EPA-420-S-88-100
88-128.1
Air Toxics Emissions From Motor Vehicles
Penny M. Carey and Joseph H. Somers
U.S. Environmental Protection Agency
2565 Plymouth Road
Ann Arbor, Michigan 48105
Paper presented at the June 1988 Air Pollution Control Association
meeting in Dallas, Texas
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INTRODUCTION
Considerable effort is underway within the Environmental
Protection Agency (EPA) to determine the magnitude of the air
toxics problem in the United States. This paper focuses on air
toxics emissions from motor vehicles, specifical ly, all air
carcinogens for which EPA has unit risk estimates and are
emitted from motor vehicles, specific pollutants and pollutant
categories which are discussed include diesel particulate,
formaldehyde, benzene, gasoline vapors, gas phase organics,
organics associated with gasoline particulate, dioxins,
asbestos, vehicle interior emissions, and metals. This paper
summarizes information contained in an EPA technical report.1
For each pollutant, information is provided regarding
emissions, ambient concentrations, and health effects. Where
adequate information was available, quantitative estimates of
cancer incidence were made for calendar years 1986 and 1995.
The risk estimates are 95 percent upper confidence limits.
Following this ls a summary of the aggregate risk from
mobile source air toxics emissions, together with the
limitations inherent in the estimate.
DIESEL PARTICULATE
Composition and Emissions
Diesel particulate is composed of an elemental carbon core
with hundreds of condensed and/or adsorbed fuel and lubricant
components and other varied combustion products. Over 90
percent of diesel particulate is less than 1 micron in size and
is therefore small enough to be inhaled and deposited deep
within the lungs.
The light-duty vehicle and truck diesel particulate
emissions standard, prior to 1987, was 0.6 gram/mile.
Effective in 1987, the standards are 0.20 gram/mile Cor
light-duty vehicles and 0.26 gram/mile for light-duty trucks.
For heavy-duty diesel engines, a standard of 0.6 gram/brake
horsepower-hour (g/bhp-hr) begins in 1988, with increasingly
more stringent standards effective in 1991 (0.25 g/bhp-hr) and
1994 (0.10 g/bhp-hr). For urban bus engines, the standard is
0.10 g/bhp~hr beginning in 1991. These increasingly stringent
standards are accounted for in the 1995 projections.
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The diesel particulate standards are not based on
potential carcinogenic risk, but rather on adverse health
effects associated with particulate matter in general, as
described in the Federal Register notice describing EPA's
revisions to the National Ambient Air Quality Standards for
particulate matter.2
Emission factors (in grams/mile) for each vehicle type and
a composite emission factor for the fleet were estimated for
calendar years 1986 and 1995. Model year data for the previous
20 years are used, i.e., Eor 1986, model year data back to 1967
are used; for 1995, model year data back to 1976 are used.
Each model year's emission factor is multiplied by that model
year's fraction of calendar year vehicle miles travelled (vmt)
and the diesel sales Eraction for that model year, and then
summed across all 20 model years to obtain calendar year
emission factors. The heavy-duty classes 7-8 emit far more
particulate on a g/mile basis than any of the other vehicle
classes. Please refer to reference 1 for a detailed discussion
of these model year inputs.
The resulting fleet emission factors for calendar years
1986 and 1995 are 0.1669 g/mile and 0.0650-0.0797 g/mile,
respectively. The range in 1995 is due to a range of diesel
sales assumed in future years. Nationwide diesel particulate
emissions (metric tons/year) for 1986 and 1995 were then
calculated by combining the calendar year emission factors with
estimated VMT for 1986 and 1995. Estimated VMT for 1986 and
1995 is 1626.84 x 109 miles and 1934.33 x 10* miles,
respectively.3 VMT is projected to increase 19 percent from
1986 to 1995.
Nationwide diesel particulate emissions in 1986 were
estimated to be 273,572-273,643 metric tons, or roughly 3.9
percent of the 1984 total suspended particulate (TSP)
emissions. Diesel particulate emissions are projected to drop
to 125,352-153,944 metric tons/year in 1995. This decrease is
due to the more stringent diesel particulate standards.
Urban diesel particulate emissions are projected to be
roughly 35,700 metric tons in 1995. Urban emissions do not
comprise the bulk of nationwide emissions because the VMT
fraction of the highest emitting heavy-duty trucks is small in
urban areas. In comparison, the Motor Vehicle Manufacturers
Association (MVMA) predicts that 30,500 metric tons of diesel
particulate will be emitted in U.S. urban areas in 1995."
