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

Carbon monoxide (CO) pollution is mainly caused by incomplete combustion of motor-vehicle fuels. The Clean Air Act Amendments of 1990 require the use of oxygenated gasoline in areas of the country where the National Ambient Air Quality Standard (NAAQS) for CO is being exceeded. This requirement was intended to protect those who are most susceptible to the adverse health effects of CO, particularly patients with cardiovascular disease. Methyl tertiary-butyl ether (MTBE) has become the most widely used oxygenate in the United States for motor-vehicle fuels. (Other oxygenates used in fuels include ethanol, tertiary-butyl alcohol, ethyl tertiary-butyl ether, and tertiary-amyl methyl ether.)

Concurrently with the start of the federal oxygenated-gasoline program in 1992, MTBE has been implicated in complaints of headaches, coughs, and nausea, principally in Alaska, but also in Montana, New Jersey, and Wisconsin. There have also been anecdotal reports of reduced fuel economy in some locations and questions about engine performance. Additional concerns have been raised about the detection of low levels of MTBE in some samples of groundwater. Due to the public concern over the potential health effects of MTBE-oxygenated fuels, more than $2 million of scientific studies have been conducted by EPA and others to investigate the reported symptoms. Unfortunately, all of these

studies have deficiencies, such as inadequate exposure assessment, insufficient sample size, subjective outcome assessment, and the possibility of selection bias.

EPA asked the National Research Council to independently review a draft interagency report from the federal government, prepared under the direction of the Office of Science and Technology Policy (OSTP) through the Committee on Environment and Natural Resources (CENR) of the president's National Science and Technology Council (NSTC). The purpose of the interagency report was to provide a review of the scientific information on oxygenated fuels and to assess effects of the winter oxygenated-fuels program on air quality, fuel economy, engine performance, water quality, and public health.

In response to EPA's request, the Research Council assembled a multidisciplinary committee—the Committee on Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels—which has prepared this report. The committee was charged with the following tasks: (1) review the draft interagency report on the toxicological effects of MTBE and other oxygenates and compare such effects and multimedia exposures with those of conventional gasoline; (2) review the draft interagency report on the impacts of MTBE and other oxygenates on vehicle emissions, air pollution, fuel economy, and engine performance and compare such impacts with those of conventional gasoline; and (3) identify priorities for research to fill data gaps. The committee was not provided with the original sources of data from which the interagency report was written, so a full and complete critique of scientific credibility, comprehensiveness, and internal consistency of the data was not possible within the constraints of this study. Although the charge to the committee did ask for a comparison of oxygenated fuels with conventional gasoline, it was in the context of reviewing the interagency report. The comparison was not possible, because the

interagency document failed to make such comparisons with respect to health effects, in large part due to the limited amount of comparative data. Comparative results were available in the interagency report on air quality, fuel economy, and engine performance and were considered by the committee. The committee was asked to address only the interagency report, which dealt with the winter oxygenated-fuels program, not the program itself. In this report, ''oxygenated fuels" is intended to refer only to the fuels used within the oxygenated-fuels program. The interagency report did not specifically examine the reformulated gasoline program, which is intended to reduce motor-vehicle emissions that lead to increased ozone concentrations in the lower atmosphere.

The committee's critique of the interagency report submitted to the NRC on March 15, 1996, and recommendations for further studies are summarized below. (The preface and executive summary of the interagency report are presented in Appendix A.)

The interagency report concludes that there have been substantial reductions in ambient CO concentrations in the past 20 years and that vehicle emission controls have been a major factor in this reduction. The committee agrees with these conclusions. However, the committee believes that the federal report should better characterize the uncertainty about the extent to which oxygenated fuels have contributed to this reduction. Although the interagency report provides a good summary of previous studies that have assessed the impacts of oxygenated fuels on winter CO concentrations, it should state clearly that winter ambient CO reductions

attributable to oxygenated fuels have been as low as zero and as high as about 10%. In addition, the implications of emission-data deficiencies in evaluating the effectiveness of the oxygenated-fuels program should be fully discussed in the interagency report.

Because of the relationship between ambient temperature and tailpipe emissions of CO, it is important that the term "winter temperature" be well defined in the interagency report. A map, or similar mechanism, giving winter temperature ranges and mean temperatures for the areas participating in the oxygenated-fuels program is essential in understanding and evaluating the data.

