Carbon monoxide (Encyclopedia of Environmental Issues, Revised Edition)
Carbon monoxide (CO), an indoor and outdoor air pollutant, is formed by incomplete combustion of fossil fuels when insufficient oxygen is present to convert carbon compounds to nontoxic carbon dioxide. Indoors, CO may come from unvented kerosene and gas space heaters; leaking or improperly vented chimneys, furnaces, gas water heaters, wood stoves, gas stoves, and fireplaces; generators and other gasoline-powered equipment; automobile exhaust from attached garages; and tobacco smoke. Outdoors, CO is present in small amounts naturally in the atmosphere, chiefly as a product of volcanic activity, but also from naturally occurring fires.
Motor vehicle exhaust contributes about 56 percent of all CO emissions in the United States. The highest levels of CO occur in areas with heavy traffic congestion, and in cities, 85-95 percent of all CO emissions may come from motor vehicle exhaust. Other nonroad engines and vehicles (such as construction equipment and boats) contribute about 22 percent of CO emissions nationwide. Additional sources of CO emissions include industrial processes (metals processing and chemical manufacturing) and residential wood burning. The highest levels of outdoor CO typically occur during the winter months, for three primary reasons: Motor vehicles need more fuel to start at cold temperatures, some emissions-control devices (oxygen sensors and catalytic converters) operate less efficiently in the cold, and temperature...
(The entire section is 539 words.)
Further Reading (Encyclopedia of Environmental Issues, Revised Edition)
Kleinman, Michael T. “Carbon Monoxide.” In Environmental Toxicants: Human Exposures and Their Health Effects, edited by Morton Lippmann. 3d ed. Hoboken, N.J.: John Wiley & Sons, 2009.
Penney, David G., ed. Carbon Monoxide Poisoning. Boca Raton, Fla.: CRC Press, 2008.
(The entire section is 39 words.)
Definition (Encyclopedia of Global Warming)
Carbon monoxide (CO) is a colorless, odorless, tasteless, and highly toxic gas composed of an atom of carbon and an atom of oxygen chemically bonded together. It is useful in the production of a wide variety of chemicals in various industries, including the automotive, construction, agrochemical, cosmetics, pharmaceutical, plastics, and textile industries. Environmental CO is primarily produced from the incomplete combustion of carbon-containing materials.
Anthropogenic (human) sources of CO include incomplete combustion of fossil fuels in internal combustion engines, from which it is released in automobile exhaust; industrial plant exhaust, including exhaust from industry oxidation of hydrocarbons; cigarette smoke; burning of biomass; and various fuel-burning household appliances, including wood-burning stoves, water heaters, clothes dryers, furnaces, fireplaces, generators, and space heaters. CO released in automobile exhaust accounts for about 60 percent of all U.S. CO emissions. Moreover, such automobile exhaust can represent up to about 95 percent of all CO emissions in U.S. cities. Natural sources of CO include coal mines, forest fires, volcanoes, vegetation, soil (including water-saturated areas such as wetlands), the ocean, and atmospheric oxidation of hydrocarbons.
In the United States, CO is considered to be the leading cause of death from poisoning. CO is toxic in that it interferes with delivery of oxygen in...
(The entire section is 266 words.)
Significance for Climate Change (Encyclopedia of Global Warming)
CO affects global warming through its ability, either directly or indirectly, to increase the levels of other gases in the atmosphere. Such other gases, including carbon dioxide (CO2), methane (CH4), and ozone (O3), directly affect global warming. After the Earth is heated by the Sun, some terrestrial heat normally leaves the planet, escaping into outer space. This process allows the Earth to maintain a constant temperature, rather than growing steadily warmer as more solar energy impinges upon it. CO2, methane, and ozone have the ability to trap the terrestrial heat attempting to leave the Earth, thereby preventing this heat from escaping into outer space. Although such gases, known as greenhouse gases (GHGs), play a role in maintaining a stable and moderate temperature on Earth, an excess of GHGs results in a terrestrial buildup of heat and an elevation of the Earth’s temperature. The atmospheric concentrations of many GHGs have significantly increased since the advent of industrialization, around 1750.
Although CO is not a GHG, it acts directly or indirectly to increase the levels of Earth’s GHGs—including methane, tropospheric ozone (the troposphere is the lowest portion of the Earth’s atmosphere), and CO2—by participating in various chemical reactions in the atmosphere. For example, CO indirectly affects the levels of methane and tropospheric ozone by reacting with the hydroxyl radical. The...
(The entire section is 398 words.)
Further Reading (Encyclopedia of Global Warming)
Houghton, John. Global Warming: The Complete Briefing. 4th ed. New York: Cambridge University Press, 2009. Discusses the science of global warming, the effects of climate change on society, and ways these effects can be mitigated.
Intergovernmental Panel on Climate Change. Climate Change, 2007—Synthesis Report: Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by the Core Writing Team, Rajendra K. Pachauri, and Andy Reisinger. Geneva, Switzerland: Author, 2008. A multi-article scientific assessment of information related to climate change, including discussion of the role of greenhouse and other gases, including carbon monoxide, and an evaluation of the environmental and economic consequences of climate change.
Kroschwitz, Jacqueline I., and Arza Seidel, eds. Kirk-Othmer Encyclopedia of Chemical Technology. 5th ed. Hoboken, N.J.: Wiley-Interscience, 2004-2007. Provides detailed information on chemical compounds (including CO), including their physical and chemical properties, the reactions they participate in, their uses, and related information.
