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.
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 mobile 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).
- 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 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 related to mobile source pollution. For additional
information, contact the lab at 2565 Plymouth Road, Ann Arbor, MI 48105, or call
(313) 668-4333.
Source: US EPA Office of Transportation and Air Quality
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Last Update - 20-Jul-98