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Executive Summary
Scope
The Clean Air Act Amendments of 1990 require the winter use of
oxygenated gasoline in certain areas of the country that exceed the
National Ambient Air Quality Standards (NAAQS) for carbon monoxide
(CO). Use of oxygenated gasoline increases combustion efficiency and
reduces CO emissions. CO, which poses a health threat to persons with
chronic heart disease, tends to be a winter pollution problem because
motor vehicle fuel combustion tends to be less efficient in cold
weather. In response to public complaints of acute health effects from
exposure to evaporative and exhaust emissions from oxygenated gasoline
in conjunction with the winter oxygenate program, the Administration
convened an interagency panel of scientists and health effects experts.
This working group was charged with developing an immediate analysis of
evidence on the acute health effects and other health issues related to
the wintertime use of oxygenated fuels.
This assessment is the first phase of a comprehensive evaluation
of oxygenated fuels now being conducted under the coordination of the
National Science and Technology Council's Committee on Environmental and
Natural Resources (CENR). The present document does not address the
potential health benefits of the oxygenated gasoline program, nor does
it compare risks of oxygenated gasoline to the risks of conventional
gasoline. The assessment focuses on exposures to oxygenates and
oxygenated gasoline which occur through inhalation; other routes of
exposure not addressed here, such as through contaminated groundwater,
may also be an issue, but are being considered in the next phase.
Methyl tertiary butyl ether (MTBE) has become the most widely used
motor vehicle fuel oxygenate in the U.S., though in some areas, ethanol
is the dominant oxygenate used for motor vehicle fuel. Typically,
MTBE-oxygenated gasoline in the winter oxygenate program contains
approximately 15% MTBE by volume. The Clean Air Act requires at least a
2.7% oxygen content for gasoline sold in CO nonattainment areas, and 15%
MTBE achieves this requirement. This assessment focuses primarily on
MTBE. Emphasis on MTBE reflects its widespread use, but does not imply
that this is the only oxygenate of concern. Because of limitations of
available data, this assessment does not address to the same degree
other oxygenates, such as ethanol, ethyl tertiary butyl ether (ETBE),
tertiary amyl methyl ether (TAME), tertiary amyl ethyl ether (TAEE), and
diisopropyl ether (DIPE). Ethanol is an exception for which the data
base is extensive, but it pertains primarily to ethanol ingestion, not
inhalation exposure to an ethanol/gasoline blend.
The second phase of this assessment process will evaluate the
scientific literature related to the potential health risks associated
with MTBE and other fuel oxygenates in gasoline relative to conventional
gasoline. It will also assess risks associated with groundwater
contamination, changes in air quality, benefits associated with reduced
emissions or improved air quality, and engine performance and fuel
economy. The second phase is anticipated to be completed by mid-1996.
This document was prepared by scientists from three federal
agencies (the Centers for Disease Control and Prevention (CDC), the
National Institute of Environmental Health Sciences (NIEHS), and the
Environmental Protection Agency (EPA)), under the guidance of an
Interagency Oxygenated Fuels Assessment Steering Committee. The initial
assessment and its peer review were completed in an intensive effort
over a two-month period. The final draft then underwent a second round
of external peer review.
Assessment Findings
Acute Health Effects
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. Community-based surveys in Alaska and Milwaukee, Wisconsin,
indicated that while most people reported no increase in acute health
symptoms after exposure to oxygenates in gasoline, a substantial number
of people attributed headaches and other health complaints to the
presence of oxygenates in gasoline. Results from Milwaukee suggest that
increased media attention to this issue could have been a factor in the
greater likelihood of citizen reports of health complaints in some
communities. Some occupationally exposed workers did report health
complaints during the oxygenate fuel season, but some evidence suggested
that the prevalence of such complaints was similar for exposure to
oxygenated gasoline or conventional gasoline. Thus, a causal
association between acute health effects and exposure to MTBE or other
oxygenates in gasoline in a relatively smaller proportion of persons has
not been demonstrated but cannot be ruled out on the basis of the
limited epidemiologic studies that have been conducted to date.