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88-128.1
Ambient Concentrations of Diesel Particulate
Urban and rural concentrations of diesel particulate are
estimated for 1986 and 1995, using a modified version of the
EPA NAAQS Exposure Model (NEM) for CO.5 This modified NEM
model was also used to calculate urban and rural concentrations
for many of the mobile source pollutants discussed in this
paper.
The modified NEM model provides an estimate of nationwide
annual person-hours of exposure to any non-reactive mobile
source pollutant of interest. Using this information, mean
exposure levels may be calculated. The CO NEM was used since
outdoor CO is almost exclusively mobile source related. Since
the CO monitor data, on which the CO NEM was based, can be
assumed to be related to mobile source emission rates, exposure
to other non-reactive mobile source pollutants can be modeled
using this relationship.
The modified NEK includes more recent 1991 CO monitoring
data, including an expanded number of monitors, relative to the
original NEM, Emission factors in gcams/rainute for the
pollutant of interest are input ta the modified iJEM. Exposures
irj three jr?obile source rricroenvironments <£treet canyons,
tunnels and parking garages), where elevated concentrations of
mobile source pollutants could be experienced, vere included in
the modified NEM. Finally, a national extrapolation procedure
designed expressly for mobile sources was devised.
The exposure levels predicted by the model are those
resulting from direct exhaust emissions, and do not account for
either the destruction or photochemical formation of the
pollutant in the atmosphere. The model also assumes that the
pollutant of interest has emission formation and dispersion
characteristics si/nilar to that of CO, For diesel particulate,
CO appears to be a particularly good surrogate.
The mean urban, rural, and nationwide diesel particulate
exposure levels predicted by the model for 1986 are 2.63, 2.38,
and 2.56 ug/m3, respectively. For 1995, the mean urban,
rural, and nationwide exposure levels are projected to drop to
1.27-1.69, 1.06-1.27, and 1.22-1.58 ug/m1, respectively.
Health Effects of Diesel Particulate
Diesel particulate was found to be mutagenic in the late
1970's. Subseguent studies revealed that nitropolynuclear
aromatic hydrocarbons (nitro~PAH), specifically, nitropyrenes,
dinitropyrenes and nitrohydroxypyrenes together account for
much of the mutagenicity observed. The organics extracted from
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diesel particulate and other known carcinogens such as coke
oven emissions were tested in a battery of bioassays. These
included bacteria and mammalian cell bioassays, arid one skin
painting study with SENCAR mice. Animal inhalation studies
were conducted at that time but gave negative or inconclusive
results. The unit risk for diesel particulate was determined
by comparing the potency of diesel particulate with the
potencies of the other carcinogens determined in these tests.
The range of upper confidence limit unit risks used in this
paper, 0.2-1.0x10' *, is based on various analyses of the
comparative potency data.6,7,8 It should be noted that the
comparative potency method is a novel approach to risk
assessment, and represents a departure from conventional risk
assessment methodologies used by EPA. The method involves a
series of assumptions which have been acknowledged by EPA and
criticized by others as introducing uncertainties much larger
than those commonly associated with the conduct of risk
assessment.6
Several animal inhalation experiments have been recently
completed which, in contrast to the earlier studies,
demonstrate that diesel exhaust, when inhaled chronically at
high concentrations, is a pulmonary carcinogen in the rat.
These studies are currently b^ing evaluated.
The unit risks used in this paper are defined as the
individual life time excess cancer risk from continuous
exposure to 1 ug carcinogen per mJ inhaled air. Assuming a
life time is 70 years, the excess lung cancer risk in 1 year is
derived by simply dividing the unit risk by 70. Using this
approach, latency is ignored. The unit risks used in this
paper are 95 percent upper confidence limits rather than best
estimates. This is consistent with current EPA practice. The
risk estimates presented should therefore be considered upper
bound estimates.
Current and Projected Cancer Risk
The unit risks are combined with the exposure estimates
and population estimates to obtain estimates of cancer
incidence for 1986 and 1995. Urban and rural populations for
1986 and 1995 were estimated based on U.S. Department of
Commerce data.'0 The annual cancer risk from diesel
particulate exposure for the U.S. population is 178-860 in 1986
and drops to 92-443 in 1995. The range is due to the range of
unit risk estimates which were used and, to a lesser extent, a
range of assumptions regarding future dieseL sales. The
roughly 40 percent decrease in risk from 1986 to 1995 can be
attributed to the more stringent future diesel particulate
standards.
