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Chlorofluorocarbons

CHLOROFLUOROCARBONS

Chlorofluorocarbons (CFCs) are a class of chemicals that contain only atoms of carbon, chlorine, and fluorine. As a group, they are unreactive, stable, and poorly soluble in water. Commercially, the most important CFCs were derivatives of methane and ethane. These included trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) and 1,2-dichloro-1,1,2,2-tetrafluoroethane (CFC-114). CFCs were first introduced in the 1930s as safe replacements for refrigerants such as sulfur dioxide, ammonia, chloroform, and carbon tetrachloride. During World War II they were used to produce aerosols of insecticides. During the next fifty years the applications expanded to include foam blowing, precision cleaning, air conditioning, refrigeration, and propellants for medicinal, cosmetic, food, and general-purpose aerosols. These uses eventually resulted in large emissions of CFCs into the atmosphere. Because of their low chemical reactivity, CFCs typically have long atmospheric residence times, and as a consequence are distributed globally.

In 1974, M. Molina and F. Rowland hypothesized that when CFCs reached the stratosphere they would break down to release chlorine atoms. The chlorine atoms would then react with stratospheric ozone, breaking it down into oxygen. Since stratospheric ozone absorbs much of the sun's ultraviolet radiation, decreased stratospherel ozone levels could lead to increased ground-level ultraviolet radiation. This could adversely affect crop growth, and also lead to increases in cataracts and nonmelanoma skin cancer. Following reports of a marked drop in "column ozone" over Antarctica (the "ozone hole") during the Antarctic winter of 1986, most of the nations of the world drafted and signed an agreement calling for the phaseout of CFCs. This agreement is known as the Montreal Protocol. Included were all CFCs and bromochlorofluorocarbons (halons), which are used in fire suppression systems.

The banning of CFCs has lead to research to identify other chemicals that can be used in the same applications but without the same environmental concerns. Two classes of chemicals that have been identified are the hydrochlorofluorocarbons (HCFCs) and the hydrofluorocarbons (HFCs). The presence of hydrogen in the molecule promotes attack by hydroxyl radicals in the atmosphere leading to more rapid breakdown and shorter atmospheric lifetimes. While HFCs do not contain chlorine and therefore can not contribute to ozone depletion, HCFCs do contain chlorine and can contribute to ozone depletion. However, due to the presence of hydrogen, their atmospheric lifetimes are much shorter than the CFCs and the corresponding ozone depletion values are smaller, typically by a factor of between 10 and 100. In subsequent amendments to the Montreal Protocol, the HCFCs have been classified as transitional substances and they are also scheduled for a phase-out, but at much later dates.

One of the reasons the CFCs have been used so extensively and in such a wide variety of applications is their low level of toxicity. The acute, median lethal concentration for a four-hour exposure to many of these materials is greater than 50,000 parts per million (ppm) (5% in the air). In longer term exposure studies, rarely are effects seen below 20,000 ppm (2% in the air). The one exception to this is the potential of all of these compounds, as well as hydrochlorocarbons and hydrocarbons, to sensitize the heart to the action of adrenaline. In the 1960s, it was first reported that teenagers were abusively inhaling CFCs to get a preanesthectic "high." However, in some cases, the individual would get excited, run around and then die, with no apparent cause of death. Subsequent research demonstrated that this effect could be reproduced in laboratory animals which are now used to test possible CFC replacements.

George M. Rusch

(see also: Ambient Air Quality [Air Pollution]; Atmosphere; Hazardous Air Pollutants; Melanoma; Skin Cancer; Ultraviolet Radiation )

Bibliography

Lashof, D. A., and Ahuja, D. R. (1990). "Relative Contributions of Greenhouse Gas Emissions to Global Warming." Nature 344:529531.

Molina, M. J., and Rowland, F. S. (1974). "Stratospheric Sink for Chlorofluoromethanes: Chlorine Atom Catalyzed Destruction of Ozone." Nature 249:810812.