Van Ham, J., et al., eds. Non-CO2 Greenhouse Gases: Scientific Understanding, Control, and Implementation. Amsterdam, the Netherlands: Kluwer Academic, 2000. Discusses the sources of non-CO2 GHGs, including chemical reactions occurring in the...
(The entire section is 196 words.)
Carbon Monoxide (Encyclopedia of Science)
Carbon monoxide is a compound of carbon and oxygen in which the ratio of the two elements is one atom of carbon to one atom of oxygen. Its formula is CO. Carbon monoxide is a colorless, odorless, tasteless, poisonous gas. Most people have heard about carbon monoxide because of its toxic effects. People who live or work in crowded urban areas may become ill with headaches and nausea because of exposure to carbon monoxide in polluted air. In higher concentrations, the gas can even cause death.
The early history of gases such as carbon monoxide is sometimes difficult to trace. Until the early 1600s, scientists did not realize that the material we call air is actually a mixture of gases. As early as the late thirteenth century, Spanish alchemist Arnold of Villanova (c. 1235311) described a poisonous gas formed by the burning of wood; this gas was almost certainly carbon monoxide.
Flemish scientist Jan Baptista van Helmont (c. 1580644; some sources give death date as 1635) nearly died as a result of inhaling gas carbonum, apparently a mixture of carbon monoxide and carbon dioxide. Credit for the discovery of carbon monoxide, however, is usually given to English chemist and theologian Joseph Priestley (1733804). During the period between 1772 and 1799, Priestley gradually recognized the difference between carbon dioxide and carbon...
(The entire section is 934 words.)
Carbon Monoxide (Encyclopedia of Public Health)
Carbon monoxide (CO) is a clear, colorless, odorless, and insidious poison that is responsible for hundreds of inadvertent and preventable deaths in the United States each year. The major environmental source of CO is incomplete combustion of carbonaceous fossil fuels. The reason for its toxicity is that it combines with the oxygen-carrying site of hemoglobin, the red protein within red blood cells that is responsible for delivering oxygen from the lung to body tissues. CO has a more than two-hundredfold greater affinity for this oxygen-carrying site than does oxygen. This means that, at sea level, exposure to 1,000 parts per million (ppm) CO in 20 percent oxygen (200,000 ppm) would lead, at equilibrium, to about 50 percent of hemoglobin sites being combined with CO rather than oxygen. Fortunately, it requires eight to twelve hours for maximum blood levels to be achieved when the body encounters a new CO concentration, otherwise mainstream cigarette smoke, which contains even higher levels of CO, might be instantaneously lethal. When CO combines with hemoglobin, the resulting chemical is called carboxy hemoglobin (COHb).
The negative effect of CO on the delivery of oxygen to the tissues extends beyond just the simple blockage of oxygen-combining sites. Each hemoglobin molecule contains four oxygen-carrying sites. Once the first oxygen molecule is released at the tissue level the second, third, and fourth come off even more rapidly. Oxygen release is delayed by CO so that there is even less oxygen delivered than would be expected purely on the basis of the amount of oxygen not being carried by hemoglobin. For this reason, overt symptoms due to lack of oxygen can be observed at COHb levels of approximately 15 to 20 percent, or even less, in healthy people. Levels of COHb over 40 percent can be lethal.
The uptake of CO increases as respiratory rates increase. This puts children at greater risk since they breathe more rapidly, in proportion to their body weight, than adults. This explains the unfortunate situation of a family in an automobile stuck in a snowstorm with the motor running being found with the adults unconscious and the children dead. The fetus is also at higher risk due to the greater affinity of CO for fetal, as compared to adult, hemoglobin.
All cases of fatal CO poisoning are readily preventable. In addition to automobile exhaust, other lethal sources of CO are often related to home heating systems. Blockage of flues, or inappropriate repair work on the home heating source or on ducts, is often responsible for CO toxicity. Symptoms of CO toxicity, such as headache, weakness, and listlessness, tend to be worse in the morning and to go away during the day if people leave the home. Many fatal cases are preceded by visits to physicians or emergency departments with only symptomatic treatment. Home CO alarms are relatively cheap and are an effective means of prevention. CO poisoning occurs more rapidly at high altitude due to the relative lack of oxygen to compete for the oxygen-combining site of hemoglobin. Conversely, symptomatic CO poisoning is treated with oxygen.
CO is also made in the human body through the normal catabolism of heme (oxygen-carrying hemoglobin), which leads to a background concentration in the blood of approximately 0.5 percent COHb. Concentrations of 2 to 3 percent COHb have been associated with an increased risk of angina attacks in susceptible individuals with preexisting arteriosclerotic heart disease. Preventing this adverse consequence is the major basis for the current U.S. ambient standard for CO. There has been a significant decline in outdoor CO levels in the United States as a result of decreased automotive emissions of carbon monoxide.
BERNARD D. GOLDSTEIN
(SEE ALSO: Ambient Air Quality [Air Pollution])
Ernst, A., and Zibrak, J. D. (1998). "Carbon Monoxide Poisoning." New England Journal Medicine 339:1603608.
Raub, J. A.; Mathieu-Nof, M.; Hampson, M. B.; and Thom, S. R. (2000). "Carbon Monoxide PoisoningA Public Health Perspective." Toxicology 145(1):14.
Tomaszewski, C. (1999). "Carbon Monoxide PoisoningEarly Awareness and Intervention Can Save Lives." Postgrad Medicine 105(1):390.