United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington DC 20460
Research and Development
EPA/600/S6-87/001 Sept. 1987
v>EPA Project Summary
Evaluation of the Carcinogenicity
of Unleaded Gasoline
Larry D. Anderson, Chao W. Chen, Vincent James Cogliano,
Aparna M. Koppikar, Robert E. McGaughy, William E. Pepelko, and
D. E. B. Potter
In the final report, the likelihood that
unleaded gasoline vapors are carcino-
genic to humans is evaluated. From
carcinogenicity data in animals, an
estimate is made of the magnitude of
cancer risk a person would experience,
if exposed for a lifetime to 1 ppm in the
ambient air, under the assumption that
gasoline vapors are carcinogenic. All
studies believed to be relevant to deter-
mining the potential carcinogenicity of
unleaded gasoline vapors are reviewed
including: (a) chronic and shorter-term
animal studies of aerosolized whole
gasoline, various gasoline fractions, and
analogous hydrocarbon mixtures; and
(b) epidemiologic studies of occupations
involving exposure to gasoline vapors.
Fifty-five epidemiologic studies involv-
ing gasoline exposure are reviewed. A
quantitative analysis of cancer incidence
in the two long-term animal gasoline
inhalation studies is performed, an
upper-bond cancer risk potency estimate
is calculated, and the uncertainties in
the estimate are discussed. The major
conclusions are: (1) although employ-
ment in the petroleum refineries is
possibly associated with cancers of the
stomach, respiratory system, and lym-
phopoietic and hematopoietic tissues,
exposure to gasoline cannot be im-
plicated as a causative agent because
of confounding exposure to other
chemicals and inadequate information
on gasoline exposure; (2) the occurrence
of liver cancer in female mice and kidney
cancer in male rats provides "sufficient"
evidence in animals that inhalation of
wholly aerosolized gasoline is carcino-
genic; and (3) gasoline vapors from
vehicle refueling might be less carcino-
genic than indicated by animal experi-
ments using wholly aerosolized gasoline,
if the less volatile components, which
are apparently responsible for acute
kidney toxicity, also contribute to the
observed carcinogenic response.
This Project Summary was developed
by ERA'S Office of Health and Environ-
mental Assessment, Washington, DC, to
announce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
This document presents an evaluation
of the likelihood that unleaded gasoline is
a human carcinogen and provides a basis
for estimating its possible public health
impact, including a potency evaluation in
relation to other carcinogens. The evalu-
ation of carcinogenicity depends heavily
on animal bioassays and epidemiologic
evidence. However, other factors, includ-
ing mutagenicity, metabolism (particularly
in relation to interaction with DNA), and
pharmacokinetic behavior have an im-
portant bearing on both the qualitative
and quantitative assessment of carcino-
genicity. This document presents an
evaluation of the animal bioassays and
relevant toxicity studies, the human
epidemiologic evidence, the quantitative
aspects of assessment, and finally, a
summary and conclusions dealing with
all of the relevant aspects of the car-
cinogenicity of unleaded gasoline.
Summary and Conclusions
Animal Studies
A lifetime inhalation bioassay of un-
leaded gasoline in rats and mice has
induced a statistically significant in-
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creased incidence (6/100) of renal car-
cinomas in the kidney cortex of male rats
and a larger, also statistically significant,
increase in the incidence (20/100) of
hepatocellular carcinomas in female mice.
Female rats and male mice had no
significant treatment-related increase in
tumors at any organ site. The increase of
renal carcinomas in male rats was
statistically significant at the highest dose
tested (2,056 ppm) but not at the two
lower doses (292 ppm and 67 ppm).
However, the combined incidence of
adenoma/carcinoma/sarcoma was also
significantly increased at the intermediate
dose. In mice, the incidence of liver car-
cinomas alone and adenoma and car-
cinoma combined was significantly
increased in the highest but not the two
lower dose groups. Moderate decrements
in the body weight gain in the high-dose
groups indicate that the maximum tol-
erated dose was reached. Glomerulo-
nephrosis occurred in nearly all of the
male rats, and mineralization of the pelvis
was correlated with dose. However, there
was no correlation between animals with
tumors and those with mineralization.