National Aeronautics and Space Administration (NASA) (1986). Present State of Knowledge of the Upper Atmosphere: An Assessment Report. Ref. Publ. No. 1162. Washington, DC: Author.

Reinhardt, C. F.; Mullin, L. S.; and Maxwell, M. E. (1973). "Epinephrine-Induced Cardiac Arrhythmia Potential of Some Common Industrial Solvents." Journal of Occupational Medicine 15 (12):953955.

World Meteorological Organization (1999). 19981999 WMO Global Ozone Research and Monitoring Project. Geneva: Author.

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Chlorofluorocarbon (CFC)

Chlorofluorocarbon (CFC)

A chlorofluorocarbon (CFC) is an organic compound typically consisting of chlorine, fluorine, carbon , and hydrogen. Freon, a trade name, is often used to refer to CFCs, which were invented in the 1930s and have been used widely as aerosol propellants, refrigerants, and solvents. Odorless, colorless, nontoxic, and nonflammable, CFCs are considered valuable industrial products and have proven an especially safe and reliable aid in food preservation. However, the accumulation of CFCs in the stratosphere that may be linked to ozone depletion has generated considerable public debate and has led to legislation and international agreements (such as the Montreal Protocol and its amendments, signed by 148 countries) that banned the production of most CFCs by the year 2000. One of the substitutes developed by industry, the hydrochlorofluorocarbons (HCFCs), still contain enough chlorine to interfere with atmospheric ozone chemistry , although in much lesser amounts than CFCs. The Copenhagen amendment to the Montreal Protocol calls for the cessation of HCFC production by 2030. Hydrofluorocarbons (HFCs) are currently considered a safer substitute due to prevent ozone loss due to their lack of chlorine and shorter reactive time. As of 2002, new automobiles in the United States contain HFC refrigerant products in the air conditioners.

In the late 1920s, researchers had been trying to develop a coolant that was both nontoxic and nonflammable. At that time, methyl chloride was used, but if it leaked from the refrigerator, it could explode. This danger was demonstrated in one case when methyl chloride gas escaped, causing a disastrous explosion in a Cleveland hospital. Sulfur dioxide was sometimes used as an alternative coolant because its unpleasant odor could be easily noticed in the event of a leak. The problem was brought to the attention of Thomas Midgley Jr., a mechanical engineer at the research laboratory of General Motors. He was asked by his superiors to try to manufacture a safe, workable coolant. (At that time, General Motors was the parent company to Frigidaire.) Midgley and his associate chemists thought that fluorine might work because they had read that carbon tetrafluoride had a boiling point of 5°F (15°C). The compound, as it turns out, had accidentally been referenced. Its actual boiling point is 198°F (92.2°C), not nearly the level necessary to produce refrigeration. Nevertheless, the incident proved useful because it prompted Midgley to look at other carbon compounds containing both fluorine and chlorine. Within three days, Midgley's team discovered the right mix: dichlorodifluoromethane, a compound whose molecules contain one carbon, two chlorine, and two fluorine atoms. It is now referred to as CFC-12 or F-12 and was marketed as Freonas were a number of other compounds, including trichlorofluoromethane, dichlorotetrafluoroethane, and chlorodifluoromethane.

Midgley and his colleagues had been correct in guessing that CFCs would have the desired thermal properties and boiling points to serve as refrigerant gases. Because they remained unreactive, and therefore safe, CFCs were seen as ideal for many applications. Through the 1960s, the widespread manufacture of CFCs allowed for accelerated production of refrigerators and air conditioners. Other applications for CFCs were discovered as well, including their use as blowing agents in polystyrene foam. Despite their popularity, CFCs became the target of growing environmental concern by certain groups of researchers. In 1972, two scientists from the University of California, F. Sherwood Rowland and Mario Jose Molino, conducted tests to determine if the persistent characteristics of CFCs could pose a problem by remaining indefinitely in the atmosphere. Soon after, their tests confirmed that CFCs do indeed persist until they gradually ascend into the stratosphere, break down due to ultraviolet radiation, and release chlorine, which in turn affects ozone production. Their discovery set the stage for vehement public debate about the continued use of CFCs. By the mid-1970s, the United States government banned the use of CFCs as aerosol propellants but it resisted a total ban for all industries. Instead, countries and industries began negotiating the process of phasing out CFCs. As CFC use is allowed in fewer and fewer applications, a black market has been growing for the chemical. In 1997, the United States Environmental Protection Agency and Customs Service, along with other governmental agencies, initiated enforcement actions to prevent (CFC) smuggling in the United States.

