Introduction: The Problem
Earth's natural ozone layer in the upper atmosphere is under attack from synthetic chemicals. These include chlorofluorocarbons (CFCs), which were used widely before 1987 when the Montreal Protocol outlawed their manufacture. Before this ban, thousands of tonnes of CFCs entered the atmosphere from air conditioners and refrigerators. CFCs may persist for years before breaking down, and each chlorine atom in CFCs can destroy many thousand ozone (O 3 ) molecules.
Near the ground, ozone is often part of urban smog, but ozone in the stratosphere benefits living things below. Its absorption spectrum has a peak in the UV-B wavelengths emitted by the sun, so most UV-B never reaches ground level (Figure 1).
More than a billion years ago, earth's atmosphere grew rich in oxygen and the ozone layer appeared. Since then life has evolved under a UV-B shadow. Recent increases in UV-B levels may have many consequences for living things. UV-B, twice as energetic as visible red light, can alter DNA, destroy proteins, bleach pigments, and cause tissue burns.
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Updated April 19, 2007