The committee agrees with the interagency report that, under many fuel-control systems, oxygenated fuels decrease CO emissions under Federal Test Procedure conditions (at 75°F). However, the data presented do not establish the existence of this benefit under winter driving conditions. Also, the report does not clearly address the effects of fleet composition, particularly high-emitting vehicles, on total CO emissions. Emissions from high emitters can be two orders of magnitude larger than those from late model, well-maintained vehicles.

EPA designed a computer model, referred to as the MOBILE model, specifically for use by states in preparing emission inventories required under the Clean Air Act. MOBILE 5a is the model version currently in use. The interagency report highlights some discrepancies between MOBILE 5a model results, vehicle-emissions data, and ambient concentrations. However, it does not provide an assessment of why those differences exist. It should do so and should also emphasize to a greater extent that the model apparently overpredicts the oxygenated-fuel effect by at least a factor of 2 based on comparisons of model predictions of CO emission reductions with observed data.

Much of the available data suggests that increased NO_x emissions have resulted from the use of oxygenated fuels. Any increase in NO_x emissions could be detrimental in ozone nonattainment areas

where exceedances have occurred during the period of the oxygenated-fuels program.

A well-designed field study with adequate statistical power should be performed at low temperatures to assess whether oxygenated fuels reduce ambient CO concentrations under such conditions. If a beneficial effect of reduction in CO is demonstrated in that field study, a carefully designed study should then investigate the effects of oxygenated fuels on emissions of NO_x, VOCs, and toxic air pollutants on winter air quality, using appropriate controls and accounting for differences in such factors as fleet population, high-emitting vehicles, inspection and maintenance programs, and local fleet characteristics. The introduction of toxic organic compounds into the air, as well as a fuel-economy penalty, should also be addressed in such a study. Also, it is equally important to perform a similar study under cold temperature conditions in an area where no oxygenated fuels are used. Results for an area using oxygenated fuels can be compared with an area not using such fuels.

The interagency report concludes that the fuel-economy penalty associated with the use of oxygenated fuels is approximately 2% to 3% and is related to changes in volumetric energy content. The committee agrees with these conclusions. The committee also agrees with the report's conclusion that engine performance is typically not adversely affected by the use of oxygenated fuels. The report should indicate clearly that, based on data from a wide variety of sources, fuel-economy changes are reliably predicted by

the change in energy content per gallon of fuel brought about by a given change in composition.

The interagency report discussed water-quality issues arising from the use of fuel oxygenates (primarily MTBE) and their movement in the hydrologic cycle. In general, the report provides a clear and factual presentation of the current understanding of MTBE fate and transport in the environment. However, the water-quality discussion does not adequately emphasize that there is only a small amount of monitoring data available on MTBE. Such data suggest that MTBE is sometimes present in precipitation, storm-water runoff, groundwater, and drinking water.

In addition, the interagency report should clarify or revise its discussion of the following issues:

•  The extent to which the results of testing for MTBE in groundwater in the Denver area might not be representative of nationwide trends.

• The discrepancy in the atmospheric degradation rates of MTBE and other oxygenates reported in the water-quality chapter versus Appendix 3 of the air-quality chapter.

• The effects of volatilization on the transport of fuel oxygenates to groundwater, especially with respect to uncertainties. The report should document the modeling approach used and the assumptions relating to MTBE concentrations in the atmosphere. The modeling approach and assumptions used to assess such effects should be documented.

• The extent to which abiotic degradation mechanisms or other

  nonbiological mechanisms (e.g., chemisorption) reduce groundwater concentrations of MTBE and other alkyl ether oxygenates.

The large majority of states do not have any programs or requirements in place to monitor MTBE or other fuel oxygenates in storm-water runoff, groundwater, or drinking water. The absence of these monitoring data prevents an accurate assessment of exposure of human or aquatic biota to MTBE, possible health effects, and implementation of control measures to prevent adverse impacts.

On the basis of the small amount of monitoring data, MTBE has been detected in less than 5% of the groundwater samples analyzed, suggesting that drinking water is not currently a major exposure pathway for MTBE for much of the population.

storm-water runoff and shallow groundwater can be contaminated with low levels of MTBE (< 20 µg/L) via precipitation or contact with small surface spills. These contamination sources should be carefully monitored to evaluate changes over time and the effect of landuse, storm-water management practices, and hydrogeologic factors on MTBE concentrations in environmental media. If EPA considerably lowers the level of its recommended health advisory concentration for MTBE, substantial concerns would arise about the potential for nonpoint sources of MTBE to adversely impact water supplies.

