There are two main causes of cancer: heredity and environment. We can do nothing to change our heredity, that is, the basic physical characteristics that were passed on to us by our parents. However, we can do a great deal to control our environment, which may account for 80-90 %of all cancers.

　　Environmental pollution can be divided into two kinds: personal and impersonal. Personal pollution may be defined roughly as unhealthy habits such as smoking, drinking, and eating the wrong foods. Clearly, with enough will power we can control this personal environment.

　　Impersonal pollution, on the other hand, refers to those things which are beyond our individual control. One example would be industrial pollution, which is very hard to control because of the expense involved. It would be expensive in terms of profits and perhaps jobs. The American automobile industry, for example, resisted the law requiring smog-control devices because it feared the added expense would reduce profits. Nevertheless, some laws protecting the environment have been quite successful in reducing pollution.