The global average level of stratospheric ozone fell by about 5 per cent during the eight years from 1979 to 1986. In the tropics the trend is nearly independent of the season, between 0.5 and 1.0 per cent per year. Outside them the losses vary seasonally and increase with the latitude, becoming greatest at the poles. Since 1969 ozone levels have fallen by about 2 per cent in temperate latitudes in summer, and around 5 to 6 per cent in winter. There have been especially sharp seasonal drops in Antarctica and recently over the Arctic.

Stratospheric ozone is a form of oxygen that acts as a filter for harmful ultraviolet radiation (UV-B) emitted by the sun. It also is one of the minor greenhouse gases, but its effect on ultraviolet radiation is far more important. Several manufactured chemicals, especially some of the chlorofluorocarbons (CFCs) and halons, accelerate the breakdown of stratospheric ozone. The consequent increased exposure of people to UV-B will lead to increased incidence of skin cancer, sunburns, eye damage, and the aging and wrinkling of skin. In addition there will be harmful effects on other species, including some important agricultural crops. Increased UV-B also has a tendency to suppress the efficiency of the body's immune system and to increase the incidence of skin infections. UV-B can also damage plant hormones and chlorophyl and reduce the rate of photosynthesis. Plant species sensitive to UV-B, such as cotton, peas, beans, melons, and cabbage, would grow more slowly, and in some cases pollen would fail to germinate. Increased UV-B levels would also damage algae and aquatic ecosystems, perhaps leading to declines in fish stocks.

The 1987 Montreal Protocol on Substances That Deplete the Ozone Layer entered into force on 1st January 1989. It calls on the signatories to reduce production and consumption of harmful chlorofluorocarbons (CFCs) by 50 per cent from 1986 to 2000; a ten- year delay is allowed for developing countries whose consumption is less than 0.3 kilograms per capita. Measures have already been taken in some countries to reduce or ban the use of the controlled CFCs in all or some products, non-essential aerosols, for example. Some recent studies point to the necessity of amending the Montreal Protocol to ban the production of CFCs and halons altogether and to freeze the production of methyl chloroform. In May 1989 in the Helsinki Declaration on the Protection of the Ozone Layer, ministers and senior officials of 81 countries and the European Community agreed to a total phase-out of ozone-depleting CFCs by the year 2000 or sooner if possible. They also agreed both to phase out halons and to control and reduce other ozone-depleting substances that contribute significantly to ozone depletion as soon as feasible. In addition, they agreed to "facilitate the access of developing countries to relevant scientific information, research results and training and to seek to develop appropriate funding mechanisms to facilitate the transfer of technology and replacement of equipment at minimum cost to developing countries". They committed themselves, "in proportion to their means and resources, to accelerate the development of environmentally acceptable substituting [sic] chemicals, products and technologies". Major producers and users of CFCs are seeking alternatives to CFCs for use in foam blowing, refrigeration, air- conditioning, solvents and other uses. Several major companies that use CFCs and halons recently agreed to eliminate these chemicals in their world-wide operations. One of the major producing companies (the DuPont Corporation) has indicated that it will phase out its production of CFCs entirely over the next few years.