Exhaust gas

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  • Crankcase blow-by
  • Evaporation of unused gasoline

Motor vehicle emissions contribute to air pollution and are a major ingredient in the creation of smog in some large cities.[1][2][3][4] A 2013 study by MIT indicates that 53,000 early deaths occur per year in the United States alone because of vehicle emissions.[5] According to another study from the same university, traffic fumes alone cause the death of 5000 people every year just in the United Kingdom.[6]


The largest part of most combustion gas is nitrogen (N2), water vapor (H2O) (except with pure-carbon fuels), and carbon dioxide (CO2) (except for fuels without carbon); these are not toxic or noxious (although carbon dioxide is generally recognized as a greenhouse gas that contributes to global warming). A relatively small part of combustion gas is undesirable noxious or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (properly indicated as CxHy, but typically shown simply as "HC" on emissions-test slips) from unburnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, ozone (O3), and particulate matter (mostly soot).

Exhaust gas temperature[edit]


Cold engines[edit]

Steam from tailpipe of cold car


  • Rich air-fuel ratio requirement in cold engines: When a cold engine is started, the fuel does not vaporise completely, creating higher emissions of hydrocarbons, nitrogen oxides and carbon monoxide, which diminishes only as the engine reaches operating temperature. The duration of this start-up phase has been reduced by advances in materials and technology, including computer-controlled fuel injection, shorter intake lengths, and pre-heating of fuel and/or inducted air.
  • Inefficient catalytic converter under cold conditions: Catalytic converters are very inefficient until up to their operating temperature. This time has been much reduced by moving the converter closer to the exhaust manifold and even moreso placing a small yet quick-to-heat-up converter directly at the exhaust manifold. The small converter handles the start-up emissions, which allows enough time for the larger main converter to heat up. Further improvements can be realised in many ways,[7] including electric heating, thermal battery, chemical reaction preheating, flame heating and superinsulation.

Passenger car emissions summary[edit]

U.S. Environmental Protection Agency estimates of average passenger car emissions in the United States for July 2000[8]
Component Emission Rate Annual pollution emitted
Hydrocarbons 2.80 grams/mile (1.75 g/Km) 77.1 pounds (35.0 kg)
Carbon monoxide 20.9 grams/mile (13.06 g/Km) 575 pounds (261 kg)
NOx 1.39 grams/mile (0.87 g/Km) 38.2 pounds (17.3 kg)
Carbon dioxide - greenhouse gas 0.916 pounds per mile (258 g/km) 11,450 pounds (5,190 kg)

Comparable with the European Emission Standards EURO III as it was applied on October 2000


Component Emission Rate Annual pollution emitted
NMOG (Volatile organic compounds) 0.075  grams/mile (0.046 g/Km) 2.1 pounds (0.95 kg)
Carbon Monoxide 3.4 grams/mile (2.1 g/Km) 94 pounds (43 kg)
NOX 0.05 grams/mile (0.0305 g/Km) 1.4 pounds (0.64 kg)
Formaldehyde 0.015 grams/mile (0.0092 g/Km) 0.41 pounds (0.19 kg)


Internal-combustion engines[edit]

Spark-ignition engines[edit]


This table DOES NOT correctly reflect graph presented on page 6 of referenced document!


Combustion-engine exhaust gases[10]
All figures are approximate
 % of total
Compound Petrol Diesel
nitrogen 71 67
carbon dioxide 14 12
water vapor 12 11
oxygen 10
Trace elements[citation needed] < 0.5 ~ 0.3
nitrogen oxides < 0.25 < 0.15
carbon monoxide 1 - 2 < 0.045
particulate matter < 0.045
hydrocarbons < 0.25 < 0.03
sulphur dioxide possible traces < 0.03
Nitromethane additive[edit]

Exhaust gas from an internal combustion engine whose fuel includes nitromethane will contain nitric acid vapour, which is corrosive, and when inhaled causes a muscular reaction making it impossible to breathe. People exposed to it should wear a gas mask.[11]

Diesel engines[edit]

See Diesel exhaust; Soot.

Gas-turbine engines[edit]


Jet engines and rocket engines[edit]

What looks like exhaust from jet engines, is actually contrail. (Jet flying over the United States Air Force Academy in Colorado Springs, Colorado).


Other types[edit]

From burning coal[edit]

Steam engines[edit]


Main motor vehicle emissions[edit]


Smog in New York City as viewed from the World Trade Center in 1988. A 2009 report indicates that, despite the City's air being close to the EPA's short-term ozone standard, that New Yorkers still face a 25 per-cent increased risk of dying from lung disease.[12]

Mono-nitrogen oxides NO and NO2 react with ammonia, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles may cause or worsen respiratory diseases such as emphysema and bronchitis. It may also aggravate existing heart disease.[13][14][15] In a 2005 U.S. EPA study the largest emissions of NOx came from on road motor vehicles, with the second largest contributor being non-road equipment which is mostly gasoline and diesel stations.[15]

Volatile organic compounds[edit]

Non road equipment is mostly gasoline and diesel stations.[16]



Ozone is beneficial in the upper atmosphere,[18] but at ground level, ozone irritates the respiratory system, causing coughing, choking, and reduced lung capacity.[19] It also has many bad effects throughout the ecosystem.[20]

Carbon monoxide (CO)[edit]

MOPITT satellite computer image of carbon monoxide March 2010.

