The Health Effects Institute
"A Partnership of the U.S. Environmental Protection Agency and Industry"
The Potential Health Effects of Oxygenates Added to Gasoline
A Review of the Current LiteratureA Special Report of the Institute's Oxygenates Evaluation Committee
EXECUTIVE SUMMARY
INTRODUCTION
Since the passage of the Clean Air Act in 1970, the United States has endeavored to reduce human exposure to a number of air pollutants that pose threats to public health, including several—carbon monoxide, certain air toxics, and ground-level ozone—that come in significant proportion from motor vehicle fuels and exhaust emissions. A number of strategies have been implemented to reduce motor vehicle emissions, including increasingly more stringent exhaust emission standards for new vehicles, improved inspection and maintenance of all vehicles, and more recently, reformulation of motor vehicle fuels. Although many areas of the United States still face serious air pollution problems, these strategies have substantially reduced emissions from motor vehicles, and levels of general air pollution as well. However, this reduction in emissions has been offset substantially by continuous and rapid increases in the number of vehicles and the number of miles driven. Thus, the country continues to be faced with the task of both further reducing pollution in some areas and maintaining the cleaner air in those areas where it has been improved.
By enacting the Clean Air Act Amendments of 1990, the Congress took further steps toward reducing emissions from motor vehicles, including mandating, for the first time, requirements to change the formulation of gasoline. Specifically, to reduce carbon monoxide emissions, the Amendments required that areas in which the carbon monoxide standard was exceeded must, in 1992, begin to use oxygenated fuel (usually referred to as oxyfuel), which would contain at least 2.7% oxygen by weight. This oxygen content has been achieved by adding, most commonly, 15% (by volume) of methyl tert-butyl ether (MTBE) or 7.8% (by volume) of ethanol. To reduce ground-level ozone in the nine areas most out of compliance with the ozone standard (and in other areas that chose to be involved in the program), the Amendments required that, beginning in 1995, reformulated gasoline be used; this is a significantly changed form of conventional gasoline that contains at least 2% oxygen by weight, has a reduced content of benzene and other aromatic compounds, and produces limited emissions of total air toxics.
THE HEALTH EFFECTS INSTITUTE'S OXYGENATES REVIEW
The introduction of fuels containing oxygenates elicited concerns from workers and the general public in some areas, including reports of unpleasant odors, headaches, or other symptoms attributed to the fuels, and questions about their effects on the cost of gasoline, the performance of engines, and the economy of fuel usage (miles per gallon). In response to continuing health questions, the U.S. Environmental Protection Agency (EPA) and the Centers for Disease Control and Prevention (CDC) supported an effort by the Health Effects Institute to conduct an intensive review of (1) the existing science of the health effects of oxygenates, (2) the risk evaluations done by the EPA in 1993 and 1994, and (3) in a qualitative way, the health effects of exposure to the new additives as they relate to the health effects of other pollutants whose levels in emissions change when fuels containing oxygenates are used. In addition, the White House Office of Science and Technology Policy is conducting a broader review of the oxygenated fuels program, which will consider air quality benefits, engine performance, fuel economy, and the costs of these new fuels. This HEI review will form the core of the health effects portion of the White House effort.
In June 1995, the HEI Board of Directors appointed distinguished scientific experts to the Oxygenates Evaluation Committee to work with the HEI scientific staff to conduct this review. The Health Effects Institute also sought out a diverse Advisory Panel of individuals from federal government agencies, state health departments, industry, and labor and environmental organizations, who were asked to provide input at the planning stage of this review and on a draft of this report. Each section of the report also was reviewed carefully by experts in the appropriate scientific disciplines.
The results of the review by the Oxygenates Evaluation Committee are summarized below.
ASSESSMENT OF EXPOSURE TO OXYGENATES
The HEI Oxygenates Evaluation Committee noted that about 70 million people live in areas where oxygenates (mainly MTBE, but a substantial portion of ethanol) have been added to fuels. Although MTBE is widely used, information on personal exposure to it is limited. However, the Committee was able to identify ranges and approximate levels of exposure for the general public in several situations, for service station attendants and mechanics, and for workers who transport and handle MTBE and fuels. Information on exposure to ethanol is much more limited, and even less is available on ethers other than MTBE.
Both the general public and service station attendants have been exposed during refueling in two ways: to inhalation levels of MTBE typically between 0.2 and 1.5 parts per million (ppm), with some peaks above 10 ppm, and to occasional skin contact by spillage. Members of the general public could be expected to be exposed to these levels very briefly while refueling, whereas service station attendants face similar levels but for longer periods (e.g., a work shift). Limited data on ethanol exposure during refueling suggest exposure levels below 1 ppm (the minimal level of detection in the studies), with some unusual peak recordings as high as 46 ppm.
