The ozone layer is a region mainly in the lower portion of the stratosphere between roughly 20 to 50 kilometers in altitude that contains about 90 percent of the total ozone in the atmosphere.
Ozone is a highly reactive gas with a pale blue color and a distinctively pungent smell whose molecules each consist of three oxygen atoms. It is produced naturally in the atmosphere by the action of ultraviolet light and lightning. It is also produced intentionally for use in many industrial and consumer applications because it is one of the most powerful oxidizing agents and by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that are emitted directly by internal combustion engines and industrial processes.
Ozone at or near ground level can be harmful to animals and plants, including damaging mucous and respiratory tissues, creating smog, and interfering with photosynthesis and thus stunting the overall growth of some plant species. On the contrary, the ozone in the ozone layer has an extremely beneficial role for both plants and animals by absorbing 97 to 99 percent of the medium-frequency ultraviolet radiation from the sun (from about 200 nm to 315 nm), which could otherwise severely damage life below.
However, the ozone layer is also fragile and has been degraded by human activity, resulting in ozone depletion and the so-called ozone hole. In particular, some chemicals, such as the chlorofluorocarbons and hydrochlorofluorocarbons, which are used in spray cans, refrigerants and fire extinguishing systems, as well as methyl bromide, which is used in fumigation, release chlorine or bromine atoms, which enter the upper atmosphere and destroy the ozone molecules there much more quickly than they can be replaced naturally.