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FORMALDEHYDE
Composition and Emissions
Formaldehyde is the most prevalent aldehyde in vehicle
exhaust arid is formed as a result of incomplete combustion of
the £uel. Formaldehyde is emitted in the exhaust of both
gasoline- and diesel-fueled vehicles. It has the chemical
formula CKzO. Formaldehyde is of interest due to its
photochemical reactivity in ozone formation and suspected
ca rcinogenici ty.
Formaldehyde exhaust emissions from motor vehicles
correlate well with exhaust hydrocarbon (HC) emissions. For
this analysis, formaldehyde emissions were expressed as a
weight percencage of exhaust HC. These percentages were then
applied to the exhaust HC output from the MOBIIiE3 emissions
model for 1996 and 1995 to obtain the FTP formaldehyde emission
factors. In this way, deterioration and other effects are
included. The percentages generally vary from 1 to 4 percent,
depending on the vehicle class. Resulting composite FTP
formaldehyde emission factors are 0.0418-0.0453 g/mile in 1986
and 0.0201-0.0224 g/mile in 1995. The range accounts foe both
the presence and absence of an Inspection/Maintenance (I/M)
program. Characteristics of the I/M program selected represent
the minimum EPA requires of I/M programs.
Mobile source formaldehyde emissions in 1986 were
estimated to be 68,003-73,697 metric tons, or roughly 28
percent o£ the total formaldehyde emissions in the U.S. Mobile
source formaldehyde emissions are expected to drop to
38,880-43,329 metric tons in 1995. This is due to the
increasing use of 3-way and 3-way plus oxidation
catalyst-equipped gasoline-fueled vehicles together with the
phase out o£ non-catalyst-equipped vehicles. The result is a
marked decrease in projected HC and, by association,
formaldehyde emissions.
Ambient Concentrations of Formaldehyde
The mean urban, rura2, and nationwide formaldehyde
exposure levels predicted by the modified NEM model for 1986
are 1.21-1.30, 0.56-0.60, and 1.04-1.13 ug/m3, respectively.
For 1995, the :rean. urban, rural, and nationwide exposure levels
are projected to drop to 0.68-0.76, 0.31-0.35, and 0.59-0.65
ug/m3, respectively. The exposure levels predicted by the
model are those resulting from direct exhaust emissions only
and do not account for either the destruction or photochemical
formation of formaldehyde in the atmosphere. Since the
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majority of formaldehyde in the atmosphere is thought to be
formed from photochemical reactions of volatile organic
compounds (VOC), the exposure levels predicted by the model are
likely underestimates.
Another approach intended to include photochemistry was
also developed. with this approach, a mobile source fraction
was applied to an annual average formaldehyde concentration
developed using available ambient monitoring data. Mobile
sources account for 29 percent of the total VOC emissions and
roughly 30 percent of the formaldehyde emitted directly.
Assuming that the VOC from all sources have the equivalent
potential to form formaldehyde, a mobile source fraction of
0.30 was selected. This fraction was applied to an urban
population weighted average of 12.71 ug/mJ (based on data
obtained in 4 cities) and a rural concentration of 1.50
ug/ro3.11 Since the summer concentrations used to calculate
the urban concentration probably represent maximum rather than
average values, the risk estimates can be used to represent a
plausible upper limit. Resulting urban and rural formaldehyde
concentrations due to mobile sources currently are 3.81 and
0.45 ug/mJ, respectively.
Health Effects of Formaldehyde
Formaldehyde can cause a number of acute adverse health
effects such as eye, nose, throat and skin irritation,
headaches and nausea, as well as death. EPA has classified
formaldehyde as a probable carcinogen in humans. An upper
confidence limit unit risk of 1.3xlQ~s was used,'2 It is
based on a single study in which rats exposed to formaldehyde
developed malignant and benign tumors in the nasal cavities;
the unit risk is based on malignant tumor formation only.13
The consideration of benign tumors would increase the
formaldehyde risk by a factor o£ 15.
Current and Projected Cancer Risk
Using the nationwide exposure levels estimated from the
modified NEM model (which do not account for photochemistry),
the resulting risk is 46-50 in 1986 and 29-31 in 1995.
