1. Chemical carcinogenic factors 1) Coal tar: its main carcinogenic substance is 3,4-benzopyrene, which exists in coal smoke of factories, cigarette smoke, automobile exhaust and roasted and smoked meat. 2) Ethylaniline: It is mainly contained in the air of aniline printing and dyeing factories. 3) Nitrosamines: The precursors of their synthesis are widely present in meat, vegetables, cereals and tobacco, and are more abundant in spoiled vegetables and food. In addition, food preservatives, coloring agents, etc. are themselves precursors of nitrosamines. Its content is higher in the soil and water quality of certain areas. 4) aflatoxin: widely present in moldy foods, especially moldy peanuts, corn and cereals contain the most. 5) Arsenic, chromium, nickel and copper: contained in pesticides and industrial materials. 2. Physical carcinogenic factors 1) Ionizing radiation: long-term exposure to X-rays, radium, cobalt, saw and other radionuclides, as well as radioactive contaminated areas, such as Nagasaki and Hiroshima in Japan. 2) Sunlight and ultraviolet radiation: Long-term exposure to bare skin. 3) Asbestos, glass wire, alumina: the production workers inhaled a large amount from the respiratory tract for a long time. 3. Biological carcinogenic factors 1) Virus: Human infection with certain types of virus, such as hepatitis B virus, may cause liver cancer. 2) Parasites: Infection with Schistosoma egypti, Schistosoma japonicum and Schistosoma chinensis may cause cancer.