In today's world, almost everyone knows that air pollution and water pollution are harmful   to people's health. However, not all the persons know that noise is also a kind of pollution, and that is harmful to human health, too. People who work and live under noisy conditions become deaf  . Today, however, scientists believe that 10 percent of workers in Britain are being deafened by the noise where they work. Many of the workers who print newspapers and books, and who weave   cloth become deaf. Quite a few people living near airports also become deaf. Recently it was discovered that many teenagers   in America could hear no better than 65-year-old people, for these young people like to listen to "pop" music and most of "pop" music is a kind of noise. Besides, noise produced by jet planes or machines will make people's life difficult and unpleasant, or even make people ill or even drive them mad  . It is said that a continuous noise of over 85 decibels   can cause deafness. Nowadays the government in many countries have made laws to reduce or control noise and make it less than 85 decibels. In China, the people's government is trying to solve   not only air and water pollution problems but also noise pollution problem. Recently many teenagers could hear even worse than 65-year-old people in America, because   _  .
they are too young to hear anything clearly their ears are still too small they haven't trained their hearing they listen to "pop" music too much