Accounting for photochemistry, using the approach outlined
above, the annual cancer risk from mobile source formaldehyde
is 131 in 1986 and 77 in 1995. Combining both approaches, the
cancer incidences due to mobile sources range from 46-131 in
1986 and 29-77 in 1995.
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BENZENE
Composition and Emissions
Benzene is an aromatic hydrocarbon with the formula
C&H*. It is present in both exhaust and evaporative
emissions. Very little exhaust benzene is unburned fuel
benzene. Same work indicates that non-benzene aromatics in the
fueLs cause about 70-80% of the exhaust benzene formed.
Benzene also forms frcm engine combustion of non-aromatic fuel
hydroca rbons.
For this analysis, benzene emissions were expressed as a
weight percentage of exhaust and evaporative HC. These
percentages were then applied to the HC output from the MOBILE3
emissions model for 1986 and 1995 to obtain the FTP benzene
emission factors. A standard, minimum I/M program was
assumed. The fraction of benzene in the exhaust varies
depending on contrai technology and fuel composition but is
generally about 3-5%. The fraction of benzene in the
evaporative emissions also depends on control technology (e.g.,
whether the vehicle has fuel injection or a carburetor) and
fuel composition (e.g., benzene level and RVP) and is generally
about IV
Resulting composite FTP benzene emission factors for 1986
and 1995 are 0.128-0.135 g/mile and 0.055-0.057 g/mile,
respectively. The range is due to consideration of both a low
and high range evaporative emissions estimate for light-duty
gasoline-fueled vehicles. Diesel vehicles account for only
about 2% of the total mobile source benzene emitted. Based on
an analysis conducted by EPA's Office of Mobile Sources, RVP
control, which would be accompanied by a small increase in both
benzene content and total aromatic content of gasoline, would
have little or no effect on overall fleet emissions or on the
number of cancer cases.'"
Mobile sources dominate the nationwide benzene emission
inventory. In 1982, mobile source benzene emissions were
roughly 250,000 metric tons, or 85 percent of the total benzene
emissions- Of the mobile source contribution, 70 percent comes
from exhaust, 14 percent from evaporative emissions and 1
percent from motor vehicle refueling.14
Ambient Concentrations of Benzene
Nationwide exposure levels from both exhaust and
evaporative emissions, estimated using the modified NEM model,
are roughly 3.1-3.2 ug/m1 and 1.7-1.8 ug/m1 for 1986 and
1995, respectively. The reason for the marked decrease is the
decrease in projected HC in 1995 and, thus, benzene emissions.
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An alternative approach, similar to that used Cor
formaldehyde, was also developed. With this approach, a mobile
source fraction was applied to estimated urban and rural
concentrations developed using available ambient monitoring
data. LMobile sources account for 35 percent of the total
benzene emissions. Therefore, a fraction of 0.85 was applied
to an urban population weighted average of 10.24 ug/m1 and a
rural concentration of 7.52 ug/m1.11'15 Resulting
estimated urban and rural benzene concentrations due to mobile
sources currently are 8.70 and 6.39 ug/rnJ, respectively.
The concentrations predicted using the ambient
apportionment approach are somewhat higher than those
calculated with the NEM modeling approach. Both approaches
contain uncertainties. The six day half-life of benzene in air
likely results in a build-up of benzene in the atmosphere that
is not accounted for with the NEM modeling approach. With the
ambient apportionment approach, the ambient data may be from
fixed site monitors recording peak. levels that could
over represent average 24-hour exposures of the population. It
is also not certain whether the cities chosen are
representative of the entire urban population.
Health Effects of Benzene
Several epidemiology studies on workers exposed to benzene
have identified benzene as a carcinogen causing leukemia in
humans. The unit risk estimate determined from these studies
and used by EPA is 8.2 x 10"14,16 EPA is presently
evaluating more recent data.
Current and Projected Health Risk
Using the nationwide exposure levels estimated from the
modified NEM model, annual cancer incidences from exhaust and
evaporative emissions are estimated to be 84-89 in 1986 and
50-52 in 1995. Refueling emissions were also considered.
Exposure to benzene during refueling includes self-service
refueling, occupational exposure
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Using the alternate approach outlined above, the annual
cancer risk from mobile source benzene is 223 in 1986.
Emissions of benzene from mobile sources are projected to
decrease roughly 40 percent from 1986 to 1995. Accounting for
this decrease and the projected population increase, the annual
lung cancer risk is 145 in 1995. Combining both approaches,
the lung cancer incidences due to mobile sources range from
92-223 in 1985 and 57-145 in 1995.
