Ozone (O3)

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Ozone (O3)


Ozone is one of the three allotropes (forms of a chemical element with different physical and chemical properties) of the element oxygen (O). The chemical symbol for ozone is O3, which means it has three atoms in each molecule. Ozone is normally invisible, but in high concentrations it can appear pale blue in color. When it is a liquid, it is colored deep blue; as a solid it is darkish purple.

Under normal temperatures and pressures, ozone is a poisonous gas. It is dangerous in small doses and deadly in large amounts. It also has a strong odor, which is why the Greek word from which it is derived—ozein—means “to smell.” People sometimes smell ozone during a rainstorm or when they are near an electrical outlet. Ozone easily reacts chemically with various substances, so it is frequently used as an oxidizing agent to bleach food products, sterilize air, and purify liquids such as water.

Historical Background and Scientific Foundations

Most of the oxygen in Earth's atmosphere exists as O2, with ozone only adding a minute fraction. Ozone occurs

most often in the upper atmosphere, the stratosphere, where it absorbs all of ultraviolet-C (UV-C) radiation from the sun, almost all of ultraviolet-B (UV-B), but very little of ultraviolet-A (UV-A). It occurs to a much lesser degree in the lower troposphere.

UV-B radiation is potentially dangerous to living things and the environment. When increasing amounts of UV-B reach Earth's surface, the deoxyribonucleic acid (DNA) of humans can be damaged. UV-A is used by humans in very small dosages to synthesize vitamin-D. However, too much UV-A can cause health problems with the immune system and the eyes (in the form of cataracts). UV radiation, in general, can cause various forms of cancer.

Impacts and Issues

The ozone layer in the stratosphere protects life on Earth from the harmful ultraviolet rays of the sun. Atmospheric ozone is affected by temperature, winds, airborne chemicals, and other climatic factors. These factors are also affected by ozone.

This interdependence between ozone and climatic factors has been a subject of debate since the 1970s and even more hotly debated since an ozone hole was found over Antarctica.

Ozone depletion is thought to be a slow but steady decrease of ozone within the stratosphere and a faster and much larger increase in the size of the ozone holes over the North and South Poles during winters and early springs. These two events are thought to be caused by ozone-depleting substances, such as bromofluorocarbon compounds (halons) and chlorofluorocarbon (CFC) compounds, which are artificially produced by humans.

Such ozone-depleting chemicals, as formerly found in many aerosol cans in the United States, have been banned (under the Montreal Protocol) or phased out by many industrial countries around the world. Consequently, scientists are hopeful that the ozone layer in the stratosphere may recover sometime in the later half of the twenty-first century. In the meantime, less ozone in the atmosphere results in cooler temperatures. Scientific studies have generally found that the stratosphere has cooled by several degrees over the past decades.

Concurrently, scientists have discovered that greenhouse gases are increasing in the troposphere. Based on experiments, a relationship has been formed between the increased concentrations in greenhouse gases within the atmosphere and the steady depletion of ozone. Ozone depletion gets worse when the stratosphere (where the ozone layer is) becomes colder. Because global warming traps heat in the troposphere (the atmospheric layer below the stratosphere), less heat will reach the stratosphere, which will make it colder.


AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.

ALLOTROPE: One of two or more forms of an element.

BROMOFLUOROCARBONS: A class of compounds, also called halons, that contain bromine, chlorine, and carbon or fluorine. Used as fire retardants in fire-extinguishing systems, these are potent greenhouse gases with high ozone depletion potential.

CHLOROFLUOROCARBONS: Members of the larger group of compounds termed halocarbons. All halocarbons contain carbon and halons (chlorine, fluorine, or bromine). When released into the atmosphere, CFCs and other halocarbons deplete the ozone layer and have high global warming potential.

DEOXYRIBONUCLEIC ACID (DNA): Large, complex molecules found in the nuclei of cells that carries genetic information for an organism's development.

STRATOSPHERE: The region of Earth's atmosphere ranging between about 9 and 30 m (15 and 50 km) above Earth's surface.

TROPOSPHERE: The lowest layer of Earth's atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth's surface.

See Also Aerosols; Antarctica: Observed Climate Changes; Atmospheric Pollution; Global Warming; Greenhouse Gases; Montreal Protocol; Ozone Depletion.



Andersen, Stephen O. Protecting the Ozone Layer: The United Nations History. Sterling, VA: Earthscan, 2004.

Keeling, R. F., ed. The Atmosphere. Boston: Elsevier, 2006.

Parker, Larry. Stratospheric Ozone Depletion. New York: Novinka Books, 2003.

Web Sites

“Ozone: Good Up High, Bad Nearby.” U.S. Environmental Protection Agency. <http://www.epa.gov/air/oaqps/gooduphigh/ozone.pdf> (accessed October 30, 2007).