The chemistry for producing chlorofluorocarbons (CFCs) was invented by the Belgian chemist Swarts in the 1890s but they were developed as refrigerants in 1928. Existing refrigerants were toxic, explosive or both so there was a pressing need to find safe alternatives. CFCs had all the appropriate properties and the first was dichlorodifluoromethane (CCl2F2 or CFC-12). Others followed and their introduction changed the socio-economic development of all hot countries. The use of CFC propellants to deliver DDT in World War II saved millions of lives.

CFCs are a family of chemicals (halocarbons) based upon hydrocarbon skeletons (most often methane), where some or all of the hydrogens have been replaced with chlorine and/or fluorine or bromine atoms. They are non-flammable, tasteless and odourless, and chemically stable. Their other important property is their volatility, having boiling points close to zero degrees Centigrade. These physical properties make them ideal for use as refrigerant gases in air conditioners, freezers and refrigerators, as well as for blowing agents for foam plastics. Some are important in metered-dose inhalers used by asthmatics and as fire-retardants (especially in museums where there are delicate artefacts, and in the electronics industry).

In the 1980s, it was recognised that CFCs were one of the main causes of ozone depletion in the stratosphere (discovery of the Antarctic "Ozone Hole" in 1985 and observations, since then, in the middle and higher latitudes). Because they do not break down in the lower atmosphere CFCs are eventually carried into the stratosphere, a process that can take 2 to 5 years. There is sufficient short-wave radiant energy in the stratosphere to cause breakdown and the release of atoms of chlorine and/or bromine. It is these two elements that cause the loss of ozone:

CF2Cl2 + hv º Cl + CF2Cl
(8 <220 nm)

Ozone (O3), which plays a vital part in protecting biota from excess UV, is constantly created and destroyed in the stratosphere. Chlorine and bromine atoms alter the balance, leading to destruction. A single chlorine molecule will, on average, remove between 10,000 and 100,000 molecules of ozone before it randomly moves down into the troposphere or is removed by reaction with molecules such as NO2 or methane, hence the ozone depleting effect of CFCs. Other ozone-depleting substances include pesticides such as methyl bromide, halons used in fire extinguishers, and methyl chloroform used in industrial processes.

A second, but perhaps less publicized, reason for discontinuing CFCs is that they can act as greenhouse gases, contributing to climate forcing. The concept of global warming potential was developed to compare the ability of each gas to trap heat in the atmosphere relative to a reference gas, carbon dioxide. Most CFCs have warming potentials several thousand times greater than CO2.

The internationally agreed "Montreal Protocol" aims to phase out the production and use of ozone-depleting substances. Production and consumption of CFCs and other ozone depleting substances have decreased dramatically due to these international control measures (see figure 1 below). However, due to past emissions and the long atmospheric lifetime of CFCs, scientists predict that ozone depletion will continue to increase, with the peak expected between 2000 and 2010. Given the full implementation of the Montreal Protocol, natural atmospheric processes are expected to repair the ozone layer around the middle of the 21st century.

The planned phase-out of CFCs resulted in a major effort to find alternatives that were effective but which were environmentally safe. Midgley (1995) cited the options as being: conservation (improved design, recovery and re-cycling); non-fluorocarbon compounds (e.g. hydrocarbons for foam-blowing); hydrochlorofluorocarbons (HCFCs); and hydrofluorocarbons (HFCs). It was estimated (cited by Midgley, 1995) that about three quarters of future demand could be met through conservation and the use of non-fluorocarbon alternatives. The remainder required the use of compounds such as HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarrbons) which would allow phase-out of CFCs, and existing equipment to be operated for its useful economic life. The HCFCs and HFCs breakdown in the troposphere so they do not reach the stratosphere. Their chemistry and toxicology were studied in a massive research programme before they went into full use. Information about the changes in production, alternatives and their chemistry and ecoxtoxicology are available at: http://www.afeas.org/. This web site also has historical and up to date information on production and release of CFCs, HCFCs and HFCs.

References:

Midgley PA. Alternatives to CFCs and their behaviour in the atmosphere. In; Volatile Organic Compounds in the Atmosphere Re H, RM H. Issues in Environmental Science and Technology, The Royal Society of Chemistry. Cambridge UK. 1995;4:pp 91-108.

DETR. The potential effects of ozone depletion in the UK. London; 1996 p. .

of Sciences RSA. Effects of ozone depletion. Ambio. 1995;24(Special Issue No. 3.).

UNEP. Environmental effects of ozone depletion: 1998 assessment. In: UNEP. UNEP. Nairobi; 1998. p. . Available from: http://www.gcrio.org/ozone/toc.html