More needs to be known about the biodegradation of MTBE and other alkyl ether oxygenates in surface water, soil, and groundwater. Biodegradation processes, in particular, have the potential to substantially reduce the impacts of point and nonpoint source releases of MTBE and other oxygenates. Current information

should be assessed to determine whether a better understanding of abiotic degradation is an important research need.

The committee finds important deficiencies in the human exposure analysis presented in the interagency draft report, which calculates cumulative exposure estimates for two hypothetical scenarios. The lifetime exposures calculated for these scenarios are 10 times higher than the maximum daily exposures estimated on the basis of empirical data as summarized in the portion of the report prepared by the Health Effects Institute (HEI). In addition to a "reasonable worst case" scenario, the report should generate a more realistic baseline exposure. Other emission products should also be considered.

The HEI-prepared portion of the interagency report provides a useful summary of MTBE exposure studies. Concentration ranges encountered in occupational and nonoccupational situations are adequately represented. Whereas the HEI report states that the data are too limited to calculate cumulative exposures for risk assessment, the committee's opinion is that the data are sufficient to bound a quantitative risk analysis and to develop a framework for conducting a comparative risk assessment of conventional and oxygenated fuels.

A quantitative framework should be established for evaluating changes in exposure from related emission products. Routine

ambient monitoring of MTBE and one of its major products of photo-oxidation, tertiary-butyl formate (TBF), should be initiated in communities where MTBE is used.

Representative personal exposure monitoring of MTBE in an exposed population is needed to describe the distribution of exposures and for input into risk analyses. Such exposure monitoring should include the characterization of each individual's time-activity patterns, especially in the microenvironments where important exposures are likely to occur. The most important factors affecting personal exposure should be determined in such a study.

The relationship between fixed-site community monitoring and personal exposures to MTBE and related air pollutants should be evaluated in order to determine the information value of using community monitoring for assessing human exposure.

The committee is in basic agreement with the evaluation of data presented in the interagency report with respect to the metabolism, disposition, and toxicokinetics of MTBE.

The interagency report also provides a thorough review of short-term animal studies conducted on the individual oxygenates MTBE and ethanol; however, the toxicity of TBF is not addressed and should be. Based on the available data, the committee is skeptical about the need for additional toxicity studies in rats based solely on motor activity. The committee notes that even at 800 ppm the effect, if any, is 100-1,000 times predicted exposures. It is also a reversible effect characteristic of this class of compounds, and there is no indication of neuropathology or persistent neurotoxicity

following exposure to MTBE or other ethers, even after long-term exposure to high levels of MTBE.

Although no human data are available to indicate that exposure to MTBE is linked to the development of acute human disease, the committee considers it noteworthy that the available data consistently indicate that exposures to gasoline containing MTBE in occupational settings are associated with an increased rate of acute symptoms. By suggesting that "a relatively smaller proportion of persons" might have problems with exposure, the interagency report appears to ignore consistent findings in exposed workers and overlooks the possibility that typical occupational exposures may pose an overall problem, whether or not a more sensitive subpopulation exists. There are virtually no data to indicate that the reported acute health effects are confined to a sensitive subpopulation. Thus, it is the consensus of the committee that the interagency report prematurely dismisses that acute health effects might be occurring in the exposed population as a whole as opposed to occurring only in a sensitive subgroup.

The committee agrees with the interagency report that adverse reproductive and developmental effects are not expected to result from the environmental levels of MTBE to which most people would be exposed.

The report does not discuss comprehensively the long-term animal studies in their totality, i.e., a weight-of-evidence approach. While the report notes that MTBE is a multispecies, multisite, and multisex animal carcinogen, it fails to make note of certain inconsistencies in the data. Because of inconsistencies and unresolved questions with regard to the animal carcinogenesis studies, "cancer potency estimates" of MTBE as proposed in the report should be considered cautiously. The committee feels that the male rat kidney-tumor data probably should not be used for this purpose in light of the new information on its probable causation, i.e., _2u-globulin nephropathy, which is thought to be

unique to the male rat and not relevant to humans. The use of the lymphoma and leukemia data should also be questioned until such time as a thorough review of this study, including an objective third-party review of the pathology, is accomplished. The most reliable data presently available for risk-assessment purposes are on the induction of benign liver tumors in female mice exposed via inhalation to 8,000-ppm MTBE.

