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(CFCs), is widely used in metered dose inhalers (MDIs) for asthma and other respiratory contenant des CFC et qui offrent même des avantages intéressants par rapport aux anciens . patient-physician relationship. It should also address . Ozone depletion occurs when chlorofluorocarbons (CFCs) and halons—gases formerly found in aerosol spray cans and refrigerants—are. ozone by chlorofluorocarbons. Effects on . atoms, the chlorofluorocarbons will react with the ozone layer. cause-and-effect relationships of UV damage to effects on montre que même une faible augmentation de l'exposition au.
Methods for sample collection have been developed to achieve greater selectivity and sensitivity. Natural occurrence The chlorofluorocarbons discussed in this monograph are not known to occur in nature. Man-made sources Almost all chlorofluorocarbons produced, except for those used as chemical intermediates, are eventually released into the environment, whether during manufacture, handling, use, or disposal.
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The significance of the release mechanisms discussed below should be evaluated with this in mind. However, this was before the Montreal Protocol was signed in September It stipulates a number of stepwise importation bans binding on signatories in order to achieve these reductions United Nations Environment Programme. The demand for CFC is also expected to grow because of its use as a solvent in the semi-conductor industry and as a replacement for chlorinated solvents under regulatory pressure CMR, At that time, the hypothesis that certain chlorofluorocarbons that accumulate in the upper atmosphere could deplete the earth's ozone layer had a major impact on the fluorochemical industry Smart, However, with the entry into force of the Montreal Protocol, which progressively limited the pro- duction of CFCs, 12, andthe release of all of these chlorofluorocarbons should decline.
Some of the major world producers of chlorofluorocarbons in a,b Country Company name Argentina Ducilo S. Australia Pacific Chemical Industries Pty. Brazil Du Pont do Brasil S. Canada Allied Canada, Inc.
Algofren Japan Asahi Glass Co. Asahiflon ; Daikin Kogyo Co. Flon Showa Denko, K. Du Pont de Nemours and Co. Venezuela Produren subsidiary to Atochem, S. Noble and Smart US ITCunless otherwise specified. Manufacturing processes The traditional method for manufacturing the fully halogenated chlorofluorocarbons is the catalytic displace- ment of chlorine from chlorocarbons with fluorine by reac- tion with anhydrous hydrogen fluoride Hamilton, ; Smart, Carbon tetrachloride, and hexachloroethane or tetrachloroethylene plus chlorine are commonly used starting materials for 1- and 2-carbon chlorofluorocar- bons.
The reaction can occur in either liquid or vapour phases. The processes use antimony pentafluoride or an equivalent catalyst, in contact with which the chlorocarbon and hydrogen fluoride react. Excess hydrogen fluoride may then be recovered and the chloroflu- orocarbon stream is neutralised to remove traces of acid and dried. The chlorofluorocarbons are then separated in a fractionating column and sent to storage.
An alternative process for the production of the methane-based chloroflu- orocarbons uses the direct reaction of methane with a mixture of chlorine and hydrogen fluoride Noble, The production processes described above give very high yields.
Losses of chlorofluorocarbons are limited to small mechanical leakage, small amounts leaving with the by-product hydrogen chloride, and miscellaneous venting. Loss during disposal of wastes The release of chlorofluorocarbons into the environ- ment during their disposal arises mainly from pre-fabri- cated refrigeration and air-conditioning equipment. Environmental contamination due to chlorofluorocarbon disposal results principally from the following: Disposal of these old appliances is usually to scrap yards or waste dumps.
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Efforts are made in some countries to remove chlorofluorocarbon refrigerants before discarding equipment. This is a minor source of environ- mental contamination compared with that of scrapped refrigerants. Waste disposal streams resulting from manufacturing operations are very minor contamination sources compared with scrapped refrigerants.
Because of the high vapour pressure of chlorofluoro- carbons at ambient temperature, all releases pass eventu- ally into the atmosphere except in cases where the com- pounds have been chemically altered. Release from transport, storage, and accidents 3. The products are shipped in a wide variety of pressure containers ranging from litre drums to m3 tank cars.
The containers are fitted with safety valves, rupture discs, and fusible plugs according to US Interstate Commerce Commission ICC specifications; also included are requirements for labelling and leak pressure testing. Loss of product during transport and storage is rela- tively minor because of the completely closed system used. Losses are further controlled by monitoring discrepancies, if any, between product billings and receipts.
In addition, the high cost of the products provides an incen- tive to control losses. However, it is probable that quantities re- leased by accident are negligible compared with quantities released by use and disposal. Major uses Chlorofluorocarbons are commercially important because of their unique physical and chemical properties and rela- tively low physiological activity.
They are mainly used as refrigerants, solvents, blowing agents, sterilants, aerosol propellants, and as intermediates for plastics. Table 5 lists the estimated use patterns of chlorofluoro- carbons in the USA for the years,and The aerosol propellant market, which consumed half of the total chlorofluorocarbon production inis currently a minor application because of governmental restrictions.
The first Chlorofluorocarbon was CFC, a single carbon with two chlorines and two Fluorines attached to it. These halogenated hydrocarbons, notably trichlorofluoromethane CFC, or F and dichlorodifluoromethane CFC, or Fhave been used extensively as aerosol-spray propellants, refrigerants, solvents, and foam-blowing agents.AIR POLLUTION - OZONE DEPLETION BY CFC GASES AND IT'S EFFECTS ON THE ENVIRONMENT
They are well-suited for these and other applications because they are nontoxic and nonflammable and can be readily converted from a liquid to a gas and vice versa. Chlorofluorocarbons or CFCs also known as Freon are non-toxic, non-flammable and non-carcinogenic. They contain fluorine atoms, carbon atoms and chlorine atoms. CFCs have been found to pose a serious environmental threat. Studies undertaken by various scientists during the s revealed that CFCs released into the atmosphere accumulate in the stratosphere, where they had a deleterious effect on the ozone layer.
Stratospheric ozone shields living organisms on Earth from the harmful effects of the Sun's ultraviolet radiation; even a relatively small decrease in the stratospheric ozone concentration can result in an increased incidence of skin cancer in humans and in genetic damage in many organisms. Total usage of CFCs has also fallen dramatically, particularly by aerosols. The only aerosols using CFCs in the developed world are asthma inhalers and these too are being phased out.
Aerosol propellants now use only 4. Whilst chlorine is a natural threat to ozone, CFCs which contain chlorine are a man-made problem.
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Although CFC molecules are several times heavier than air, winds mix the atmosphere to altitudes far above the top of the stratosphere much faster than molecules can settle according to their weight.
CFCs are insoluble in water and relatively unreactive in the lower atmosphere but are quickly mixed and reach the stratosphere regardless of their weight. The chlorine atom then hits an ozone molecule consisting of three oxygen atoms and takes one of the oxygen molecules, destroying the ozone molecule and turning it into oxygen.