Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels (1996)

The use of oxygenated gasoline was mandated under the Clean Air Act Amendments of 1990 in areas that did not meet the federal ambient air standard for carbon monoxide (CO). Motor vehicle emissions are the primary source of ambient CO levels in most areas. The Clean Air Act requires at least a 2.7% oxygen content for gasoline sold in CO nonattainment areas, and this level of oxygen is typically achieved by the addition of 15% methyl tertiary butyl ether (MTBE) or 7.5% ethanol (by volume). The higher oxygen content of oxygenated gasoline compared to conventional gasoline is intended to lead to a more complete combustion of the gasoline and therefore to reduced tailpipe emissions of CO.

Soon after the oxygenated gasoline program was introduced nationally in the winter of 1992-1993, anecdotal reports of acute health symptoms were received by health authorities in various areas of the country. Such health concerns had not been anticipated but have subsequently focused attention on possible health risks

associated with using oxygenated gasoline. These health concerns have been joined by complaints of reduced fuel economy and engine performance, as well as the detection of low levels of MTBE in some samples of groundwater. The reformulated gasoline program, which is intended to reduce motor vehicle emissions that lead to higher ozone levels during the summer months and air toxics year round, and which also makes use of fuel oxygenates, was not specifically examined in this report.

In order to address public concerns and to take full advantage of the extensive expertise across the Federal government, as well as outside experts where appropriate, the U.S. Environmental Protection Agency (EPA) requested the assistance of the Office of Science and Technology Policy (OSTP) through the Committee on Environmental and Natural Resources (CENR) of the President's National Science and Technology Council (NSTC), to coordinate a comprehensive assessment of these issues. Working groups that prepared this evaluation were comprised of technical and scientific experts from across several Federal agencies, as well as representatives from state government, industry, and environmental groups.

This assessment is a scientific state-of-understanding report of the fundamental basis and efficacy of the EPA's winter oxygenated gasoline program. The assessment considers not only health effects, but also air quality, fuel economy and engine performance, and groundwater and drinking water quality. The potential health effects of oxygenated gasoline were evaluated in two separate reports, one prepared by an Interagency group of health scientists and the second by the Health Effect Institute (HEI) and panel of experts. Both the Interagency report and the HEI report, as well as a comparison between these two documents, are included. Each of the chapters in this report underwent extensive external peer-review prior to the submission of the entire report for review by the National academy of Sciences (NAS). The findings and comments from the NAS review will be incorporated into this assessment.

An expanded summary of the health effects of oxygenated gasoline will be prepared based on information in the Interagency and HEI reports and the comments from the NAS review.

When the Interagency Steering Committee began this effort, it was their intention that a full risk assessment and cost benefit analysis of using oxygenated gasoline in place of conventional gasoline be included in the report. However, because of serious limitations in the data, the Steering Committee concluded that such an analysis was not possible at this time. Several research needs on oxygenated gasoline were identified that would reduce uncertainties and allow a more thorough assessment of human exposure, health risks and benefits, and environmental effects.

Oxygenates have been used as octane enhancers in gasoline since the late 1970s, due to the phaseout of lead. During the 1980s, oxygenates came in to wider use as some states implemented oxygenated gasoline programs for the control of carbon monoxide (CO) pollution in cold weather. People with coronary artery disease are particularly sensitive to the adverse effects of this air pollutant. The first winter oxygenated gasoline program in the United States was implemented in Denver, Colorado in 1988. The 1990 Clean Air Act Amendments required the use of oxygenated gasoline in several areas of the country that failed to attain the National Ambient Air Quality Standard (NAAQS) for CO. During the winter months of 1992-1993, many new oxygenated gasoline programs were implemented to increase combustion efficiency in cold weather and thereby reduce CO emissions.

Methyl tertiary butyl ether (MTBE) has become the most widely

used motor vehicle oxygenate in the U.S., though in some areas, ethanol is the dominant oxygenate used in motor vehicle fuels. Other fuel oxygenates that are in use or may potentially be used include ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), diisopropyl ether (DIPE), tertiary butyl alcohol (TBA), and methanol. Because of limitations in available data, there is less emphasis in this report on these other oxygenates. The Clean Air Act requires at least 2.7% by weight oxygen content for gasoline sold in CO nonattainment areas, and 15% by volume MTBE or 7.5% by volume ethanol achieve this requirement.

