The ozone layer, which is part of the stratosphere, is comprised of the major atmospheric gases nitrogen, oxygen and argon, but also contains a significantly higher concentration of the trace gas ozone than the other layers of the atmosphere. The other trace gases include carbon dioxide, neon, helium, methane and the manmade chlorofluorocarbons, or CFCs. The CFCs reaching the stratosphere from the Earth's surface have become a cause for global concern because of the role they play in the chemical reaction that removes ozone from the atmosphere.
Ozone, along with the other trace gases, is found throughout the atmosphere, but about 90 percent of the ozone is concentrated in the stratosphere. The ozone layer is in the lower portion of the stratosphere and begins at an altitude of around 9 1/2 miles and stretches upward to about 21 1/2 miles.
Higher than normal concentrations of ozone at the Earth's surface, such as may be found in large cities during periods of poor air quality, can be corrosive and cause health issues, but its presence in the ozone layer serves as a protective screen against harmful ultraviolet, or UV, radiation. Because of its shorter wavelength, UV radiation is highly energetic and can cause harm to biological tissues. UV radiation is considered the primary cause of skin cancer and can also cause eye cataracts, changes in animal DNA and reductions in the size of plant leaves.
A widening hole in the ozone layer over Antarctica was discovered in the 1980s. The global concern generated by the alarming discovery led to the signing of the 1987 Montreal Protocol. The signatory nations agreed to limit and to eventually discontinue the production of the CFCs contributing to the eroding of the protective UV radiation screen created by the ozone layer.