GASOLINE VAPORS
Totally vaporized gasoline has been found to cause a
statistically significant increase in kidney tumors in male
rats and liver tumors in female mice.17 EPA has classified
gasoline vapors as a 32 probable human carcinogen. An upper
confidence limit based on the rat data is 1.18 x
lO"1."'"1 The significance of the rat kidney tumors has
been questioned since the male rat may be unique in its
response to gasoline vapors; however, EPA is retaining the B2
classification.11 As more data become available, the
classification may be reconsidered.
Exposure to gasoline vapors during refueling was estimated
based on an American Petroleum Institute (API) study that
involved measuring gasoline and vapor levels in the region of
the face of a person refueling a vehicle tank. The
exposure in a typical urban area for these refueling emissions
was also estimated using the Industrial Source Complex
dispersion model to calculate annual concentrations. Based on
these exposures, the risk from gasoline vapors (excluding
benzene) was estimated as 65 lung cancer incidences per year.
GAS PHASE ORGANICS
Gas phase organics, or volatile organic compounds (VOC),
are present in both exhaust and evaporative emissions. Over
300 VOC have been identified. The majority of VOC consist of
unsaturated and saturated hydrocarbons along with benzene,
aLkyl benzenes, aliphatic aldehydes and a variety of polycyclic
aromatic hydrocarbons.
Of all the VOC emitted from motor vehicles, only benzene,
formaldehyde, benzo(a)pyrene (B(a)P), ethylene, and
1,3-butadiene have unit risks. Gas phase B(a)P was considered
with particle-associated B(a)P since the majority of B(a)P is
in the particulate phase. Therefore, this section will focus
on ethylene and 1,3-butadiene.
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Ethylene
Composition and Emissions. Ethylene emissions are present in
vehicle exhaust. Ethylene in evaporative emissions is
negligible and was not considered. Ethylene emissions were
expressed as a weight percentage of exhaust HC. These
percentages were then applied to the exhaust HC output from the
MQBILE3 emissions model for 1986 and 1995 to obtain the FTP
ethylene emission factors. The percentages generally vary from
6 to 13 percent, depending on the vehicle class. Resulting
composite FTP ethylene emission factors, with and without I/M,
are 0.2367-0.2607 g/mile in 1986 and 0.0946-0.1092 g/mile in
1995.
Ambient Concentrations of Ethylene. The mean urban, rural,
and nationwide formaldehyde exposure levels predicted by the
modified NEM model for 1986 are 6.77-7.49, 3.46, and 5.94-6.48
ug/m3, respectively. For 1995, the mean urban, rural, and
nationwide exposure levels are projected to drop to 3.32-3.59,
1.60, and 2.89-3.09 ug/mJ, respectively. These exposure
estimates are rather uncertain, since ethylene is
photochemically reactive.
Health Effects of Ethylene. The upper confidence limit unit
risk of 2.7 x 10"4 for ethylene was provided in an EPA study
although it was not developed by EPA.20 The unit risk is
extremely tentative, since there is no available direct
evidence that ethylene is carcinogenic. The unit risk was
estimated based on assumptions regarding its potency relative
to ethylene oxide, a metabolite of ethylene and an animal
carcinogen.
Current and Projected Cancer Risk. The annual cancer risk
from ethylene exposure for the U.S. population is 55-60 in 1986
and 29-31 in 1995. Since the unit risk is so tentative, a
lower bound cisk o£ zero is also used. The resulting ranges of
cancer risk are 0-60 in 1986 and 0-31 in 1995.
1,3-Butadiene
Composition and Emissions. 1,3-Butadiene is a photochemically
reactive compound present in vehicle exhaust. The report upon
which this paper is based assumed 1,3-butadiene was roughly
0.94 percent of the total FID exhaust HC. This was estimated
based on very limited data in which 1,3-butadiene and n-butane
were reported together. Assumptions had to be made about the
percentage attributable to 1,3-butadiene. Recently,
1,3-butadiene data for light-duty three-way catalyst-equipped
vehicles has become available. Based on a preliminary review
of the available data, 1,3-butadiene constitutes roughly 0.35
percent of the total FID exhaust HC.21 Due to lack of data
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foe the cither vehicle classes, this percentage was simply
applied to the MOBCLE3 composite exhaust KC emission factor.
