Lung cancer is caused by a variety of factors. Research indicates that lung cancer results from a combination of genetic defects, external and internal environmental factors, and personal factors. Numerous studies have confirmed that long-term and heavy smoking is the most significant carcinogenic factor for lung cancer. There is a positive correlation between the amount and duration of smoking and the incidence of lung cancer. The second factor is related to working in asbestos-related industries, which very much relates to the work environment, and also has a certain relation to smog. Indoor environmental pollution is also a very significant carcinogenic factor for lung cancer. This includes smoke from burning coal indoors, fumes from cooking oils in kitchens, and carcinogenic substances released by indoor decorating materials. Fourthly, internal human factors such as the immune status, metabolic abnormalities, genetic factors, and chronic lung infections are also related. The fifth factor involves irregular diet and rest habits, irritating foods, alcohol consumption, and heavy smoking.