Carbon monoxide
Carbon monoxide (CO) is a colorless, odorless, and highly toxic gas consisting of one carbon atom and one oxygen atom. It is produced primarily through the incomplete combustion of carbon-based materials, with significant contributions from human activities such as vehicle emissions, industrial processes, and the use of fuel-burning appliances. Natural sources of CO include forest fires, volcanic activity, and certain biological processes. Due to its high affinity for hemoglobin, CO impedes the transport of oxygen in the bloodstream, making it a leading cause of poisoning in the U.S.
In addition to its toxicity, carbon monoxide has implications for climate change. While it is not classified as a greenhouse gas, CO indirectly contributes to the levels of gases like methane and tropospheric ozone, both of which are potent greenhouse gases. As CO interacts with hydroxyl radicals in the atmosphere, it limits the availability of these radicals to break down methane and ozone, allowing their concentrations to rise. Furthermore, CO can lead to the formation of carbon dioxide, another significant greenhouse gas. Understanding CO's role in both health and environmental contexts is crucial for addressing its impacts on public safety and climate change.
Carbon monoxide
Definition
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. The US Environmental Protection Agency (EPA) reported in 2023 that CO released from highway vehicle exhaust accounted for 31 percent of all US CO emissions. 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 the delivery of oxygen in the body. Normally, oxygen binds to a blood protein called hemoglobin, which then transports the oxygen throughout the body. CO has a higher affinity for hemoglobin than does oxygen. Therefore, when CO is inhaled, it binds to hemoglobin, displacing oxygen or preventing it from binding and thereby preventing the hemoglobin from delivering the oxygen to the cells that need it.
Significance for Climate Change
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 the 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 the earth’s GHGs—including methane, tropospheric ozone (the 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 hydroxyl radical is a reactive molecule, consisting of an oxygen atom chemically bonded to a hydrogen atom, that is responsible for decreasing the levels of many atmospheric pollutants, including methane and tropospheric ozone. When atmospheric carbon monoxide reacts with the hydroxyl radical, it decreases the amount of this radical available to react with and remove methane and tropospheric ozone. As a result, methane and ozone build up in the atmosphere.
CO also indirectly affects the levels of tropospheric ozone by its involvement in reactions producing substances that can generate tropospheric ozone. For example, when CO reacts with the hydroxyl radical, one of the products formed in a series of reactions is the hydroperoxyl radical (a reactive molecule consisting of an atom of hydrogen and two atoms of oxygen). The hydroperoxyl radical can participate in reactions that form tropospheric ozone. Finally, CO directly affects the levels of atmospheric CO2 through its reaction with the hydroxyl radical. When CO reacts with the hydroxyl radical in the atmosphere, it forms CO2. CO2 is one of the most potent GHGs.
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