Damage to Ozone Layer Gets Worse In the middle of winter, when snow is falling in many parts of the United States, scientists have sounded a warning to people who plan to spend many hours in the sun this summer．The warning: The sun's summertime rays are more dangerous than once thought. A team of scientists from 80 nations recently reported  to the United Nations that a layer of ozone（臭氧层）in the atmosphere, which protects humans from harmful levels of ultraviolet（紫外线）radiation, will be thinner over the United States this summer．The thinner layer allows more ultraviolet rays from the sun to reach Earth．The extra amount of ultraviolet radiation could cause an increase in the number of cases of skin cancer.Scientists first became concerned about the layer in the mid-1980s when a hole was discovered in the layer above Antarctica during the winter．The hole was caused in chemicals used in refrigerators and air conditioners．When these chemicals are sent out into the atmosphere, they produce gases that destroy the ozone. Concern about the protective ozone layer rose more recently when data from satellites and ground stations showed that ozone levels were dropping over areas other than Antarctica．Low ozone levels were recorded in the spring and summer over the United States and over other populated areas in the world. Although many countries have already begun stopping the use of ozone-destroying chemicals, the new findings are expected to advance the timetable for a total ban of the chemicals. 小题1:The scientists have observed the ozone layer ______．
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