Preface

摘要

About one-third of all human cancer cases are due to tobacco smoking. An unbalanced diet is responsible for another third of cancer cases. The causes of the remaining third are diverse, such as aging, occupational exposure, hormonal stimulation of increased cell division, DNAviral infection by various viruses; heavy air pollution might be contributing a little as well. On the other hand, cancer rates have been going down (when adjusted for age), apart from cancer due to smoking (which is still going up, but should go down as people give up smoking). In the past, a source of cancer was occupational exposure to some synthetic chemicals. The chemical industry was expanding rapidly by the end of the 19th century and consequently some workers were exposed to new chemicals. For example, some were exposed to high doses of -naphthylamine for many years, and a high percentage of those workers got bladder cancer. It was at this time that the effect of some industrial chemicals to cause cancer in humans was understood.