Additional research on the toxicokinetics of MTBE and other oxygenates would be useful as a basis for extrapolating health-effects data from animals to humans and for identifying suitable biologic markers of exposure for use in any future epidemiological studies.

The committee agrees with the report that additional short-term animal studies on MTBE that determine blood levels in addition to evaluating central nervous system (CNS) function could be useful.

The committee believes there should be better coordination between clinical observations, epidemiologic studies, and exposure-chamber experiments regarding research on acute human health effects and exposure to oxygenated fuels. Improved clinical characterization of symptoms reported to be attributed to exposures from MTBE in gasoline is also needed and should take into account the actual types of symptoms experienced by individuals, the settings in which the symptoms occur (e.g., refueling versus driving) and the duration of symptoms. Developing objective measures of outcomes representing the key symptoms initially defined by the Alaska studies is desirable where possible. Research in the development of improved methodology in the field of acute health effects, including validation and reliability of instruments used to measure symptoms without physical findings, should be supported. One or more analytical epidemiologic studies examining the association

between MTBE exposure and acute health effects should be conducted. These studies should include individual-level quantitative exposure assessments, outcome assessments through questionnaires whose reliability and validity have been established with pilot data or through objective measurements, and high-quality data on potential confounders, including demographics, weather conditions, concurrent exposures, and automobile characteristics. (See Chapter 5 for study details.) Due to difficulties (e.g., high cost, long follow-up time, large required sample size) in conducting an analytical epidemiologic study to investigate the relationship between cancer and other chronic diseases and long-term exposure to MTBE, attention should be turned to ecologic designs, despite their widely acknowledged deficiencies. However, the committee does not recommend that ecological studies be undertaken at the present time, but rather that missing routine environmental-monitoring data on air and water begin to be collected, so that such studies can be conducted in the future.

The committee agrees with the interagency report that the data are too limited to evaluate the effectiveness of the winter oxygenated-fuel program in lowering CO exposures to a level that would not affect cardiovascular morbidity and mortality.

Epidemiologic studies are needed to evaluate the effectiveness of

the winter oxygenated-fuel program in lowering CO exposures to a level that would reduce carboxyhemoglobin levels and risk of exacerbation of cardiovascular disease. Epidemiologic studies are also needed to determine the exposures to aldehydes and air toxics, produced from the combustion of oxygenated fuels and from conventional gasoline, that would cause irritant and other short-term health effects.

With respect to the noncarcinogenic effects of MTBE resulting from long-term, continuous inhalation, the interagency report's use of EPA's reference concentration (RfC) of 3 mg/m³ (0.8 ppm) seems appropriate. This value should not, however, be used to judge the safety of acute exposures until the possible association between MTBE exposures and acute symptoms is resolved. With respect to carcinogenic risks, all estimates presented in Table 7.3 of the interagency report that are based on rat kidney tumors and lymphomas and leukemias should be eliminated until the investigations recommended above are completed (see Chapter 5). Moreover, maximum likelihood estimates (MLEs) are not reliable and should not be presented. The failure of the interagency report to make any comparison of the risks of MTBE-containing fuels and nonoxygenated fuels is a serious deficiency and should be corrected. All available data should be considered, including various surrogate measures of exposure, to compare risks and then the degree of uncertainty associated with those comparisons should be stated. The presentation of cancer risk estimates for MTBE in isolation from all other risks has little value for risk managers. The assessment contained in the HEI report comes close to the type of

evaluation the committee thinks is useful for decision-making. The overall conclusion of the HEI report, that MTBE-containing fuels do not pose health risks substantially different from those associated with nonoxygenated fuels, seems reasonably well supported but requires additional quantitative documentation.

The National Research Council committee suggests that the portion of the report that was prepared by HEI be used as the framework and database (with appropriate additions and reinterpretations as described by the National Research Council committee) for a comprehensive government risk assessment. However, greater effort should be made to provide some indication of the magnitudes of the health risks that are said to be increased and decreased (relative to conventional fuels) by the use of oxygenated fuels.

The interagency report does not present an assessment of the costs and benefits of the oxygenated-fuels program. Although the committee was not charged to conduct such an assessment, it concludes that the interagency report should address this issue. Despite uncertainties in estimating program-related costs and even greater uncertainties in estimating benefits, full evaluation of the winter oxygenated-fuel program requires that the interagency report address and document program costs and benefits at least at a broad level.