Resulting composite 1,3-butadiene emission factors with and
without I/M for 1986 and 1995 are 0.0089-0.0098 g/mile and
0. 0O45-Q.0053 g/mile, respectively.
Ambient Concentrations of i,3-Butadiene. The modified NEM
model was used to estimate exposure. Urban and rural exposure
from mobile sources in 1986 is estimated to be 0.26-G.28
ug/m1 and 0.12-0.13 ug/m3, respectively. In 1995, urban
and rural exposure from mobile sources is estimated to be
0.16-0.18 ug/m* and 0.07-0.09 ug/cnJ, respectively.
Since the NEM-pcedicted exposure estimates are rather
uncertain, due to the photochemical reactivity of
1,3-butadiene, available monitoring data foe 1986 were reviewed
and compared to the exposure estimates. Average 6-9 a.m.
summer values for 18 cities range from 0.24-1.98 ug/m3.21
The 1986 NEM estimate oE urban exposure from motor vehicles
lies within this range.
Health Effects of 1,3-Butadiene. Exposure to 1,3-butadiene
results in a wide spectrum of cancers in mice and rats, EPA
has classified 1,3~butadiene as a probable B2 human
carcinogen. The 95 percent upper confidence limit unit risk
for 1,3-butadiene is 2.8 x 10~4, based on a mouse inhalation
study.21 The use of this study for risk estimation has been
questioned because of the high dose levels used and the
possibility that the animals were infected with a virus known
to cause one of the most prevalent tumor types observed in the
exposed groups,14
Current and Projected Health Risk. The risk in 1986 is 221-244
cancer incidences and drops to 146-171 cancer incidences in
1995.
ORGANICS ASSOCIATED WITH GASOLINE PARTICULATE
Gaso.line-f ueled vehicles emit far less particulate than
their diesel counterparts. Data indicate that the classical
polycyclic aromatic hydrocarbons (PAHs) may be responsible for
the mutagenicity of these organics rather than the nitro-PAH's.
Three approaches were used to estimate the risk. The
first approach assumes the risk from B(a)P emissions adequately
represents the risk of all gasoline particle-associated
organics. B(a)P emission data and the B(a)P unit risk were
used. Using this approach, the resulting annual cancer risk is
1.3 in 1985, and is projected to drop to 0.78 in 1995.
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The second approach uses estimated emission rates of
gasoline particle-associated organics (as an unspeciated
mixture) together with an upper confidence limit unit risk for
these organics. Exposures were estimated using the modified
NEM model. Estimated composite emission factors, with and
without I/M, are 0.0075-0.0082 g/mile in 1986 and 0.0048-0.0058
g/mile in 1995.
An upper confidence limit unit risk based on one
catalyst-equipped vehicle (1978 Ford Mustang) is 2.5 x
10"".b The bioassays used to estimate the unit risk were
the same as those used to estimate the unit risk for diesel
particulate, and the same approach was used. The vehicle had
exceptionally high exhaust emissions, comparable to those from
a non-catalyst-eguipped vehicle. The mutagenic activity of the
particle-associated organics from this vehicle, however, as
indicated by the Ames Salmonella bioassay (strain TA-98), is on
the low end of the range when compared with other
catalyst-equipped vehicles. As a result, the vehicle and the
unit risk should be considered of uncertain
representativeness. Using this approach, the risk in 1986 is
163-176 cancer incidences and drops to 115-136 cancer
incidences in 1995.
The third approach uses B(a)P emission factors for
gasoline-fueled vehicles together with the products of
incomplete combustion (PIC) unit risk {which is expressed per
unit of exposure of B(a)P) to calculate cancer incidence.20
The resulting cancer risk is 122 in 1986 and drops to 72 cancer
incidences in 1995,
For this analysis, a range of risk estimates was chosen,
which encompasses the results of all three approaches. The
resulting range of cancer incidences is 1.3-176 in 1986 and
0.78-136 in 1995.
ASBESTOS
Asbestos is used in brake linings, clutch facings, and
automatic transmissions. Asbestos emissions from vehicles with
front disc brakes and rear drum brakes ranged from 4-28
ug/rnile. Based on these emission rates, maximum annual average
asbestos levels in urban areas due to motor vehicles are
estimated to range from 0.25-1.75 nanograms per cubic meter
(ng/m1). The individual upper bound cancer risk from urban
levels of asbestos is estimated to range from 9 x 10to
3.6 x 10"7 pec ng/mJ exposure.25 Assuming an urban
population of 180 milLion, the resulting cancer risk is
estimated to range from 0.405-113.4 cancer incidences per year.
