U.S. ENVIRONMENTAL PROTECTION AGENCY EPA 4oo-F-92-oo5
OFFICE OF MOBILE SOURCES
Automobiles and Carbon Monoxide
What is Carbon Monoxide?
Carbon monoxide (CO) is a colorless, odorless, poisonous gas. A product of incom-
plete burning of hydrocarbon-based fuels, carbon monoxide consists of a carbon
atom and an oxygen atom linked together.
Why is Carbon Monoxide a Public Health Problem?
Carbon monoxide enters the bloodstream through the lungs and forms
carboxyhemoglobin, a compound that inhibits the blood's capacity to carry oxygen
to organs and tissues. Persons with heart disease are especially sensitive to car-
bon monoxide poisoning and may experience chest pain if they breathe the gas
while exercising. Infants, elderly persons, and individuals with respiratory dis-
eases are also particularly sensitive. Carbon monoxide can affect healthy indi-
viduals, imparing exercise capacity, visual perception, manual dexterity, learning
functions, and ability to perform complex tasks.
In 1992, carbon monoxide levels exceeded the Federal air quality standard in 20
U.S. cities, home to more than 14 million people.
How is Carbon Monoxide Formed?
Carbon monoxide results from incomplete combustion of fuel and is emitted di-
rectly from vehicle tailpipes. Incomplete combustion is most likely to occur at low
air-to-fuel ratios in the engine. These conditions are common during vehicle start-
ing when air supply is restricted ("choked"), when cars are not tuned properly, and
at altitude, where "thin" air effectively reduces the amount of oxygen available for
combustion (except in cars that are designed or adjusted to compensate for alti-
tude).
Nationwide, two-thirds of the carbon monoxide emissions come from transporta-
tion sources, with the largest contribution coming from highway motor vehicles.
In urban areas, the motor vehicle contribution to carbon monoxide pollution can
exceed 90 percent.
What's Been Done to Control Carbon Monoxide Levels?
The Clean Air Act gives state and local governments primary responsibility for
regulating pollution from power plants, factories, and other "stationary sources."
The U.S. Environmental Protection Agency (EPA) has primary responsibility for
"mobile source" pollution control.
FACT SHEET OMS-3
January, 1993
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• Automobiles and Carbon Monoxide.
The EPA motor vehicle program has achieved considerable success in reducing
carbon monoxide emissions. EPA standards in the early 1970's prompted
automakers to improve basic engine design. By 1975, most new cars were equipped
with catalytic converters designed to convert carbon monoxide to carbon dioxide.
Catalysts typically reduce carbon monoxide emissions upwards of 80 percent. In
the early 1980s, automakers introduced more sophisticated converters, plus on-
board computers and oxygen sensors to help optimize the efficiency of the catalytic
converter.
Today's passenger cars are capable of emitting 90 percent less carbon monoxide
over their lifetimes than their uncontrolled counterparts of the 1960s. As a result,
ambient carbon monoxide levels have dropped, despite large increases in the num-
ber of vehicles on the road and the number of miles they travel. With continued
increases in vehicle travel projected, however, carbon monoxide levels will begin to
climb again unless even more effective emission controls are employed.
Carbon monoxide emissions have dropped, despite increased travel
Percent of 1983 Level
150 -r
125 --
100 --i
75 --
50 --
25 --
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
I carbon monoxide emissions D vehicle miles traveled
What Else Is Being Done?
Carbon monoxide emissions from automobiles increase dramatically in cold weather.
This is because cars need more fuel to start at cold temperatures, and because
some emission control devices (such as oxygen sensors and catalytic converters)
operate less efficiently when they are cold.
Until 1994, vehicles were tested for carbon monoxide emissions only at 75° F. But
recognizing the effect of cold weather, the 1990 Clean Air Act calls for 1994, and
later, cars and light trucks to meet a carbon monoxide standard at 20° F as well.
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• Automobiles and Carbon Monoxide.
The 1990 Clean Air Act also stipulates expanded requirements for Inspection and
Maintenance programs. These routine emission system checks should help iden-
tify malfunctioning vehicles that emit excessive levels of carbon monoxide and other
pollutants. The inspections will be complemented by requirements for on-board
warning devices to alert drivers when their emission control systems are not work-
ing properly.
Another strategy to reduce carbon monoxide emissions from motor vehicles is to
add oxygen-containing compounds to gasoline. This has the effect of "leaning out"
the air-to-fuel ratio, thereby promoting complete fuel combustion. The most com-
mon oxygen additives are alcohols or their derivatives.
Several Western U.S. cities have successfully employed wintertime oxygenated
gasolines for many years. The 1990 Clean Air Act expands this concept and re-
quires that oxygenated gasolines be used during the winter months in certain
metropolitan areas with high carbon monoxide levels (see a listing on the reverse
side of this page).
For More Information:
The Office of Mobile Sources is the national center for research and policy on air
pollution from highway and off-highway motor vehicles and equipment. You
can write to us at the EPA National Vehicle and Fuel Emissions Laboratory,
2565 Plymouth Road, Ann Arbor; MI 48105. Our phone number is (313) 668-4333.
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• Automobiles and Carbon Monoxide.
Cities* Participating in Wintertime Oxygenated Fuels Program
Albuquerque, NM
Baltimore, MD
Chico, CA
Colorado Springs, CO
Denver-Boulder, CO
El Paso, TX
Fort Collins-Loveland, CO
Fresno, CA
Grants Pass, OR
Greensboro-Winston Salem-High Point, NC
Klamath County, OR
Las Vegas, NV
Los Angeles-Anaheim-Riverside, CA
Medford, OR
Minneapolis-St-Paul, MN-WI
Missoula, MT
Modesto, CA
New York-N. New Jersey-Long Island, NY-NJ-CT
Philadelphia-Wilmington-Trenton, PA-NJ-DE-MD
Phoenix, AZ
Portland-Vancouver, OR-WA
Provo-Orem UT
Raleigh-Durham, NC
Reno, NV
Sacramento, CA
Salt Lake City, UT
San Diego, CA
San Francisco-Oakland-San Jose, CA
Seattle-Tacoma, WA
Spokane, WA
Stockton, CA
Washington, DC-MD-VA
* The 1990 Clean Air Act requires oxygenated fuels in designated CO nonattainment areas where
mobile sources are a significant source of CO emissions.
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