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 lowers harmful carbon monoxide and other emissions from vehicles, 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. Starting in 1992, MTBE was 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 use of 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.
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. But MTBE was the main oxygenate in RFG in some areas outside 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 on air quality, MTBE has some properties that can cause problems. MTBE is much more soluble in water than most other components of gasoline. If it gets in the ground, it can travel faster and farther through groundwater 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. Even fairly small amounts of MTBE in water can give it an unpleasant taste and odor, making the water undrinkable. MTBE also does not break down (biodegrade) easily. As a result, it is harder to clean up once contamination occurs.
In the late 1990s, many community drinking water supplies in areas that used a lot of MTBE were found to have detectable levels of MTBE. Since then, MTBE use in gasoline has been phased out because of concerns about groundwater contamination. 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.
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, companies have switched from using MTBE to using ethanol instead in their gasoline for a number of reasons, including state laws banning MTBE use, concerns over possible legal liability, and government tax subsidies for using ethanol. According to the US Environmental Protection Agency (EPA), “MTBE has not been used in significant quantities” in gasoline since 2005.
However, groundwater in some areas of the country might still contain MTBE. And MTBE is still being used as a gasoline additive in other parts of the world.
How are people exposed to MTBE?
People could be exposed to MTBE in different ways:
• From water contaminated with MTBE (drinking it, inhaling vapors from it, or through skin contact while showering, bathing, or swimming)
• By breathing in air while pumping gasoline containing MTBE or pouring it into engines such as in lawn mowers
• By breathing exhaust fumes while in a vehicle or near a road
The use of MTBE in gasoline has been phased out in the United States, so water contamination is the most likely source of exposure for most people. MTBE can enter water sources through leaking underground or above-ground gas storage tanks and pipelines, as well as from gasoline spills. It can then contaminate drinking water that comes from ground sources, such as wells, and from surface sources, such as lakes or reservoirs. MTBE can remain in underground water for a long time.
MTBE does not build up in the body. It is broken down and exhaled or excreted in urine, usually within a couple of days.
Does MTBE cause cancer?
Researchers use 2 main types of studies to try to figure out 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 find out if a substance could possibly 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 with 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 sometimes it can be hard to know what the results of these studies mean, because many other factors that might affect the results are hard to account for.
In most cases neither type of study provides enough 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.
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 who might have been exposed to MTBE at work often have also been exposed to a number of other chemicals, which makes studying this issue difficult.
What expert agencies say
Several national and international agencies study 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, 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 reviewed MTBE in 1998, and voted not to include it in the lists of compounds that are known or reasonably anticipated to be human carcinogens, because it felt that the evidence in lab animals was not sufficient.
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. In its latest report on MTBE (published in 1999), IARC 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. EPA’s Office of Water has concluded that there isn’t enough evidence to estimate potential health risks of MTBE at low exposure levels in drinking water, but that the evidence supports the conclusion that MTBE is a potential human carcinogen at high doses.
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. But 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’s not clear if these results would apply to the lower levels of exposure people might have.
How is MTBE regulated?
No federal regulations limit or ban the use of MTBE in gasoline, although many states have passed such laws, and companies have effectively stopped using it as a gasoline additive.
The Environmental Protection Agency (EPA) is responsible for ensuring the safety of drinking water in the United States. The EPA has not set a maximum contaminant level of MTBE in drinking water at this time, but it 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. This level is not based on safety, but the EPA states that keeping MTBE below this level should provide protection against any health effects with a wide margin of safety.
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-lasting health effects. The main concern for many people is exposure in drinking water. The levels at which MTBE makes water 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.
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.
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