The purpose of this report is to provide a review of the scientific literature on oxygenated fuels and to assess effects of the winter oxygenated fuels program on air quality, water quality, fuel economy and engine performance, and public health. The request from EPA for this assessment was prompted by public complaints of headaches, nausea, and other acute symptoms attributed to wintertime use of oxygenated fuels, as well as complaints of reductions in fuel economy and engine performance.

This report does not specifically examine the reformulated gasoline program which is intended to reduce motor vehicle emissions that lead to higher ozone levels during the summer months and air toxics year round, and which also makes use of fuel oxygenates. The report identifies areas where the data are too limited to make definitive conclusions about the costs, benefits, and risks of using oxygenated gasoline in place of conventional gasoline. Several research needs on oxygenated gasoline were identified that would reduce uncertainties and allow a more thorough assessment of human exposure, health risks and benefits, and environmental effects.

• A general decline in urban concentrations of CO over the past twenty years is attributed to stringent EPA mandated vehicle emission standards and improved vehicle emission control technology. The effects of meteorology must be accounted for in assessments of air quality benefits of oxygenated gasoline.

• In some cities with winter oxygenated gasoline programs, a reduction in ambient CO concentrations of about 10% is observed and is attributed to the use of the oxygenate.

• Studies of the effects of fuel oxygenates on vehicle emissions show a consistent reduction of CO emissions at ambient temperatures above about 50°F. At temperatures below 50°F, the magnitude of the reduction is decreased and more uncertain. Some studies show increased CO emission when oxygenated fuels are used at low temperature.

• The EPA MOBILE 5a model appears to overestimate the benefits of oxygenated gasoline on fleetwide CO emissions by a factor of two.

• Oxygenates also reduce total hydrocarbon exhaust emissions. Fuel oxygenates decrease vehicle emissions of air toxics, benzene and 1,3-butadiene, but increase the emissions of aldehydes (acetaldehyde from use of ethanol or ETBE and formaldehyde from use of MTBE).

• The amount of pollutant emissions is smaller in newer technology vehicles (fuel injected and adaptive learning, closed loop three-way catalyst systems) than in older technology vehicles (carbureted and oxidation catalysts). Also, the percentage reductions in CO and hydrocarbon emissions from use of fuel oxygenates are found to be smaller in the newer technology vehicles compared to older technology and higher emitting vehicles.

• Emissions of nitrogen oxides are not changed significantly by low concentrations of fuel oxygenates but some studies indicate increased nitrogen oxide emissions with oxygenate concentrations greater than 2 percent by weight oxygen.

• During the winter season, the oxygenates are not removed rapidly from the urban atmosphere, although some scavenging by precipitation is expected. Consequently, the oxygenates are likely to be dispersed and diluted throughout the troposphere, where they ultimately would be removed by slow photooxidation.

• Releases of gasoline containing oxygenates to the subsurface from storage tanks, pipelines, and refueling facilities provide point sources for entry of high concentrations of fuel oxygenates into groundwater. Underground storage tank improvement programs underway by the states and EPA should result in a reduction in the release of gasoline and fuel oxygenates to groundwater from these potential point sources.

• Exhaust emissions from vehicles and evaporative losses from gasoline stations and vehicles are sources of oxygenate release to the atmosphere. Because of their ability to persist in the atmosphere for days to weeks and because they will partition into water, fuel oxygenates are expected to occur in precipitation in direct proportion to their concentration in air. Hence, fuel oxygenates in the atmosphere provide a nonpoint, low concentration source to the hydrologic cycle as a result of the dispersive effect of weather patterns and occurrence in precipitation.

• Volatilization of the alkyl ether oxygenates will occur slowly from ponds and lakes, and from slow-moving and deep streams and rivers; volatilization can be rapid from shallow and fast-moving streams and rivers. Alkyl ether oxygenates are much less biodegradable

than ethanol or the aromatic hydrocarbon constituents of gasoline and, therefore, will persist longer in groundwater. They also adsorb only weakly to soil and aquifer material. Consequently, dissolved alkyl ether oxygenates will move with the groundwater flow and migrate further from a point source of contamination.

• The USEPA draft drinking water lifetime health advisory for MTBE ranges from 20 to 200 µg/L; a revised health advisory is expected later this year. Health advisories have not been developed for other fuel oxygenates.