See also Atmospheric pollution; Global warming; Greenhouse gases and greenhouse effect; Ozone layer and hole dynamics; Ozone layer depletion

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chlorofluorocarbons

chlorofluorocarbons (klōr´əflŏŏr´əkär´bənz, klôr´–) (CFCs), organic compounds that contain carbon, chlorine, and fluorine atoms. CFCs are highly effective refrigerants that were developed in response to the pressing need to eliminate toxic and flammable substances, such as sulfur dioxide and ammonia, in refrigeration units and air conditioners. The most common commercial CFCs, marketed under the trade name Freon, are trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12). Commercial CFCs are nonflammable, noncorrosive, nontoxic, and odorless, and their vapor pressures and heats of vaporization made them very suitable for refrigeration applications. They were also widely used as aerosol propellants, cleansing agents for electrical and electronic components, and foaming agents in shipping-plastics manufacturing.

In the mid-1970s, scientists at the Univ. of California, Irvine predicted that CFCs could cause ozone depletion in the upper atmosphere; this was later confirmed by ground-based and satellite studies. When CFCs are released into the atmosphere, they move via air currents to altitudes ranging from 15 to 25 mi (25–40 km). There, they are dissociated by ultraviolet light as given by the reaction: CF2Cl2 → CF2Cl + Cl. The resulting free chlorine atoms (Cl) decompose ozone (O3) into oxygen (O2), Cl + O3 → ClO + O2, and are regenerated by interaction with free oxygen atoms (O), ClO + O → Cl + O2. When chlorine is regenerated, it is free to continue to break down other ozone molecules. This process continues for the atmospheric lifetime of the chlorine atom (one to two years), during which it destroys an average of 100,000 ozone molecules. Chlorine radicals are removed from the stratosphere after forming two compounds that are relatively resistant to dissociation by ultraviolet light: hydrogen chloride (HCl) and chlorine nitrate (ClONO2). Dissociation is slow enough so that these compounds can diffuse down to the troposphere, where they react with water vapor and are removed in rain.

Bromine radicals react like chlorine radicals to remove ozone from the stratosphere and sometimes react in concert with chlorine. Bromine is much more destructive than chlorine because the compounds hydrogen bromide (HBr) and bromine nitrate (BrONO2) are much more susceptible to dissociation by ultraviolet light; thus, many more ozone molecules are destroyed before the bromine molecules can diffuse downward. Fluorine radicals combine to form hydrogen fluoride (HF) and other stable compounds that do not affect the ozone layer.

Ozone is vital to human and animal survival because it is responsible for the absorption of the sun's ultraviolet light. Without this protection, blindness and skin cancers could result from penetrating ultraviolet light. In 1987 an international treaty, the Montreal Protocol, called for reducing CFC use by 50% by 2000. A 1992 amendment to the treaty called for the end of CFC production in industrial countries by 1996, and by 1993 CFC emissions had dropped dramatically.

Halons are organic compounds that are similar to CFCs. They contain carbon, fluorine, and bromine and may contain chlorine. Halons have been used primarily as propellants in fire extinguishers. Because of their bromine content they are even more destructive to ozone than CFCs, and an amendment to the Montreal Protocol banned their use by 1994.

