The ozone layer  is a thin layer of gas around the earth. It forms a special "blanket" around the planet. This "blanket" protects the earth from dangerous rays produced by the sun. Without the ozone layer, the earth's climate could warm to dangerous levels. This increase in temperature would threaten our survival. Scientists recently have discovered a hole in the ozone layer above the Antarctica. Now there is great concern that this warning has started, endangering all life on the earth. What has destroyed part of the ozone layer? Scientists do not know for sure. They suspect that our popular spray cans   have played a part in the damage. Many spray cans contain dangerous gases. These gases might have destroyed the ozone layer fifteen miles above the earth's surface, The chemicals used in refrigerators   and air conditioners may contribute to the problem, too. Many scientists believe that the destruction of the ozone layer is a serious problem for people everywhere. Global warming could change weather patterns around the world. Farmers and ranchers  could find it more and more difficult to produce enough food for the earth's people. If more of the sun's dangerous rays reached the earth, there also could be an increase in skin cancers. Some skin cancers might cause people to die. Although some scientists do not believe there is a serious danger, obviously no one wants to take any chance on such a possibility. Can scientists repair the hole in the ozone layer? They really don't know the answer to that question. Can people do anything to save the ozone layer that the earth still has? Scientists stress the need to limit the use of spray cans and other items that contain the dangerous gases. This will not be an easy task. People all over the world use these items. Solving the problem certainly will require commitment and cooperation from everyone. The following may do harm to the ozone layer EXCEPT   _  .
spray cans refrigerators ranchers air conditioners