The benefits and problems associated with MTBE
Requiring cleaner-burning gasoline is one of the most significant pollution reduction initiatives ever taken in areas with poor air quality. Commonly referred to as reformulated gasoline (RFG), it is required by the 1990 Clean Air Act Amendments (CAAA) in many of the most polluted areas of the United States that do not meet the health standard for ozone. Other areas, such as the Dallas/Fort Worth (D/FW) region, that do not meet air quality standards but are not required to implement RFG programs, chose to voluntarily implement them in an effort to reduce ground-level ozone.
Initially recognized as the fuel of the future, RFG reduces levels of ozone. However, one of its ingredients is methyl tertiary-butyl ether, (MTBE). Many health questions have been raised about MTBE, in turn raising questions about the future of RFG.
MTBE as an additive
MTBE, the most commonly used additive in RFG in many areas, is a colorless, flammable liquid with a strong odor. Within the United States, 27 companies produced 9.1 billion pounds of MTBE in 1992. MTBE producing companies include BP Amoco PLC, Atlantic Richfield Co. (ARCO) Chemical, Enron Corp. Exxon and Mobil Corp. Added to gasoline, MTBE improves combustion and reduces emissions of carbon monoxide (CO). The ozone forming potential of MTBE is lower than alkenes, aldehydes, nontoluene aromatics and ethene. Some of the chemical and physical properties of MTBE are shown in Table 1.
Chemical and physical properties of MTBE
Common synonyms - MTBE; 2-methoxy, 2-methyl propane
Molecular formula - C5
Physical state - Colorless liquid
Molecular weight - 88.15 g/mole
Melting point - -109o
Boiling point - 55.2o
Water solubility - 51.26 g/L at 25o
Vapor density (air = 1) - 3.1
Henry's Law Constant - 5.5 x 10-4 atm m3 /mole at 25o
Source: Office of Pollution Prevention and Toxics, EPA, Chemical Summary for MTBE (EPA 749-F-94-017a), August, 1994; Andrew Stocking, Stephen Koenigsberg, and Michael Kavanaugh, "Remediation and treatment of MTBE," Environmental Protection (April 1999, Vol. 10, No. 4) 38.
The U.S. Environmental Protection Agency (EPA) reported that total annual environmental releases of MTBE in 1992 were reported to be 3 million pounds, of which 2.8 million pounds were emitted into the air and 100,000 pounds into surface water. Releases can occur at manufacturing sites for MTBE, storage sites, transfer of the mixed fuel, and from spills or leaks at refueling stations.
MTBE has a vapor pressure of 245 mmHg, with a Henry's Law Constant of 5.5 x 10-4
/mole. Thus, it is highly volatile. The hydroxyl radical (HO) reacts with MTBE, however. Therefore, it is believed that it will not last long in the atmosphere. The Hazardous Substances Data Bank lists a rate constant for MTBE's reaction with the HO to be 2.84 x 10-12
at 25 degrees Celsius. Additionally, MTBE will not absorb light greater than 210 nanometers (nm); "therefore, direct photolysis by ultraviolet (UV) absorption is not expected to be environmentally significant." Models estimate that 99.99 percent of MTBE will partition into the atmosphere.
Adverse health effects
Exposure to MTBE can occur in a number of ways. Inhalation, ingestion and skin contact are a few. Overall, the general public's contact with MTBE is minimal. However, people are at risk under certain circumstances. For example, workers at industrial manufacturing plants of MTBE are the most affected. Recent studies show high MTBE concentrations in lake water from gasoline leaked from recreational marine vehicles. All gasoline burning automobiles will emit varying ratios of formaldehyde, benzene and 1,3-butadiene, and these all have cancer causing potential. While MTBE improves air quality through decreased emissions of benzene (and CO), formaldehyde is known to increase emissions. Formaldehyde reacts photochemically in ozone formation, and it is a suspected human carcinogen. Non-cancer causing effects of formaldehyde include sensory irritation as well as behavioral and physiological impairments such as thirst, dizziness, headache and apathy. Because MTBE is a volatile
organic compound (VOC), it can contribute to photochemical smog.
Leaks from underground storage tanks are not expected to reach the air, although MTBE in the soil is expected to readily leach into groundwater. EPA has stated that neither aerobic nor anaerobic biodegradation impact MTBE, thus impacting the nation's water supply. However, more recent studies demonstrate that MTBE can be removed through these techniques (see Andrew Stocking, Stephen Koenigsberg and Michael Kavanaugh, "Remediation and treatment of MTBE," Environmental Protection, April 1999, p. 36-41). EPA's Blue Ribbon Panel, which was appointed by EPA Administrator Carol Browner in November, 1998 to study the benefits and concerns of gasoline oxygenates, reported that groundwater contamination is primarily the result of leaking underground storage tanks. Additional infiltration occurs from nonregulated systems, such as farm fuel storage tanks. Detection of MTBE in wells and reservoirs was also reviewed by the panel. Thousands of samples from California, Maine, and two U.S. Geological Service surveys
that looked at a dozen northeastern states revealed detectable amounts of MTBE in at least 5 percent to 10 percent of potable water supplies. The majority of samples were found to be well within health standards, with approximately 1 percent having concentrations greater than 20 parts per billion (ppb). However, there was concern regarding the unpalatable taste and odor of the water associated with low-level contamination and the long-term accumulation of MTBE concentrations. EPA's existing MTBE drinking water advisory for taste and odor is 20 to 40 micrograms per liter.
