Today cigarette smoking is a widespread habit all over the world. It was reported recently that smokers make up one quarter of the world population， while in China， 33.9 precent above the age of 15 are smokers. It's a very serious problem because not only the old， the youth， but also middle school students and girls have been engaged in smoking. Many heavy smokers say they cannot leave off， although they know smoking is harmful to their health and they have tried again and again to stop. Scientific research shows that smoking is not only harmful to smokers themselves， but also a threat to public health， especially to women and children. The poisonous gas from smoking is one of the major causes of many kinds of diseases. Therefore， many countries have worked out laws forbidding smokers to smoke in public places such as cinemas， hospitals and stations