参考答案：世界上许多大城市都在遭受诸多环境问题的困扰，空气污染就是其中之一。遭受空气污染几乎是当今世界城市生活无法逃避的一部分。尽管过去20年来许多工业化国家在大气污染控制上取得了一些成绩，空气质量，特别在发展中国家的大城市中，仍在恶化。
不同城市空气污染问题也迥然不同，受到包括工业化和社会经济发展水平和速度等因素的影响。随着城市人口的增加，遭受空气污染的人数也会越来越多。1990年人口在300万或300万以上的城市只有69个，但是现在的这个数字已达到80多个。联合国数据估计显示，到2010年大约有50%的世界人口居住在城市。

解析：[听力原文]
Many of the major cities of the world are confronted with environmental problems. Air pollution is one of them. Exposure to air pollution is now an almost inescapable part of urban life throughout the world. Although some progress has been made in controlling air pollution in many industrialized countries over the last two decades, air quality--particularly in the larger cities of the developing countries--is worsening.
The air pollution problems of the cities differ greatly and are influenced by a number of factors, including the level and rate of industrialization and socioeconomic development. As the population of urban areas rises, the number of people exposed to urban air pollution will also increase. In 1990, 69 cities had populations of 3 million or more, and now over 80 cities will probably be in this category. The United Nations’ estimates indicate that by the year 2010, about 50 percent of the global population will be living in urban areas.