Carbon monoxide poisoning is the most common type of fatal air poisoning in many countries.[21] Carbon monoxide is colorless, odorless and tasteless, but highly toxic. It combines with hemoglobin to produce carboxyhemoglobin, which is ineffective for delivering oxygen to bodily tissues. In the U.S. 60% of carbon monoxide is caused by on road vehicles.[22]

Hazardous air pollutants (toxics)[edit]


Particulate matter (PM10 and PM2.5)[edit]

The health effects of inhaling airborne particulate matter have been widely studied in humans and animals and include asthma, lung cancer, cardiovascular issues, and premature death. Because of the size of the particles, they can penetrate the deepest part of the lungs.[25] A 2011 UK study estimates 90 deaths per year due to passenger vehicle PM.[26] In a 2006 publication, the U.S. Federal Highway Administration (FHWA) state that in 2002 about 1 per-cent of all PM10 and 2 per-cent of all PM2.5 emissions came from the exhaust of on-road motor vehicles (mostly from diesel engines).[27]

Carbon dioxide (CO2)[edit]

Carbon dioxide is a greenhouse gas. Motor vehicle CO2 emissions are part of the anthropogenic contribution to the growth of CO2 concentrations in the atmosphere which is believed by some scientists to play a part in the normal cycle of climate change[citation needed]. Motor vehicles are calculated to generate about 20% of the European Union's man-made CO2 emissions, with passenger cars contributing about 12%.[28] European emission standards limit the CO2 emissions of new passenger cars and light vehicles. The European Union average new car CO2 emissions figure dropped by 5.4% in the year to the first quarter of 2010, down to 145.6 g/km.[29]

Pollution reduction[edit]



Health studies[edit]

Researchers from the University of California, Los Angeles School of Public Health say preliminary results of their statistical study of children listed in the California Cancer Registry born between 1998 and 2007 found that traffic pollution may be associated with a 5% to 15% increase in the likelihood of some cancers.[32] A World Health Organization study found that diesel fumes cause an increase in lung cancer.[33]

Localised effects[edit]

The California Air Resources Board (C.A.R.B.) found in studies that 50% or more of the air pollution (smog) in Southern California is due to car emissions.[citation needed]

See also[edit]

Automobile exhaust


  1. ^ "EPA Tools Available as Summer Smog Season Starts" (Press release). Boston, Massachusetts: United States Environmental Protection Agency. 30 April 2008. 
  2. ^ "Sprawl Report 2001: Measuring Vehicle Contribution to Smog". Sierra Club. 2001. 
  3. ^ "Smog - Causes". The Environment: A Global Challenge. Retrieved 25 October 2013. 
  4. ^ "Smog — Who Does It Hurt? What You Need to Know About Ozone and Your Health (EPA-452/K-99-001)" (PDF). United States Environmental Protection Agency. July 1999 
  5. ^ Caiazzo, Fabio; Ashok, Akshay; Waitz, Ian A.; Yim, Steve H.L.; Barrett, Steven R.H. (November 2013). "Air pollution and early deaths in the United States. Part I: Quantifying the impact of major sectors in 2005". Atmospheric Environment (Elsevier) 79: 198–208. Bibcode:2013AtmEn..79..198C. doi:10.1016/j.atmosenv.2013.05.081. Retrieved 25 October 2013. 
  6. ^ By Roland Pease. "Traffic pollution kills 5,000 a year in UK, says study". BBC News. 
  8. ^ "Average Annual Emissions and Fuel Consumption for Passenger Cars and Light Trucks". Transportation and Air Quality. United States Environmental Protection Agency. 
  11. ^ turbofast.com
  12. ^ Bravender, Robin (2009-03-12). "Study links smog exposure to premature death". The New York Times. 
  18. ^ http://www.epa.gov/ozone/basicinfo.html
  19. ^ http://www.epa.gov/airquality/ozonepollution/health.html
  20. ^ http://www.epa.gov/groundlevelozone/ecosystem.html
  21. ^ Omaye ST. (2002). "Metabolic modulation of carbon monoxide toxicity". Toxicology 180 (2): 139–150. doi:10.1016/S0300-483X(02)00387-6. PMID 12324190. 
  24. ^ "Air Toxics from Motor Vehicles". Transportation and Air Quality (United States Environmental Protection Agency). 
  26. ^ Mazzi, Eric A.; Dowlatabadi, Hadi (2007). "Air Quality Impacts of Climate Mitigation: UK Policy and Passenger Vehicle Choice". Environmental Science & Technology 41 (2): 387. doi:10.1021/es060517w. 
  30. ^ EPA Plain English Guide to the Clean Air Act
  33. ^ http://www.iarc.fr/en/media-centre/pr/2012/pdfs/pr213_E.pdf

External links[edit]