• MTBE was detected in 7% of 592 storm-water samples in 16 cities surveyed between 1991-1995. When detected, concentrations ranged from 0.2 to 8.7 µg/L, with a median of 1.5 µg/L. A seasonal pattern of detections was evident, as most of the detectable concentrations occurred during the winter season. MTBE was detected both in cities using oxygenated gasoline to abate CO nonattainment and in cities using MTBE-oxygenated gasoline for octane enhancement.

• At least one detection of MTBE has occurred in groundwater in 14 of 33 states surveyed. MTBE was detected in 5% of approximately 1500 wells sampled, with most detections occurring at low (µg/L-level) concentrations in shallow groundwater in urban areas.

• Drinking water supplied from groundwater has been shown via limited monitoring to be a potential route of human exposure to MTBE. MTBE has been detected in 51 public drinking water systems to date based on limited monitoring in 5 states, however, when detected the concentrations of MTBE were for the most part below the lower limit of the USEPA health advisory. Because of the very limited data set for fuel oxygenates in drinking water, it is not possible to describe MTBE's occurrence in drinking water nationwide nor to characterize human exposure from consumption of contaminated drinking water.

• There is not sufficient data on fuel oxygenates to establish water quality criteria for the protection of aquatic life.

• The presence of MTBE and other alkyl ether oxygenates in groundwater does not prevent the application of conventional (active) methods to clean up gasoline releases; however, the cost of remediation involving MTBE will be higher. Also, the use of intrinsic (passive) bioremediation to clean up gasoline releases containing MTBE may be limited because of the difficulty with which MTBE is biodegraded.

• Theoretical predictions based on energy content indicate that reductions in fuel economy resulting from the addition of allowable levels of oxygenates to gasoline should be in the range of only 2-3%. On-road measurements agree with these estimates.

• Automobile engine performance problems due solely to the presence of allowable levels of oxygenates in gasoline are not expected because oxygenated gasolines and nonoxygenated gasolines are manufactured to the same specifications of the American Society for Testing and Materials.

• Complaints of acute health symptoms, such as headaches, nausea, dizziness, and breathing difficulties, were reported in various areas of the country after the introduction of oxygenated gasoline containing MTBE. Limited epidemiological studies and controlled exposure studies conducted to date do not support the contention that MTBE as used in the winter oxygenated fuels program is

causing significant increases over background in acute symptoms or illnesses in the general public or workers; however, they do not rule out the possibility that a small percentage of the population may be sensitive to MTBE alone or in gasoline.

• Human exposure data to MTBE are too limited for a quantitative estimate of the full range and distribution of exposures to MTBE among the general population. Less information is available on exposures to oxygenates other than MTBE.

• The assessment found that chronic noncancer health effects (neurologic, developmental, or reproductive) would not likely occur at an environmental or occupational exposures to MTBE. The observation of acute and reversible neurobehavioral changes in rats exposed to relatively high levels of MTBE is indicative of a neuroactive or possibly neurotoxic effect.

• Current data are too limited to quantitate health benefits of reduced ambient CO from wintertime use of oxygenated fuels.

• Experimental studies indicate that MTBE is carcinogenic in rats and mice at multiple organ sites after oral or inhalation exposure. The mechanisms by which MTBE causes cancer in animals are not well understood. Tertiary butyl alcohol and formaldehyde, the primary metabolites of MTBE biotransformation, are also carcinogenic in animals.

• MTBE has been tested for genotoxicity with generally negative results, whereas formaldehyde is genotoxic in a variety of experimental systems. While there are no studies on the carcinogenicity of MTBE in humans, based on animal data there is sufficient evidence to conclude that MTBE is either possibly or probably a human carcinogen. However, estimates of human risk from MTBE contain large uncertainties in both human exposure and cancer potency.

• The interpretation of any health risks associated with the addition of MTBE to gasoline requires a comparison to the health risks associated with conventional gasoline. The net effects of oxygenated

gasoline on emissions and ambient concentrations of air toxics have not been adequately characterized. Consequently, comparative risks between oxygenated and nonoxygenated gasolines have not been established.

• It is not likely that the health effects associated with ingestion of moderate to large quantities of ethanol would occur from inhalation of ethanol at ambient levels to which most people may be exposed from use of ethanol as a fuel oxygenate. Potential health effects from exposure to other oxygenates are not known and require investigation if their use in fuels is to become widespread.