Factories often make the air in big cities very dirty. Millions of tons of waste and poisonous （有毒）gases（气）are sent into the air with smoke. Oil is burnt in cars, trucks and buses. They produce waste gases almost as harmful as the gases from factories. In big cities during cold winter months, many old people die from the polluted air. In places around big factories, the bad air kills trees and vegetables.

　　 Another kind of polltion is noise. Very loud noises can make people ill, hurt their ears, or even drive them mad（发疯的）. Houses near the airports sometimes have their windows broken by the noise of jet planes（喷气式飞机）passing overhead.

　　 Man is fighting a battle（战争）against pollution. The battle won't be won until everyone knows how serious the danger is, and does something to stop it.