£fc U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF MOBILE SOURCES
Air Toxics from Motor Vehicles
What are Air Toxics?
Air toxics are air pollutants that cause adverse health effects. The U.S.
Environmental Protection Agency (EPA) has focused most of its air toxics
efforts to date on carcinogens, which are compounds that cause cancer.
Non-cancer health effects such as reproductive and neurological problems are
also of concern to EPA.
How dangerous are air toxics? It's hard to say. Some air toxics have been
proven to cause cancer in humans. However, most air toxics are identified
through laboratory experiments in which animals receive very high doses of
the compound being studied. People almost never breathe such high doses.
But lower exposures may still pose risks. One fact is clear: vehicles are such
an integral part of our society that virtually everyone is exposed to their
emissions.
Air Toxics from Vehicles and Their Fuels
Motor vehicles emit several pollutants that EPA classifies as known or
probable human carcinogens. Benzene, for instance, is a known human
carcinogen, while formaldehyde, acetaldehyde, 1,3-butadiene and diesel
particulate matter are probable human carcinogens. Studies are underway
to determine whether other toxic substances are present in mobile source
emissions. For example, EPA and industry are investigating whether
oxygen-containing fuel additives such as methyl tertiary butyl ether (MTBE)
cause any adverse health effects. EPA is also working with the vehicle and
fuel industries to test motor vehicle emissions for the presence of dioxin.
EPA estimates that mobile (car, truck, and bus) sources of air toxics account
for as much as half of all cancers attributed to outdoor sources of air toxics.
This estimate is not based on actual cancer cases, but on models that predict
the maximum number of cancers that could be expected from current levels of
exposure to mobile source emissions. The models consider available health
studies, air quality data, and other information about the types of vehicles
and fuels currently in use. Nonroad mobile sources (such as tractors and
snowmobiles) emit air toxics as well.
FACT SHEET OMS-2
February, 1995
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Air Toxics from Motor Vehicles
How are Air Toxics from Motor Vehicles Formed?
Some toxic compounds are present in gasoline and are emitted to the air
when gasoline evaporates or passes through the engine as unburned fuel.
Benzene, for example, is a component of gasoline. Cars emit small
quantities of benzene in unburned fuel, or as vapor when gasoline evaporates.
A significant amount of automotive benzene comes from the incomplete
combustion of compounds in gasoline such as toluene and xylene that are
chemically very similar to benzene. Like benzene itself, these compounds
occur naturally in petroleum and become more concentrated when petroleum
is refined to produce high octane gasoline.
Formaldehyde, acetaldehyde, diesel particulate matter, and 1,3-butadiene are
not present in fuel but are by-products of incomplete combustion.
Formaldehyde and acetaldehyde are also formed through a secondary process
when other mobile source pollutants undergo chemical reactions in the
atmosphere.
Reducing Air Toxics from Motor Vehicles
The emissions that come out of a vehicle depend greatly on the fuel that goes
into it. Consequently, programs to control air toxics pollution have centered
around changing fuel composition as well as around improving vehicle
technology or performance. One of the first, and most successful programs
has been the removal of lead from gasoline. The lead phaseout began in the
mid-1970s. It will be complete January 1, 1996 when lead is banned from
gasoline. The removal of lead from gasoline has essentially eliminated mo-
bile source emissions of this highly toxic substance.
More recent fuel and emission control system changes include:
• Limits on gasoline volatility
Volatility is a measure of how easily a liquid evaporates. As described
earlier, some toxics such as benzene are present in gasoline and get into
the air when gasoline evaporates. Limits on gasoline volatility have been
imposed over the last several years to control evaporative emissions of both
hydrocarbon and toxic compounds (most air toxics are hydrocarbons so
programs designed to reduce hydrocarbon emissions also reduce air toxics).
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Air Toxics from Motor Vehicles
• Reformulated gasoline
The 1990 Clean Air Act requires reformulated gasoline to be introduced in
the nation's most polluted cities beginning in 1995. From 1995-1999, these
gasolines must provide a minimum 15% reduction in air toxics emissions
over typical 1990 gasolines. This increases to a 20% minimum reduction
beginning in the year 2000. The air toxics reductions will be achieved
mainly by reducing gasoline volatility and by reducing the benzene content
of the gasoline.
• Limits on diesel sulfur
Regulations limiting the amount of sulfur in diesel fuel took effect in 1993.
Today's lower-sulfur diesel fuels are important in reducing emissions of
particulate matter and other air toxics from diesel-fueled buses and trucks.
• More stringent standards and test procedures
To date, there are no specific standards for air toxics emissions from motor
vehicles. However, the 1990 Clean Air Act does set specific emission
standards for hydrocarbons and for diesel particulate matter. Air toxics
are present in both of these pollutant categories. As vehicle manufacturers
develop technologies to comply with the hydrocarbon and/or particulate
standards (e.g., more efficient catalytic converters), air toxics are reduced
as well. Requirements under the Act for testing carbon monoxide
emissions at cold temperatures will also have an indirect but important
effect in reducing air toxics emissions in the critical first moments of
vehicle operation.
• Control of emissions in actual customer use
From a pollution perspective, what matters most is not new vehicle
emission standards but actual emissions from vehicles on the road. The
Clean Air Act establishes several programs to make sure vehicle emission
controls are functioning properly in actual use. These include
requirements for periodic emission inspections and for computerized
diagnostic systems that alert drivers and mechanics to malfunctioning
emission controls.
In summary, the many vehicle and fuel changes in the last 25 years have
greatly reduced air toxics emissions from highway vehicles. New cars today
are capable of emitting 90% less air toxics on a per-mile basis than the
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Air Toxics from Motor Vehicles
uncontrolled models of 1970; new trucks and buses are designed to emit less
than half the air toxics of their 1970 counterparts. Overall air toxics
emissions will continue to decrease through the 1990s as older vehicles leave
the fleet and as new regulatory programs take effect. However, the number
of vehicles on the road and the number of miles they travel is continuing to
grow. Without additional controls, growth in vehicle travel will offset
progress in reducing air toxics by early in the next century.
What More Can Be Done?
The 1990 Clean Air Act requires EPA to specifically regulate air toxics from
motor vehicles in the form of standards for fuels, or vehicles, or both.
Additional hydrocarbon and particulate matter controls such as tightening
tailpipe standards even further will reduce air toxics emissions somewhat.
California for example, sets tighter hydrocarbon limits for vehicles sold in
that state than the federal requirements for vehicles sold elsewhere. Other
states can choose to adopt the California standards, which provide some air
toxics benefits. Expansion of existing regulatory programs (such as more
widespread use of reformulated gasoline or wider requirements for emission
inspections) could also help reduce air toxics. Specific vehicle emission
standards for one or more toxic compounds are also an option.
Changes in gasoline and diesel fuel composition (such as reducing sulfur,
benzene or other aromatic chemical compounds) can also reduce air toxics
emissions.
A switch to alternative, non-petroleum fuels that are cleaner than today's
gasoline and diesel fuels offers another strategy for reducing air toxics.
Choices include alcohols, natural gas, propane, and electricity. These fuels
are inherently cleaner than conventional gasoline and diesel because they do
not contain toxics such as benzene. In addition, they are made of simpler
chemical compounds which yield lower levels of complex combustion
by-products such as 1,3-butadiene.
For Further Information
The EPA National Vehicle and Fuel Emissions Laboratory is the national center
for research and policy on air pollution from highway and off-highway motor
vehicles and equipment. For more information, write us at 2565 Plymouth Road,
Ann Arbor, MI 48105; or phone (313) 668-4333.
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