The highest exposures to MTBE were noted among workers who manufacture or transport it. Those involved in the transport of neat MTBE experience median short-term (e.g., 30-minute) exposures of 13.8 ppm, with a wide range up to 1,050 ppm. For those who transport MTBE in fuel mixtures, the median was 2.4 ppm with peaks in excess of 100 ppm.The Oxygenates Evaluation Committee also reviewed the information on groundwater contamination by MTBE. It concluded that MTBE and other oxygenates can move into underground water from contaminated soil (for example, as a result of leaks from underground storage tanks). Although the measurements of MTBE in shallow wells were limited, they support the possibility that underground water could become contaminated. Thus, both ingestion and absorption through skin contact with oxygenates are potential routes of exposure.
HEALTH EFFECTS OF OXYGENATES
The HEI Oxygenates Evaluation Committee reviewed a number of health endpoints for potential effects of MTBE, and identified two areas in which the most significant questions arose.
Short-Term Effects Symptoms such as nausea, headaches, and sensory irritation were reported in some communities after oxyfuel and reformulated gasoline containing MTBE had been introduced. Based on the community and human exposure studies conducted, it appears that most people do not experience unusual symptoms or significant acute medical consequences in response to short-term exposure to gasoline containing MTBE. However, exposure to MTBE may cause acute symptoms in some individuals. In addition, on the basis of effects on motor activity observed in rats exposed to relatively high levels of MTBE (800 ppm), the Oxygenates Evaluation Committee concluded that MTBE, at these high levels, is toxic to the nervous system. Because 800 ppm MTBE was the lowest exposure concentration at which motor activity was evaluated, and because complex central nervous system functions that may be more sensitive to MTBE were not tested, MTBE's neurotoxicity at the lower levels to which most people are exposed is unknown.
Cancer Effects Tumors have been observed at multiple sites in rats and mice after exposure to high levels of MTBE. The Oxygenates Evaluation Committee considered these findings to be cause for concern. At the same time, the Committee noted that the mechanisms that caused these tumors and the likelihood that these or other tumors will occur in humans exposed at substantially lower levels are both unknown.
In addition to examining the health effects of MTBE, the Committee investigated the health effects of ethanol. Extensive evidence indicates that ingesting ethanol at moderate to high levels affects the nervous system and prenatal development processes, and that ingestion over long periods of time increases the risk of certain cancers. It is unlikely that these effects would result from the very low exposure levels (by inhalation) in refueling situations, because the preexisting levels of ethanol in the blood from normal metabolic processes would not be significantly affected.
Few studies have been conducted on the health effects of other oxygenates such as ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), or diisopropyl ether (DIPE). They deserve substantial investigation if they are likely to be placed in widespread use.
Another important question about oxyfuel and reformulated gasoline is whether their health effects actually differ from those of conventional gasoline, and if so, in what ways. The short-term effects reported after exposure to MTBE are not unlike effects reported in some individuals after exposure to gasoline vapors and motor vehicle emissions. Also, some constituents of conventional gasoline and motor vehicle emissions have been shown to cause tumors in animals, although as with MTBE, the mechanisms by which the tumors developed and whether humans are also at risk are both uncertain.
ASSESSMENT OF EXPOSURE TO GASOLINE AND MOTOR VEHICLE EMISSIONS AND THEIR POTENTIAL HEALTH EFFECTS
Oxygenates have been added to gasoline to reduce the emission of certain air pollutants. Because the oxygenates change the mixture of both evaporative and tail pipe emissions, they also may change the health effects of exposure to the emissions. This factor must be taken into consideration when evaluating the impact of oxygenates in fuels. The primary motor vehicle emissions that are potentially affected by adding oxygenates are carbon monoxide (CO) and certain air toxics. (Although motor vehicle emissions also include precursors of ozone, and reformulated gasoline is designed to reduce these precursors, the primary means to accomplish this is by reducing other components of gasoline and not by adding oxygenates.)
Extensive testing of motor vehicle emissions with and without oxygenates demonstrates that oxygenates consistently reduce tail pipe emissions of CO by 10% to 25%. In addition, the number of days on which the ambient CO standard is exceeded has decreased in many areas since oxyfuel was introduced. However, ambient pollutant levels are affected by several factors, including changes in weather patterns and in the number and types of vehicles driven and miles traveled. Thus, assessing the trends in pollutant levels, and estimating the contribution that oxygenates have made to those trends would require multiyear analyses and was beyond the scope of this review.
Reducing the levels of CO is of particular interest. When it enters the body, CO combines with hemoglobin in blood, which reduces the ability of the blood to deliver oxygen to the tissues. People with coronary artery disease are particularly sensitive to this effect because they have impaired ability to increase coronary blood flow. This becomes a problem during exercise when the heart needs more oxygen. When blood flow through the heart is not sufficient to meet the oxygen demand, the heart becomes ischemic, resulting in chest pain (angina pectoris), or electrocardiographic changes, or both. Even relatively low CO levels may bring on ischemia more quickly for some individuals with coronary artery disease when they are exercising moderately. Although the evidence of effects of CO in human controlled-exposure studies is strong, little information has been gathered on precisely how many people in this sensitive group are actually exposed to CO at levels above the standard.