Hydrochlorofluorocarbons (HCFCs) are organic compounds that are similar to CFCs but less destructive to ozone. HCFCs consist of carbon, hydrogen, chlorine and fluorine. They are used as replacements for CFCs, but are to be phased out by the year 2020, as specified by the Montreal Protocol as amended, when they are expected to be replaced by hydrofluorocarbons (HFCs). HFCs are organic compounds that contain hydrogen, carbon and fluorine. HFCs, which do not contain chlorine, do not have any potential for the destruction of ozone, and so are suitable replacements for CFCs.

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chlorofluorocarbon

chlorofluorocarbon (CFC) Chemical compound in which hydrogen atoms of a hydrocarbon, such as an alkane, are replaced by atoms of fluorine, chlorine, carbon and sometimes bromine. CFCs are inert, stable at high temperatures and are odourless, colourless, nontoxic, noncorrosive and nonflammable. Under the trade name of Freons, CFCs were widely used in aerosols, fire-extinguishers, refrigerators, and in the manufacture of foam plastics. The two most common are Freon 11 (trichlorofluoromethane, CFCl3) and Freon 12 (dichlorodifluoromethane, CF2Cl2). CFCs slowly drift into the stratosphere and are broken down by the Sun's ultraviolet radiation into chlorine atoms that destroy the ozone layer. It often takes more than 100 years for CFCs to disappear from the atmosphere. Growing environmental concern led to an international agreement in 1990 to reduce and eventually phase out the use of CFCs, and to develop safe substitutes.

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chlorofluorocarbon

chlorofluorocarbon (CFC) One of a range of chemically inert compounds in which a chlorinated hydrocarbon is treated with hydrogen fluoride (e.g. CFC-12 is CCl2F2, CFC-21 is CHCl2F). CFCs have been used widely in fire extinguishers and as solvents, refrigerants, aerosol propellants, and in the manufacture of foam plastics. Their use is being phased out because of their capacity for absorbing long-wave electromagnetic radiation (i.e. they are ‘greenhouse’ gases) and because their chemical stability allows them to survive in the atmosphere long enough to enter the stratosphere, where they are decomposed by ultraviolet radiation, liberating chlorine that is implicated in the thinning of the ozone layer.

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chlorofluorocarbons

chlorofluorocarbons (CFCs) Compounds obtained by replacing the hydrogen atoms of hydrocarbons by chlorine and fluorine atoms. Their high stability to temperature makes them suitable for a variety of uses, including aerosol propellants, oils, polymers, etc. They are often known as freons. Their widespread use in aerosols and refrigerator coolants has led to increased concentrations in the upper atmosphere, where photochemical reactions cause them to break down and react with ozone, which results in depletion of the ozone layer. For this reason CFCs are now being replaced with less damaging alternatives. See also pollution.

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chlorofluorocarbon

chlo·ro·fluor·o·car·bon / ˌklôrōˌfloŏrōˈkärbən/ (abbr.: CFC) • n. any of a class of compounds of carbon, hydrogen, chlorine, and fluorine, typically gases used chiefly in refrigerants and aerosol propellants. They are harmful to the ozone layer in the earth's atmosphere.

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chlorofluorocarbon

chlorofluorocarbonBrian, cyan, Gaian, Geminian, Hawaiian, ion, iron, Ixion, lion, Lyon, Mayan, Narayan, O'Brien, Orion, Paraguayan, prion, Ryan, scion, Uruguayan, Zion •andiron •gridiron, midiron •dandelion • anion • Bruneian •cation, flatiron •gowan, Palawan, rowen •anthozoan, bryozoan, Goan, hydrozoan, Minoan, protozoan, protozoon, rowan, Samoan, spermatozoon •Ohioan • Chicagoan • Virgoan •Idahoan •doyen, Illinoisan, IroquoianEwan, Labuan, McEwan, McLuhan, Siouan •Saskatchewan • Papuan • Paduan •Nicaraguan • gargantuan •carbon, chlorofluorocarbon, graben, hydrocarbon, Laban, radiocarbon •ebon • Melbourne • Theban •gibbon, ribbon •Brisbane, Lisbon •Tyburn •auburn, Bourbon •Alban • Manitoban • Cuban •stubborn •Durban, exurban, suburban, turban, urban

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