ozone layer or ozonosphere, region of the stratosphere containing relatively high concentrations of ozone, located at altitudes of 12-30 mi (19-48 km) above the earth's surface. Ozone in the ozone layer is formed by the action of solar ultraviolet light on oxygen.

The ozone layer prevents most ultraviolet (UV) and other high-energy radiation from penetrating to the earth's surface but does allow through sufficient ultraviolet rays to support the activation of vitamin D in humans. The full radiation, if unhindered by this filtering effect, would destroy animal tissue. Higher levels of radiation resulting from the depletion of the ozone layer have been linked with increases in skin cancers and cataracts and have been implicated in the decline of certain amphibian species.

In 1974 scientists warned that certain industrial chemicals, e.g., chlorofluorocarbons (CFCs) and to a lesser extent, halons and carbon tetrachloride, could migrate to the stratosphere. There, sunlight could free the chlorine or bromine atoms to form chlorine monoxide or other chemicals, which would deplete upper-atmospheric ozone. A seasonal decrease, or "hole," discovered in 1985 in the ozone layer above Antarctica was the first confirmation of a thinning of the layer. The hole occurs over Antarctica because the extreme cold helps the very high clouds characteristic of that area form tiny ice particles of water and nitric acid, which facilitate the chemical reactions involved. In addition, the polar winds, which follow a swirling pattern, create a confined vortex, trapping the chemicals. When the Antarctic spring sun rises in August or September and hits the trapped chemicals, a chain reaction begins in which chlorine, bromine (from the halons), and ice crystals react with the ozone and destroy it very quickly. The effect usually lasts through November. There is a corresponding hole over the Arctic that similarly appears in the spring, although in some years warmer winters there do not result in a major depletion of the ozone layer. A global thinning of the ozone layer results as ozone-rich air from the remaining ozone layer flows into the ozone-poor areas.

Minimum ozone levels in the Antarctic decreased steadily throughout the 1990s, and less dramatic decreases have been found above other areas of the world. In 2000 (and again in 2003 and 2006) the hole reached a record size, extending over more than 10.5 million sq mi (27 million sq km), an area greater than that of North America. In 1987 an international agreement, the Montreal Protocol, was reached on reducing the production of ozone-depleting compounds. Revisions in 1992 called for an end to the production of the worst of such compounds by 1996, and CFC emissions dropped dramatically by 1993. Recovery of the ozone layer, however, is expected to take 50 to 100 years. Damage to the ozone layer can also be caused by sulfuric acid droplets produced by volcanic eruptions.

ozone, an allotropic form of the chemical element oxygen (see allotropy). Pure ozone is an unstable, faintly bluish gas with a characteristic fresh, penetrating odor. The gas has a density of 2.144 grams per liter at STP. Below its boiling point (-112°C;) ozone is a dark blue liquid; below its melting point (-193°C;) it is a blue-black crystalline solid. Ozone is triatomic oxygen, O₃, and has a molecular weight of 47.9982 atomic mass units (amu). It is the most chemically active form of oxygen. It is formed in the ozone layer of the stratosphere by the action of solar ultraviolet light on oxygen. Although it is present in this layer only to an extent of about 10 parts per million, ozone is important because its formation prevents most ultraviolet and other high-energy radiation, which is harmful to life, from penetrating to the earth's surface. Ultraviolet light is absorbed when its strikes an ozone molecule; the molecule is split into atomic and diatomic oxygen: 0₃+ ultraviolet light →0+0₂. Later, in the presence of a catalyst, the atomic and diatomic oxygen reunite to form ozone. Some environmental scientists fear that certain man-made pollutants (e.g., nitric oxide, NO) may interfere with this delicate balance of reactions that maintains the ozone's concentration, possibly leading to a drastic depletion of stratospheric ozone. Ozone is also formed when an electric discharge passes through air; for example, it is formed by lightning and by some electric motors and generators. Ozone is produced commercially by passing dry air between two concentric-tube or plate electrodes connected to an alternating high voltage; this is called the silent electric discharge method. Ozone is used commercially as a disinfectant and decontaminant for air and water, and as a bleaching agent for waxes, oils, and other organic compounds. The major commercial use is in the production by ozonolysis of azelaic acid (used in making plastics); it is also used in the synthesis of cortisone and certain synthetic sex hormones. Ozonization, the reaction of ozone with the double or triple bonds of unsaturated organic molecules, is useful in determining the structure of organic compounds.

or ozonosphere

Region in the upper atmosphere, about 6–30 mi (10–50 km) high, with significant concentrations of ozone, formed by the effect of solar ultraviolet (UV) radiation on oxygen and also present in trace quantities elsewhere in earth's atmosphere. Ozone strongly absorbs solar UV radiation, causing atmospheric temperature to climb to about 30°F (0°C) at the top of the layer, and preventing much of this radiation from reaching earth's surface, where it would injure many living things. Chlorofluorocarbons, or CFCs, and some other air pollutants that diffuse into the ozone layer destroy ozone. In the mid-1980s, scientists discovered that a “hole”—an area where the ozone is up to 50percnt thinner than normal—develops periodically in the ozone layer above Antarctica. This severe regional depletion, explained as a natural seasonal depletion, appears to have been exacerbated by the effects of CFCs, and may have led to an increase in skin cancer caused by UV exposure. Restrictions on the manufacture and use of CFCs and other ozone-destroying pollutants were imposed in 1978.

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Pale blue gas (O₃) that is irritating, explosive, and toxic. Like ordinary oxygen gas (O₂), it contains oxygen atoms, but the bonding of three atoms per molecule gives it distinctive properties. It is formed in electrical discharges and accounts for the distinctive odour of the air after thunderstorms or near electrical equipment. Usually manufactured on the spot by passing an electric discharge through oxygen or air, it is used in water purification, deodorization, bleaching, and various chemical reactions that require a strong oxidizing agent (see oxidation-reduction). Small amounts that occur naturally in the stratospheric ozone layer absorb ultraviolet (UV) radiation that otherwise could severely damage living organisms. Near Earth's surface, ozone contributes to air pollution, ozone produced by auto emissions in the presence of sunlight being a deleterious component of smog, and also accelerates the deterioration of rubber.

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