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OTHER AIR TOXICS EMISSIONS"
Other mobile source air toxics emissions examined include
dioxins, vehicle interior emissions, ethylene dibromide (EDB),
and cadmium. These pollutants appear to be present in only
trace quantities and no significant risk is apparent.
SUMMARY
The risks presented in this s-udy are summarized in Table
I. The aggregate risk in 1985 for the total U.S. population is
estimated to range from 529 to 1874 cancer incidences and drops
roughly 40 percent by 1995. Reasons for the projected decrease
in risk in 1995 include: 1) more stringent diesel particulate
standards for both light- and heavy-duty vehicles, and 2) the
increasing use of 3-way catalyst-equipped vehicles coupled with
the phase out of non-catalyst-equipped vehicles.
As seen in Table I, there is a wide range of risk
estimates associated with each pollutant. For diesel
particulate, the range is due to the range of potency (or unit
risk) estimates which were used and, for 1995, a range of
assumptions regarding future diesel sales. For formaldehyde,
the low end of the range attempts to account only for
formaldehyde directly emitted from the exhaust of motor
vehicles. The high end of the range attempts to account for
both formaldehyde directly emitted and formaldehyde formed in
the atmosphere from other mobile source volatile organic
compound (VOCJ emissions.
For benzene, the lower limit is based on ambient
concentrations predicted by a model, whereas the upper limit is
based on actual monitoring data, with a mobile source fraction
assigned based on the mobile source emissions contribution.
For ethylene, the uncertainty is based on the unit risk
estimate. The range for gasoline particle-associated organics
and asbestos is due to a number of different assumptions
regarding both emission factors and unit risk estimates.
Mobile source emissions are extremely complex. Hundreds
of compounds, both in the gas phase and associated with
particles are present. The lack of emissions data and/or
health data and/or exposure data prevented quantitative risk
estimates for any additional pollutants. Of particular concern
are pollutants which are formed photochemica1ly from mobile
source emissions. This category of pollutants could have
considerable impact but not enough is known to make a
quantitative estimate.
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Table I. Summary of risk estimates.3
U.S. Cancer Incidences/Year6
Motor Vehicle Pollutant 193 6 1995c
Diesel Particulate
178-
860
92-576
Formaldehyde
46-
131
29- 77
Benzene
92-
223
57-145
Gasoline Vapors
65
NDd
Other Gas Phase Organics
1,3-Butadiene
244
171
Ethylene
0-
60
0-31
Gasoline Particulate
1.3 -
176
0.78-136
Dioxins
ND
ND
Asbestos
0.41-
113 . 4
ND
Vehicle Interior Emissions
ND
ND
Cadmium
0. 18
0
Ethylene Dibromide
1.8
0.54
Total: 629-1874 350-1137
4 The risk estimates are 95% upper confidence limits.
" The risk estimates for gasoline vapors, asbestos, cadmium
and ethylene dibromide are for urban exposure only. Risks
for the other pollutants include both urban and rural
exposure.
c The total risk in 1995 is slightly underestimated. Due to
inadequate information and the sensitivity of 1995 risk to
control decisions which have not yet been made, projected
risk estimates were not made for some of the pollutants.
d ND=Not Determined.
NOTE: The risk estimates are upper bound estimates;
therefore, they are not intended to represent actual
numbers of cancer cases but rather can be used to
rank the mobile source pollutants and to guide
further study.
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88-128.1
The risk estimates are upper bound estimates; therefore,
they are not intended to represent actual numbers of cancers
but rather can be used to rank the mobile source pollutants and
to guide further study. Currently, 1,3-butadiene emissions
data are being collected and additional research on gasoline
particle-associated orgamcs is anticipated, EPA is also
investigating factors leading to formaldehyde formation in the
atmosphere. EPA's Integrated Air Cancer Project is a long-term
research project currently underway, the goal being to identify
the principal airborne carcinogens and their sources.
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88-128.1
REFERENCES
1. P. M. Carey, "Air toxics emissions from motor vehicles,"
EPA technical report EPA-AA-TSS-PA-8 6-5, 1987.
2. "Revisions to the National Ambient Air Quality Standards
Cor particulate matter," Federal Register, Vol. 52, No.