The three controlled human-exposure studies of MTBE among healthy
human volunteers provide a consistent picture: exposure to pure MTBE in
air at concentrations as high as 50 ppm under laboratory conditions did
not cause increased symptoms or any notable measurable responses. (In
oxygenated gasoline, however, MTBE is present as part of a complex
gasoline mixture, and this mixture has not been tested under controlled
exposure conditions.) These findings also do not rule out the
possibility that a subpopulation of people in the general population may
be especially sensitive to MTBE alone or in gasoline, or that effects
might be associated with exposure to evaporative or combustion emissions
from oxygenated gasoline or with some other factor that has not yet been
characterized. Studies in animals have not provided evidence of overt
neurotoxicity due to MTBE exposure at air concentrations from 100 ppm to
3000 ppm MTBE, but neuroactive properties are displayed at higher
concentrations.
With regard to human exposures to MTBE, the assessment concludes that
data are too limited for a quantitative estimate of the full range of
exposures to MTBE among the general population. Less information is
available on exposures to oxygenates other than MTBE. The limited data
available on air quality and micro environments (e.g., at gasoline
pumps, inside cars, in personal garages) were used to estimate
reasonable worst-case (high-end) potential exposures on the basis of
certain assumed activity patterns and approximate micro-environmental
concentrations.
Studies of MTBE metabolism in experimental animals demonstrated that
the metabolism and elimination of MTBE and its metabolites proceeded
rapidly regardless of the route of administration. Among humans,
clearance of most of the internal dose of MTBE is rapid, but a small
fraction is slowly eliminated from the body.
The available scientific evidence regarding human exposure to
oxygenated gasoline and acute health symptoms was considered
insufficient to develop estimates of exposure-related effects.
Chronic Health Effects
MTBE has been extensively tested for genetic toxicity with
generally negative results. Limited positive responses in vitro were
attributed to the in vitro metabolite, formaldehyde. TAME has been
tested less extensively, also with negative results. Among the
metabolites of MTBE, only formaldehyde has demonstrated mutagenicity.
Considering the magnitude and duration of exposures in the animal
studies and the association of developmental effects with maternal
toxicity, it is concluded that MTBE is not expected to pose a
reproductive or developmental health hazard under the intermittent,
low-level exposures experienced by humans. The EPA reference
concentration for other chronic noncancer health effects for MTBE (0.83
ppm) is higher than the reasonable worst-case annual average daily
exposure estimate (0.019 ppm).
While there are no studies on the carcinogenicity of MTBE in humans,
MTBE should be regarded as posing a potential carcinogenic risk to
humans based on animal cancer data. Experimental studies in rats and
mice indicate that MTBE is carcinogenic at multiple sites after oral or
inhalation exposure. The primary metabolites of MTBE, tertiary butyl
alcohol (TBA) and formaldehyde, are also carcinogenic in animals. No
studies have been reported on the carcinogenicity of ETBE or TAME.
Based on the animal carcinogenicity data, estimates of human cancer
risk for lifetime exposure to MTBE (70 years) were calculated,
recognizing that there are large uncertainties in the distribution of
exposures to MTBE in the population and in the estimates of human cancer
potency. To deal with these uncertainties several assumptions were made
in order to estimate potential low dose responses in humans. Thus,
depending on the validity of the assumptions used in making these
estimates, the actual cancer risks could even be nearly zero.
It is not known whether the cancer risk of oxygenated gasoline
containing MTBE is significantly different from the cancer risk of
conventional gasoline. The estimated upper bound cancer unit risks of
MTBE are similar to or slightly lower than those of fully vaporized
conventional gasoline, which has been listed by EPA as a probable human
carcinogen based on animal carcinogenicity data. However, because of a
lack of health data on the nonoxygenated gasoline vapors to which humans
are actually exposed, it is not possible to have a reasonably good
estimate of population cancer risk to conventional gasoline. The
estimated upper bound cancer unit risk of MTBE (i.e., cancer potency) is
approximately an order of magnitude lower than that of benzene, a
constituent of gasoline that is classified as a known human carcinogen,
and more than 100 times less than that of 1,3-butadiene, a carcinogenic
emission product of incomplete fuel combustion. The comparative risk
among oxygenated and nonoxygenated gasoline types has not been established.
The data were generally inadequate to evaluate the health risks of
oxygenates other than MTBE, a factor which makes other oxygenates and
gasoline mixtures to which they are added all the more important to
investigate further.
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