Adding oxygenates also appears to cause an overall reduction in air toxics emissions, but different air toxics are affected differently; benzene emissions are reduced, formaldehyde and acetaldehyde emissions are increased in some cases, and 1,3-butadiene emissions are either reduced slightly or not changed. All four of these air toxics emitted from motor vehicles—benzene, 1,3-butadiene, formaldehyde, and acetaldehyde—are classified as either known or probable human carcinogens. However, uncertainties in estimating personal exposure to each of the air toxics, and in projecting the carcinogenic risk from each for humans, are too great to allow a meaningful assessment of the overall impact of the changes in the emission of air toxics on the public's risk from using fuels containing oxygenates.
OVERALL ASSESSMENT
Based on its review of existing evidence on the exposure to and health effects of oxygenates used in gasoline, the HEI Oxygenates Evaluation Committee drew the following conclusions about the oxygenates themselves.
Introducing oxygenates into gasoline to reduce CO emissions has increased exposure to MTBE for the general public during brief higher-level exposures while refueling and during more sustained but lower-level exposures while driving, and for service station employees during higher-level exposures over entire work shifts. These exposures can occur by both inhalation and skin contact. Workers who handle or transport neat MTBE can experience significantly higher average inhalation exposure levels than people in other situations. Few data on exposure to other oxygenates have been gathered.
MTBE has been measured in some underground water; its presence in water may result in exposure by ingestion or skin contact should water supplies become contaminated.
The potential health effects from exposure to gasoline containing MTBE include (1) headache, nausea, and sensory irritation in some (possibly sensitive) individuals, based on reports after exposure to oxygenates; (2) acute, reversible neurotoxic effects, based on changes in motor activity in rats at high exposure levels; and (3) cancer, based on increases in the frequency of tumors at multiple organ sites in rats and mice at high exposure levels. Although questions persist about how to interpret each of these observed effects, they nevertheless point to a potential human health risk.
The health effects from exposure to ethanol by ingesting moderate to large quantities have been extensively investigated. Under these conditions, ethanol can increase the risks of certain cancers, adversely affect the developing embryo, produce neurotoxicity, and cause various other types of damage. However, it is unlikely that such effects would occur at the very low ambient levels to which most people are exposed by inhalation.
Potential health effects from exposure to other oxygenates are not known and require investigation if their use in fuels is to be widespread.
In addition to these conclusions about the oxygenates themselves, after qualitatively assessing the health effects of gasoline and motor vehicle emissions with and without oxygenates, the Oxygenates Evaluation Committee has come to the following conclusions about gasoline containing oxygenates.
The potential health effects of exposure to components of conventional gasoline (without oxygenates) include short-term and cancer effects similar to those that could result from exposure to gasoline containing oxygenates.
Adding oxygenates to gasoline can reduce the emission of CO and benzene from motor vehicles, and thereby potentially lower certain risks to members of the population. At the same time, using oxygenates increases exposure to aldehydes, which are carcinogenic in animals, and to the oxygenates themselves.
Adding oxygenates is unlikely to substantially increase the health risks associated with fuel used in motor vehicles; hence, the potential health risks of oxygenates are not sufficient to warrant an immediate reduction in oxygenate use at this time. However, a number of important questions need to be answered if these substances are to continue in widespread use over the long term.
In addition to its conclusions about possible health effects, the Oxygenates Evaluation Committee noted a general lesson to be learned from introducing oxygenates to the general public. Although it is not possible to have complete information about a substance before it is used, the diverse experiences after introducing oxygenated fuels argue strongly that any future new use of a substance should (1) be preceded by a sufficiently comprehensive research and testing program (including mechanistic and human studies), and (2) be accompanied by rigorous exposure assessment and epidemiologic studies.
RESEARCH NEEDS
The HEI Oxygenates Evaluation Committee has identified a number of research needs to reduce uncertainties about the health effects of oxygenates by themselves and as parts of gasoline mixtures. These are summarized in detail in the section, Research Priorities for Oxygenates. To some extent, a number of these needs are being addressed by ongoing or planned research programs at several organizations. Specifically, the Oxygenates Evaluation Committee identified these highest priority research needs:
A comprehensive set of studies to assess personal exposure to oxygenates in public and occupational settings;
Human environmental chamber studies to evaluate metabolism, symptoms, and neurotoxic effects in potentially sensitive individuals after exposure to MTBE and MTBE-gasoline mixtures;
Epidemiologic and animal studies to improve our understanding of the potential risk of human cancer from exposure to MTBE alone and in association with gasoline vapors and vehicle exhaust; and
Comprehensive assessments of other ethers (e.g., ETBE, TAME, DIPE) if they are to be placed in widespread use.
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