126, Wednesday, July 1, 1987, 24634.
3. Draft MOBILES fuel consumption model, 1985.
4. "Analysis of the Environmental Protection Agency's diesel
particulate study and a diesel particulate emissions
projection," prepared by the Motor Vehicle Manufacturers
Association of the United States, Inc. and the Engine
Manufacturers Association, September 22, 1986.
5. M. N. Ingalls, Southwest Research Institute, "Improved
mobile source exposure estimation," EPA contractor report
EPA-460/3-85-002, 1985.
6. R. E. Albert, J. Lewtas, S, Nesnow, T. W. Thorslund, and
E. Anderson, "Comparative potency method for cancer risk
assessment." application to diesel particulate emissions,"
Risk Anal. 3 (2): 101 (1983).
7. J. E. Harris, "Potential risk of lung cancer from diesel
engine emissions," report to the Diesel Impacts Study
Committee, National Research Council, National Academy
Press, Washington, D.C., 1981.
8. R. G. Cuddihy, W. C. Griffith, C. R. Clark and R. O.
McClellan, "Potential health and environmental effects of
light-duty diesel vehicles II," Inhalation Toxicology
Research Institute, Lovelace Biomedical and Environmental
Research Institute, prepared for U.S. Department of
Energy, LMF-89, UC-46, 1981.
9. "Detailed analysis of procedures used by EPA in their risk
assessment of diesel particulate emissions," prepared by
Environ Corporation for Motor Vehicle Manufacturers
Association and Engine Manufacturers Association, June
1986.
10. "Statistical abstract of the United States," 105th
Edition, U.S. Department of Commerce, Bureau of the
Census, 1985.
11. W. F. Hunt, Jr., R. B. Faoro, T. C. Curran and J. Muntz,
EPA, Office of Air Quality Planning and Standards,
"Estimated cancer incidence rates for selected toxic air
pollutants using ambient air pollution data," 1985.
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88-128.1
12. "Assessment of health risks to garment workers and certain
home residents from exposure to formaldehyde," EPA, Office
of Pesticides and Toxics Substances, Draft final report,
1987.
13. W. D. Kerns, K. L. Pavkov, D. J, Donofrio, E. J. Gralla and
J. A. Swenberg, "Carcmogenicty of formaldehyde in rats and
mice after long-term inhalation exposure," Cancer Res. 43:
4382 (1983).
14. EPA memo from Charles L. Gray, Jr., Director, Emission
Control Technology Division to Richard D. Wilson, Director,
Office of Mobile Sources, "Mobile source benzene emissions
and a preliminary estimate of their health impacts," May
15, 1986.
15. EPA memo from William F. Hunt, Jr., Chief, Data Analysis
Section, OAQPS to Stan Meiburg, Special Assistant for
Program Development, OAQPS, "Comparison of additive
individual lifetime risks for five cities," April 3, 1986.
16. "Evaluation of air pollution regulatory strategies for
gasoline marketing industry," EPA report 450/3-84-012b,
1984.
17. H. N. MacFarland, C. E. Ulrich, C. E. Holdsworth, D. N.
Kitchen, w. H. Halliwell and S. C. Blum, J. Am. Coll.
Toxicol. 3 (4) (1984).
18. "Level I options package for a gasoline marketing
decision," EPA Office of Air Quality Planning and Standards
and Office of Mobile Sources report, 1985.
19. EPA memo from William Pepelko, Toxicologist, Carcinogen
Assessment Group to Charles L. Gray, Jr., Director,
Emission Control Technology Division, "Review of MVMA
comments on gasoline vapor risk assessment," December 23,
1987.
20. E. Haemisegger, A. Jones, B. Steigerwald and V. Thomson,
"The air toxics problem in the United States: an analysis
of cancer risks for selected pollutants," EPA report,
Office of Air and Radiation, 1985.
21. 1987 characterization reports submitted to EPA by the
automotive manufacturers. Preliminary data also submitted
by Southwest Research Institute under EPA contract and EPA
ORD's Mobile Source Emissions Research Branch.
22. EPA memo from Edward J. Lillis, Chief, Air Management
Technology Branch to list of addressees, "Urban toxics,"
1987.
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23. "Mutagenicity and carcinogenicity assessment of
1,3-butadiene," final report prepared by EPA, Office of
Health and Environmental Assessment, EPA/6GQ/8-85/004F,
19 S 5 .
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