to get your water tested, you can also call the EPA's Safe Drinking Water Hotline at 1-800-426-4791 to get the phone number for the office in your state that certifies drinking water laboratories.
Public water systems have ways of removing MTBE from drinking water, if needed, although this process can be expensive. In some cases, alternate sources of water may be used. Some home water filters can remove MTBE from drinking water, but it is important to read the product label carefully and/or contact the filter manufacturer to be sure a particular filter does so.
More information from your American Cancer Society
The following related information may also be helpful to you. These materials may be viewed on our Web site or ordered from our toll-free number, at 1-800-227-2345.
National organizations and Web sites
In addition to the American Cancer Society, other sources of information and support include*:
Environmental Protection Agency
Safe Drinking Water Hotline: 1-800-426-4791
Web site: www.epa.gov
MTBE main page: www.epa.gov/mtbe
MTBE in Drinking Water: www.epa.gov/safewater/contaminants/unregulated/mtbe.html
Agency for Toxic Substances and Disease Registry
Toll-free number: 1-888-422-8737 (1-888-42-ATSDR)
Web site: www.atsdr.cdc.gov
Public Health Statement for MTBE: www.atsdr.cdc.gov/toxprofiles/phs91.html
ToxFAQs™ for Methyl tert-Butyl Ether (MTBE): www.atsdr.cdc.gov/tfacts91.html
Agency for Toxic Substances and Disease Registry. ToxFAQs™ for Methyl tert-Butyl Ether (MTBE). September 1997. Accessed at www.atsdr.cdc.gov/tfacts91.htmlon April 30, 2010.
Belpoggi F, Soffritti M, Filippini F, Maltoni C. Results of long-term experimental studies on the carcinogenicity of methyl tert-butyl ether. Ann NY Acad Sci. 1997;837:77-95.
Belpoggi F, Soffritti M, Maltoni C. Methyl-tertiary-butyl ether (MTBE)--a gasoline additive--causes testicular and lymphohaematopoietic cancers in rats. Toxicol Ind Health. 1995;11:119-149.
Bird MG, Burleigh-Flayer HD, Chun JS, et al. Oncogenicity studies of inhaled methyl tertiary-butyl ether (MTBE) in CD-1 mice and F-344 rats. J Appl Toxicol. 1997;17 Suppl 1:S45-55.
Environmental Protection Agency. MTBE (methyl-t-butyl ether) in Drinking Water. 2007. Aceesed at www.epa.gov/safewater/contaminants/unregulated/mtbe.html on April 30, 2010.
Environmental Protection Agency. Drinking Water Advisory: Consumer Acceptability Advice and Health Effects Analysis on Methyl Tertiary-Butyl Ether (MtBE). 1997. Accessed at www.epa.gov/waterscience/criteria/drinking/mtbefact.pdf on April 30, 2010.
Environmental Protection Agency. Integrated Risk Information System: Methyl tert-butyl ether (MTBE) (CASRN 1634-04-4). 1993. Accessed at www.epa.gov/ncea/iris/subst/0545.htm on April 29, 2010.
Environmental Protection Agency. Methyl Tertiary Butyl Ether (MTBE): Overview. 2008. Aceesed at www.epa.gov/mtbe/faq.htm on April 30, 2010.
International Agency for Research on Cancer (IARC). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Vol. 73: Some chemicals that cause tumours of the kidney or urinary bladder in rodents and some other substances. 1999. Accessed at http://monographs.iarc.fr/ENG/Monographs/vol73/mono73-18.pdf on April 29, 2010.
Mennear JH. Carcinogenicity studies on MTBE: critical review and interpretation. Risk Anal. 1997;17(6):673-681.
US Department of Health and Human Services. Public Health Service, National Toxicology Program. Report on Carcinogens, Eleventh Edition. Appendix C: Agents, Substances, Mixtures, or Exposure Circumstances Reviewed but not Recommended for Listing in the Report on Carcinogens. 2005. Accessed at http://ntp.niehs.nih.gov/ntp/roc/eleventh/append/appc.pdf on April 29, 2010.
Last Revised: 02/18/2011