The same pattern of glomerulonehpritis,
as well as positive tumor responses,
occured with chronic inhalation exposure
to synthetic fuels (RJ-5 and JP-10).
Chronic inhalation studies with jet fuels
used by the Air Force and Navy (JP-4 and
JP-5) have resulted in the same nephro-
toxic lesions, but no information is avail-
able about the carcinogenic response.
In a series of exposures of male rats to
a variety of distillate fractions and to
individual components of gasoline, toxicity
was correlated with the paraffin com-
pounds present in the 145° to 280°F
distillate fractions and not with aromatic
compounds in the mixture. The most toxic
compounds were branched-chain alipha-
tics, generally in the C6-C9 range, al-
though some larger molecules such as
2,2,4,4-tetramethyl octane also showed
a high level of activity. The acute and
subchronic renal toxicity of decalin, a
volatile hydrocarbon of the same general
type as those found in gasoline, is con-
fined to male rats and did not occur in
female rats or in mice, dogs, or guinea
pigs.
The renal toxicity pattern observed with
exposure to hydrocarbon mixtures in-
volving protein accumulation in renal
tubules is clearly different than the kidney
lesions occurring spontaneously in old
rats, and occurs in males of both Fischer
344 and Sprague-Dawley strains, but not
in females of these strains or in mice or
monkeys. Mutagenesis tests of unleaded
gasoline have been carried out in
Salmonella, yeast, mouse lymphoma in
vivo cytogenetics, in mouse dominant
lethal systems, and in a rat kidney cell
DNA repair model. Various gasoline
feedstocks have been tested in mouse
lymphoma and in vivo cytogenetics
assays. The results of most of these
assays have not met the criteria for posi-
tive responses.
Epldemlologlc Studies
Fifty-five studies were reviewed to
determine if there is any epidemiologic
evidence for an association between
gasoline exposure and cancer risk. Since
unleaded gasoline was only introduced
in the mid-1970's, even recent epidemio-
logic studies are not likely to show an
unleaded gasoline effect because of the
long latency period generally associated
with cancer. Therefore, this review was
not limited to unleaded gasoline exposure,
but addressed any potential gasoline
exposure.
None of the studies reviewed provided
qualitative as well as quantitative esti-
mates of gasoline exposure.
Seven studies were identified that
evaluated the association between em-
ployment in the gasoline service industry
and cancer risks; the industy here in-
cludes gasoline service station owners
and attendants, garage workers, gasoline
and fuel truck drivers, and those who
reported working with gasoline. One study
cited in the literature provided some
evidence of an association between
gasoline service station employment and
risk of primary liver cancer. The remain-
ing six studies were judged inadequate.
Twenty-five studies were reviewed that
evaluated the association between em-
ployment in a petroleum refinery (a work
environment with potential gasoline ex-
posure) and cancer risk. Judged in-
dividually, these studies provided inade-
quate evidence of an association. How-
ever, judged collectively these studies
provide suggestive evidence of an associ-
ation between employment in a petroleum
refinery and risk of stomach cancer,
respiratory system cancer (i.e., lung,
pleura, nasal cavity, and sinuses), and
cancer of the lymphatic and hematopoietic
tissues.
Nineteen case-control studies were
reviewed which evaluated employment
in the petroleum industry as a cancer risk
factor. Another study cited in the literature
provided limited evidence of an associ-
ation between petroleum industry em-
ployment and risk of bladder cancer.
Also reviewed were four protocols of
epidemiologic studies in progress. These
studies may provide evidence of an as-
sociation between gasoline exposure and
cancer risk; however, these findings are
3 to 5 years in the future.