Several areas of concern have been identified regarding the health effects of MTBE. Sensitive individuals have reported headaches, nausea and sensory irritation. Animal studies on rats have demonstrated changes in motor activity, as well as cancer. Based on this evidence, researchers seem to agree that while MTBE is successful in reducing air pollution, there are risks associated with the increased exposure to aldehydes and oxygenates.
MTBE may have completed its course as a gasoline additive, but its future as a contaminant is only beginning. There are many areas for further research regarding MTBE since this chemical compound is not fully understood. For example, a more comprehensive assessment needs to be conducted to evaluate risks for human exposure to MTBE in both the public environment as well as those individuals in occupational settings. Neurotoxicity studies on exposed mice result in hypoactivity, muscular weakness, hyperpnea, prostration, lacrimation and a general anesthetic effect. The indications that mice seem to experience neurological damage due to MTBE prove that it would be worthwhile to evaluate its impact on human nervous systems.
Other studies are also needed. According to an EPA assessment, there is no known information as to MTBE's chronic effect on humans. When exposed to high doses of MTBE, lab studies on rats indicate changes in blood chemistry, such as decreases in blood glucose, blood urea nitrogen and calcium, with elevations in cholesterol and aspartate aminotransferase. It is important to know how humans respond.
Only a few carcinogenicity studies have been performed, with laboratory rats as the test subjects. Inhalation of high-level MTBE vapors increased the occurrence of kidney and liver cancer in rats. Rats administered oral doses of MTBE has increased tumors of the testes and lymphohematopoietic tumors.
Likewise, reproductive toxicity is also unknown for humans exposed to MTBE. However, studies on mice have noted dead fetuses and skeletal variations.
As a VOC hazardous air pollutant, MTBE is an environmental contaminant. As such, its transport, fate and effects on humans need to be researched and understood.
Despite the potential air quality benefits to be derived from MTBE, its environmental risks, according to some, are outweighing its potential benefits. On March 25, 1999, Gov. Gray Davis banned MTBE in California. The phase-out will occur by the end of 2002. It is anticipated that MTBE manufacturers will be impacted financially as the market for MTBE begins to drop. The largest market for MTBE is California, which often sets the environmental policy trend for the rest of the United States. It is anticipated that other states will follow. It is also reported that analysts and the environmental community are applauding MTBE's phase-out. If the oxygenate requirement continues, MTBE producers will be forced to replace MTBE with another additive such as ethanol, which could be an economic challenge.
From a national perspective, EPA's Blue Ribbon Panel recommended several actions regarding MTBE and its effect on air quality, water quality and supply and cost of fuel. Of particular note, the panel stated that MTBE's "persistence and mobility in water is more likely to contaminate ground and surface water than the other components of gasoline." Furthermore, the panel stated that MTBE should be reduced from drinking water supplies and encouraged Congress to expedite gasoline standards that do not pose threats to water quality. Based on these concerns, on March 20, 2000, the White House moved to begin a reduction phase-out of MTBE stating that 31 states could already be contaminated. Acting under section 6 of the Toxic Substances Control Act, on March 24, 2000, EPA published an advance notice of intent to begin rulemaking to "eliminate or limit the use of MTBE as a fuel additive." A target phase-out date was not identified.
In order to provide cleaner air to our nation's citizens, new technologies are essential. Because nearly everyone uses gasoline, the motivation for a cleaner burning fuel is sound, and does not infringe on the daily lives of average Americans. However, the use of any technology, RFG included, demands a full impact analysis. MTBE's use in RFG is known to reduce air pollution, but it is necessary to understand how this additive may impact public health and the environment from other perspectives.
Selected works cited
Chemical Industry Institute of Toxicology, "The mechanism of male rat kidney tumors induced by MTBE and its relevance in assessing human risk" CIIT Activities
, Vol. 16, No. 10, 1996.
Chemical Summary for MTBE
, Office of Pollution Prevention and Toxics, US Environmental Protection Agency: Washington, DC, (EPA 749-F-94-017a) August, 1994.
US Environmental Protection Agency, Achieving Clean Air and Clean Water: The Report of the Blue Ribbon Panel on Oxygenates in Gasoline
, September 15, 1999.
Program Summary: Research on Oxygenates Added to Gasoline
, Health Effects Institute: Cambridge, Massachusetts, 1996.
Reuters News Service. "MTBE ban heard round the world," Houston Chronicle
(March 27, 1999) Section C: 1,3.
The Potential Health Effects of Oxygenates Added to Gasoline
, Health Effects Institute: Cambridge, Massachusetts, 1996.
EPA Office of Underground Storage Tanks MTBE resource sheet - www.epa.gov/swerust1/mtbe/index.htm
University of California-Davis MTBE research - www.tsrtp.ucdavis.edu/mtbe
State of California's response to MTBE - www.calepa.ca.gov/programs/mtbe
The Health Effects Institute research of oxygenates - www.healtheffects.org/Pubs/oxyprog.htm
EPA MTBE data regarding production, fate in environment and health - www.epa.gov/opptintr/chemfact/s_mtbe.txt
EPA Blue Ribbon Panel Web site - www.epa.gov/otaq/consumer/fuels/oxypanel/blueribb.htm
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This article appeared in Environmental Protection magazine, July 2000, Vol. 11, No. 7, p. 49.
This article originally appeared in the 07/01/2000 issue of Environmental Protection.