Air pollution People and air pollution are often in the same places. This means that cities with large populations have the biggest problem of dirty air. Air pollution is caused by many different things. A major kind of air pollution is the gas from cars. Statistics (统计资料) show that 93 percent of all auto trips are within cities. Another major kind of dirty air is the burning of coal and oil for energy. This energy is needed to make electricity. Of course, much more electricity is used in the city than in the country. On the average, we throw away more rubbish than the year before. The burning of rubbish makes air pollution even worse. Many major industries are also responsible for the dirty air in the around cities. The fumes (烟) from iron, steel, chemical, and petrol production add particles (微粒) to the air. The effects of air pollution range from mild headaches to death. The levels of pollution found in heavy traffic may cause headaches for loss of clear vision. Wherever coal and oil are used for fuel, fumes may kill trees and plants and cause metal to corrode (腐蚀). In some of the larger cities, these fumes endanger the lives of human beings by contributing to lung diseases and causing early death. 1. The key point of the passage is that .