Ozone Layer, a region of the atmosphere from 19 to 48 km (12 to 30 mi) above the earth's surface. Ozone concentrations of up to 10 parts per million occur in the ozone layer. The ozone forms there by the action of sunlight on oxygen. This action has been taking place for many millions of years, but naturally occurring nitrogen compounds in the atmosphere apparently have kept the ozone concentration at a fairly stable level. Concentrations this great at ground level are dangerous to breathe and can damage the lungs. However, because the ozone layer of the atmosphere protects life on earth from the full force of the sun's cancer-causing ultraviolet radiation, it is critically important. Thus, scientists were concerned when they discovered in the 1970s that chemicals called chlorofluorocarbons, or CFCs long used as refrigerants and as aerosol spray propellants—posed a possible threat to the ozone layer. Released into the atmosphere, these chlorine-containing chemicals rise and are broken down by sunlight, whereupon the chlorine reacts with and destroys ozone molecules—up to 100,000 per CFC molecule. For this reason, the use of CFCs in aerosols has been banned in the United States and elsewhere. Other chemicals, such as bromine halocarbons, as well as nitrous oxides from fertilizers, may also attack the ozone layer. Destruction of the ozone layer is predicted to cause increases in skin cancer and cataracts, damage to certain crops and to plankton and the marine food web, and an increase in carbon dioxide due to the decrease in plants and plankton.

Beginning in the early 1980s, research scientists working in Antarctica have detected a periodic loss of ozone in the atmosphere high above that continent. The so-called ozone “hole,” a thinned region of the ozone layer, develops in the Antarctic spring and continues for several months before thickening again. Studies conducted with high-altitude balloons and weather satellites indicated that the overall percentage of ozone in the Antarctic ozone layer is actually declining. Flights over the Arctic regions found a similar problem developing there.

In 1987 the Montréal Protocol, a treaty for the protection of the ozone layer, was signed and later ratified by 36 nations, including the United States. A total ban on the use of CFCs during the 1990s was proposed by the European Community (now called the European Union) in 1989, a move endorsed by U.S. President George Bush. In December 1995 over 100 nations agreed to phase out developed countries' production of the pesticide methyl bromide, predicted to cause about 15 percent of ozone depletion by the year 2000. Production of CFCs in developed countries ceased at the end of 1995 and will be phased out in developing countries by 2010. Hydrochlorofluorocarbons, or HCFCs, which cause less damage to the ozone layer than CFCs do, are being used as substitutes for CFCs on an interim basis, until 2020 in developed countries and until 2016 in developing countries. To monitor ozone depletion on a global level, in 1991 the National Aeronautics and Space Administration (NASA) launched the 7-ton Upper Atmosphere Research Satellite. Orbiting earth at an altitude of 600 km (372 mi), the spacecraft measures ozone variations at different altitudes and is providing the first complete picture of upper atmosphere chemistry.