A. Purpose
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), an air pollutant associated with potential health risks. CO interferes with the body's ability to utilize oxygen by combining with hemoglobin, which then prevents it from transporting oxygen efficiently to organs of the body. Persons with chronic heart disease are at particular risk for adverse health effects of CO, but other groups, including the elderly, pregnant women, infants, and persons with anemia or cardiopulmonary disease, are also likely to be at increased risk due to CO exposure. 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. Gasoline containing 2.7% oxygen (by weight) 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. In addition, oxygenates usually displace aromatics in gasoline, such as benzene, toluene and xylene, which are a source of octane. These aromatics have been targeted for reduction in the reformulated gasoline program.
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 the possible health risks associated with using oxygenated gasoline. The purpose of this interagency assessment of the potential health risks of oxygenated gasoline is to explore the question of whether evidence from recent health studies of oxygenated gasoline or oxygenates, especially MTBE, and acute illness warrants a reconsideration by EPA of potential health risks of the oxygenated gasoline program during the coming winter months. This assessment is being prepared for the Office of Science and Technology Policy, under the auspices of the Interagency Oxygenated Fuels Assessment Steering Committee. The potential health benefits of the oxygenated program are not addressed in this assessment; such benefits will be considered in the more comprehensive interagency assessment of oxygenated fuels that is currently under way.
A team of scientists from three Federal agencies, the Centers for Disease Control and Prevention (CDC), the National Institute for Environmental Health Sciences (NIEHS), and the Environmental Protection Agency (EPA) was assembled to complete this evaluation. Members of this team were chosen by members of the Interagency Steering Committee or their designates on the basis of the members' expertise in different scientific disciplines and knowledge about issues related to oxygenated gasoline.
B. Scope
The interagency team assembled for this short-term assessment was asked to review studies that have been made available since the enactment of the Clean Air Act Amendments of 1990. Because much of this recent research has not been published, the team evaluated unpublished as well as published reports. Because of the limitations of available data, this assessment focuses primarily on MTBE in oxygenated gasoline and its principal metabolite, tertiary butyl alcohol (TBA). Other ethers and alcohols, including ethanol, ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), tertiary amyl ethyl ether (TAEE), and diisopropyl ether (DIPE), are less extensively used (or may potentially be used) in oxygenated gasoline. Although this assessment does not address these other oxygenates as extensively as MTBE, this does not imply that MTBE is the only oxygenate of concern. The focus of this assessment is oxygenated gasoline, not reformulated gasoline. Reformulated gasoline is intended to reduce motor vehicle emissions of hydrocarbons which lead to higher ozone (smog) levels during summer months and air toxics year round. Reformulated gasoline has 2.0% oxygen (by weight), typically achieved by the addition of 11% MTBE or 5% ethanol (by volume), as well as lower concentrations of certain volatile organic compounds than conventional gasoline.
This assessment focuses on exposures that occur through inhalation; ingestion of oxygenates from contaminated water supplies may also be important but was considered beyond the scope of this assessment. The data base on ethanol is extensive, but it pertains primarily to ingestion, not inhalation. A full consideration of the relevance of the literature regarding the health effects of ethanol by ingestion to exposure by inhalation from evaporative and combustion mixtures of ethanol in gasoline also was considered beyond the scope of this assessment.
This assessment also attempts to identify areas where the scientific data base is particularly limited. It should also be noted that most of the information available on the health effects of MTBE pertains to MTBE alone rather than the mixture of MTBE and gasoline. This assessment does not attempt to assess the risks or benefits of MTBE-oxygenated gasoline in relation to conventional gasoline.
In addition, a more comprehensive review of the fundamental basis and efficacy of the Environmental Protection Agency's winter oxygenated gasoline program is currently being conducted by a combination of technical and scientific experts from across several Federal agencies, under the coordination of the National Science and Technology Council's Committee on Environment and Natural Resources. This more comprehensive review will consider not only health effects, but also air quality benefits, fuel economy and engine performance, ground water and drinking water quality, and an economic analysis of benefits. In addition, an independent review of the health effects of the use of oxygenates in gasoline is currently being conducted by the Health Effects Institute (HEI) and a panel of experts, in response to a request from the EPA. It is expected that the HEI review will be the core of the health effects section of the more comprehensive interagency review.
Table of Contents
Office of Science & Technology Policy (OSTP)
President's Council of Advisors on Science and Technology (PCAST)
