In many urban areas, the air is visibly polluted with smog, water vapor mixed with chemicals that result from human activities. These activities include burning fuels and using chemicals in vehicles, homes, and industries. Smog may contain nitrogen oxides, NOx, sulfur oxides, SOx, organic chemicals, small particles, and ozone, O3. Ozone is produced when nitrogen oxides react with oxygen in air under sunlight. Animals and plants are harmed by respiring the pollutants in smog. Other results of air pollution include ozone thinning, global warming, and acid precipitation.

Ozone Thinning

Ozone in the lower atmosphere is harmful, but ozone floating 20 km above Earth in a zone called the stratosphere shields the planet's surface from harmful ultraviolet radiation, or UV light. Unfortunately, several kinds of human-made chemicals contribute to the destruction of the ozone layer. The most important of these are chlorofluorocarbons (KLAWR-oh-FLOOR-uh-KAHR-buhnz), or CFCs. Gaseous CFCs were once widely used as coolants in refrigerators and air conditioners, as propellants in aerosol spray cans, and in the manufacture of plastic-foam products. After floating up to the stratosphere, CFCs act as catalysts for reactions that break down ozone. Scientists have estimated that a single CFC molecule can help destroy up to 100,000 ozone molecules.

Beginning in the 1980s, scientists discovered thinning areas in the ozone layer over Earth's polar regions, as shown in Figure 22-5, and declining ozone levels elsewhere in the layer. An international study estimated that a 10 percent drop in ozone levels would cause 300,000 new cases of human skin cancer worldwide. UV light also harms plants and photosynthetic algae, so ozone depletion could modify entire ecosystems over time.

Damage to the ozone layer led most countries to stop producing CFCs by 1995. Environmental scientists estimate that it will take the ozone layer 50 to 100 years to recover completely. The effort to protect the ozone layer shows that scientists, the public, and policy makers can work together to solve environmental problems.

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