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cause skin cancer, injure eyes, harm the immune system, and upset the fragile balance of an entire ecosystem.

Although, two decades ago, most scientists would have scoffed at the notion that industrial chemicals could destroy ozone high up in the atmosphere, researchers now know that chlorine creates the hole by devouring ozone molecules. Years of study on the ground, in aircraft, and from satellites has conclusively identified the source of the chlorine: human-made chemicals called chlorofluorocarbons (CFCs) that have been used in spray cans, foam packaging, and refrigeration materials.

All About Ozone

Ozone is a relatively simple molecule, consisting of three oxygen atoms bound together. Yet it has dramatically different effects depending upon its location. Near Earth's surface, where ozone comes into direct contact with life forms, it primarily displays a destructive side. Because it reacts strongly with other molecules, large concentrations of ozone near the ground prove toxic to living things. At higher altitudes, where 90 percent of our planet's ozone resides, it does a remarkable job of absorbing ultraviolet radiation. In the absence of this gaseous shield in the stratosphere, the harmful radiation has a perfect portal through which to strike Earth.

Although a combination of weather conditions and CFC chemistry conspire to create the thinnest ozone levels in the sky above the South Pole, CFCs are mainly released at northern latitudes—mostly from Europe, Russia, Japan, and North America—and play a leading role in lowering ozone concentrations around the globe.

Stratospheric ozone occupies the region of the atmosphere between 10 and 50 kilometers from Earth's surface and provides a shield against damaging ultraviolet radiation.

Worldwide monitoring has shown that stratospheric ozone has declined for at least two decades, with losses of about 10 percent in the winter and spring and 5 percent in the summer and autumn in such diverse locations as Europe, North America, and Australia. Researchers now find depletion over the North Pole as well, and the problem seems to be getting worse each year. According to a United Nations report, the annual dose of harmful ultraviolet radiation striking the northern hemisphere rose by 5 percent during the past decade.

Although, two decades ago, most scientists would have scoffed at the notion that industrial chemicals could destroy ozone high up in the atmosphere, researchers now know that chlorine creates the hole by devouring ozone molecules.

During the past 40 years, the world has seen an alarming increase in the incidence of malignant skin cancer; the rate today is tenfold higher than in the 1950s. Although the entire increase cannot be blamed on ozone loss and increased exposure to ultraviolet radiation, there is evidence of a relationship. Scientists estimate that for each 1 percent decline in ozone levels, humans will suffer as much as a 2 to 3 percent increase in the incidence of certain skin cancers.

Exploring Earth's Atmosphere

Like many lines of scientific inquiry, research leading to the prediction and discovery of global ozone depletion and the damaging effects of CFCs followed a path full of twists and turns. Investigators did not set our to determine whether human activity affects our environment nor did they know much about chemical pollutants. Instead, they began with basic questions about the nature of Earth's atmosphere—its composition, density, and temperature distribution.



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