Following a peak of awareness in the early 1990s, when it was championed by then US Vice-President Al Gore, the environmental issue of ozone depletion has largely disappeared from the public consciousness and the media. Nevertheless, Many of the important steps involve choosing natural hydrocarbon refrigerants where they are available.

The basics

When certain ozone-destroying substances (known as ODS) are released into the air in gaseous form, air currents may eventually carry them up to the stratosphere (a layer 10-30km above the earth's surface). The molecules of these chemicals are broken apart by strong ultraviolet light to release chlorine or bromine atoms, which in turn destroy ozone (O3) molecules. It has been estimated that a single chlorine atom in the stratosphere can destroy an estimated 100,000 molecules of ozone. Nature has its own process of ozone production and destruction, but ozone creation reactions have been unable to keep up with the recent rate of loss.

Complex and not yet fully understood interactions take place between ozone depletion and global warming; however, it is broadly agreed that the warming of the earth's lower atmosphere, in turn, leads to a cooling of the upper stratosphere. These lower temperatures result in a reduction in the rate of ozone formation. Some ODS, including chlorofluorocarbons (CFCs), and some ozone-safe alternatives are powerful greenhouse gases. As is often the case, a high level of consumer discernment is a valuable asset.

The alarm about ozone depletion was raised in 1974 when scientist James Lovelock detected CFCs in the atmosphere all around the world, but no substantial international action was taken until 1984, when conclusive evidence of stratospheric ozone loss had been produced. This culminated in 1987 in the signing of the Montreal Protocol by many countries.

One function of the ozone layer is to filter out harmful UV-B ultraviolet radiation from the sun; ozone's ability to absorb this type of radiation has been shown in laboratory experiments. Separate research has shown that for every one per cent drop in ozone, one to two per cent more UV-B will reach the earth's surface; in Antarctica a clear correlation between reduced ozone levels and higher UV-B levels has been found.

Exposure to UV-B can cause DNA damage in humans, animals and other living things including food crops. In people it has been linked to increased incidence of eye cataracts and skin cancers. Marine life is particularly vulnerable to UV-B, which affects the growth, photosynthesis, protein and pigment content of phytoplankton, thus destabilising the marine food chain. It is likely to reduce the yield and growth of plants and it will even shorten the lifetime of materials such as plastics, rubber, wood, paper and cotton.

The thinning of the ozone layer is monitored by a measurement system called the Total Ozone Mapping Spectrometer (TOMS). Depletion is so marked over Antarctica that the term "ozone hole" is now used, and there is significant ozone loss at all latitudes outside the tropics. Southern hemisphere exposure to the sun becomes more of a risk the further south you travel, and around mid October last year TOMS showed the hole had extended to populated areas near the tip of South America.

It's important not to confuse the ozone layer with ground-level ozone, produced by sources such as car exhausts and photocopiers. This manmade ozone is a noxious pollutant which reacts with sunlight to produce photochemical smog. There is a limited number of known manmade ODS, including:

• Chlorofluorocarbons (CFCs). May be used as a refrigerant, aerosol propellant, in industrial cleaners and fire extinguishers; for dry cleaning and the manufacture of electronics, styrofoam, polyurethane blow-in insulation and foam boards (polystyrene, polyurethane, polyethylene, isocynanurate and phenolic).
• Hydrochlorofluorocarbons (HCFCs). Used as a refrigerant, in the manufacture of electronics, and production of foam boards and styrofoam.
• Bromofluorocarbons (BFCs) also known as halons. Used in fire extinguishers.
• Hydrobromofluorocarbons (HBFCs). Used in fire extinguishers.
• Carbon tetrachloride, also known as tetrachloromethane. Used as a feedstock in the manufacture of CFCs.
• Bromochloromethane. A toxic, relatively new chemical used until recently in fire extinguishers.
• Methyl bromide, also known as bromomethane. A toxic chemical used as a soil pesticide and as a fumigant in quarantine and pre-shipment.
• 1,1,1 trichloroethane, also known as methyl chloroform. May be used in liquid paper (Tippex), and as an industrial solvent.

All these chemicals are subject to a global phase-out (with certain exemptions) and in some cases they are being used in developing countries but not in developed countries.