Quantitative — Data from the API study
on kidney tumors in male rats and liver
adenomas and carcinomas in female mice
were used to derive an estimate of the
incremental upper-limit unit risk due to
continuous human exposure to 1 ppm of
unleaded gasoline. Since the animals
breathed an aerosol of whole gasoline
under laboratory conditions, whereas
humans are expected to breathe only the
more volatile components of the mixture,
the estimates are uncertain. If tumor
induction is caused by the same, relatively
nonvolatile C6-C9 branched hydrocarbons
that are primarily responsible for the
nephrotoxicity in male rats, then the
quantitative estimates of the risk of
breathing gasoline vapors may be overly
conservative. The carcinogenic potency
estimate for unleaded gasoline was
derived from a continuous exposure study,
whereas the actual human exposure is
periodic in most cases. The available
information is not adequate to determine
if this will result in an overestimation or
an underestimation of risk. The estimates
from the mouse and rat data are similar:
2.1 x 10 3 (ppm)'1 from mouse data and
3.5 x 10~3 (ppm)"1 from rat data.
The presence of 2% benzene in the
unleaded gasoline mixture could theo-
retically contribute to the response, al-
though the mouse liver and rat kidney
have not been the target organs in animal
experiments with benzene. Based on
those experiments, it is estimated that
the contribution of benzene to the re-
sponse observed in the API unleaded
gasoline studies could be on the order of
20%. However, there is no qualitative
evidence that benzene actually is con-
tributing to the response.
Conclusions
On the basis of a small but definite
kidney tumor response in male rats and a
significant hepatocellular response in
female mice, using EPA's Guidelines for
Carcinogen Risk Assessment to classify
the weight of evidence for carcinogenicity
in experimental animals, there is suf-
ficient evidence to conclude that gasoline
vapors are carcinogenic in animals. The
similar pattern of response in rats to the
synthetic fuels RJ-5 and JP-10, and the
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renal toxicity observed in chronic bioas-
says with JP-4 and JP-5, support the
findings with unleaded gasoline, indicat-
ing that some agent or combination of
agents common to these mixtures is
responsible for the observed effects.
The relevance of the rat kidney response
to human carcinogenicity has been ques-
tioned on the basis of experiments
showing that early-occurring kidney toxi-
city is apparently caused by the interaction
of gasoline hydrocarbon components with
a unique protein (alpha-2-microglobulin)
produced in large quantities only by the
mafe rat and not other species. If this
toxicity were the cause of the kidney
tumor response, the case for human
carcinogenicity would be weakened.
However, given the current evidence, the
Carcinogen Assessment Group cannot
disregard the rat kidney tumor response
as an indication of potential human car-
cinogenicity for several reasons: (a) the
link between hydrocarbon nephropathy
and tumor induction is not proven; (b)
with very few exceptions, chemicals
causing cancer in humans also cause
cancer in animals, indicating a similarity
of response across the animal kingdom;
and (c) the kidney of experimental animals
is a demonstrated target organ for more
than 100 carcinogenic chemicals.
The EPA Science Advisory Board and
the Health Effects Institute have in-
dependently reviewed the earlier draft of
this report. Both groups agreed that the
evidence for carcinogenicity in animals
meets the EPA Guidelines criteria for
sufficient evidence in animals and in-
adequate evidence in humans. They both
pointed out the uncertain relevance of rat
kidney tumors as an indication of human
response and the difficulty in making
quantitative estimates of gasoline vapor
potency from the animal study of whole
gasoline when the identity of the car-
cinogenic component is unknown.
The epidemiologic studies collectively
provide limited evidence that occupational
exposure in the petroleum industry is
associated with certain types of cancer.
However, the evidence for evaluating
gasoline as a potential carcinogen is
considered inadequate under the EPA
Guidelines criteria for epidemiologic
evidence.
Based on sufficient evidence in animal
studies and inadequate evidence in
epidemiologic studies, the overall weight
of evidence for unleaded gasoline is EPA
category B2, meaning that unleaded
gasoline is a probable human carcinogen.
The carcinogenic potency of unleaded
gasoline, using data from the most sensi-
tive species tested, is 3.5 x 10"3 per ppm.
This is a plausible upper bound for the
increased cancer risk from unleaded
gasoline, meaning that the true risk is
not likely to exceed this estimate and
may be lower.
This Project Summary was prepared by staff of the Office of Health and
Environmental Assessment, Washington, DC 20460.
William £. Pepelko is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of the Carcinogenicity of Unleaded
Gasoline," (Order No. PB 87-186 151 /AS; Cost: $36.95, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Office of Health and Environmental Assessment
U.S. Environmental Protection Agency
Washington, DC 20460
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