The ozone layer, the fragile layer of gas surrounding our planet between 7 and 30 miles above the earth's surface, is being rapidly depleted. Seasonally occurring holes have appeared in it over the Poles and, recently, over densely populated temperate regions of the northern hemisphere. The threat is serious because the ozone layer protects the earth from the sun's ultraviolet radiation, which is harmful to all living organisms.     Even though the layer is many miles thick, the atmosphere in it is tenuous and the total amount of ozone, compared with other atmospheric gases, is small. Ozone is highly reactive to chlorine, hydrogen, and nitrogen. Of these chlorine is the most dangerous since it is very stable and long-lived. When chlorine compounds reach the stratosphere, they bond with and destroy ozone molecules, with consequent repercussions for life on Earth. 　　In 1958, researchers began noticing seasonal variations in the ozone layer above the South Pole. Between June and October the ozone content steadily fell, followed by a sudden increase in November. These fluctuations appeared to result from the natural effects of wind and temperature. But while the low October levels remained constant until 1979, the total ozone content over the Pole was steadily diminishing. In 1985, public opinion was finally roused by reports of a “hole” in the layer. 　　The culprits responsible for the hole were identified as compounds known as chlorofluorocarbons, or CFCs. CFCs are compounds of chlorine and fluorine. Nonflammable, nontoxic and non-corrosive, they have been widely used in industry since the 1950s, mostly as refrigerants and propellants and in making plastic foam and insulation. 　　In 1989 CFCs represented a sizeable market valued at over $1.5 billion and a labor force of 1.6 million. But with CFCs implicated in ozone depletion, the question arose as to whether we were willing to risk an increase in cases of skin cancer, eye ailments, even a lowering of the human immune defense system―all effects of further loss of the ozone layer. And not only humans would suffer. So would plant life. Phytoplankton, the first link in the ocean food chain and vital to the survival of most marine species, would not be able to survive near the ocean surface, which is where these organisms grow. 　　In 1990, 70 countries agreed to stop producing CFCs by the year 2000. In late 1991, however, scientists noticed a depletion of the ozone layer over the Arctic. In 1992 it was announced that the layer was depleting faster than expected and that it was also declining over the northern hemisphere. Scientists believe that natural events are making the problem worse. The Pinatubo volcano in the Philippines, which erupted in June 1991, released 12 million tons of damaging volcanic gases into the atmosphere.