The etiology of cancer includes both internal and external causes. External causes refer to various possible carcinogenic factors from the surrounding environment, including chemical carcinogenic factors, physical carcinogenic factors, biological carcinogenic factors and various chronic stimuli, etc. Internal factors refer to the decrease of human body’s anti-cancer ability or various internal factors that are conducive to the function of external cancer-causing factors. I. External carcinogenic factors 1. Chemical carcinogenic factors (1) Coal tar: its main carcinogenic substance is 3 and 4 benzene and castor, which are found in coal smoke of factories, cigarette smoke, automobile exhaust and baked and smoked meat. (2) amino azo dyes: mainly dimethyl amino azo benzene, which is found in artificial cream yellow dyes. (3) Ethylrongamine: mainly contained in the air of aniline printing and dyeing factories. (4) Nitrosamines: their synthetic precursors are widely present in meat, vegetables, cereals and tobacco, with higher levels in spoiled vegetables and foods. In addition, food preservatives, coloring agents, with the preparation itself is the precursors of nitrosamines. Certain areas of the soil and water quality in its high content. (5) aflatoxin: widely present in moldy foods, especially moldy peanuts, corn and cereals contain the most. (6) Arsenic, chromium, nickel and copper: contained in pesticides and industrial materials. Physical carcinogenic factors (1) Ionizing radiation: exposure to medical and industrial x-rays, radioactive isotopes such as radium, oxygen, cobalt and saw, and radioactive contaminated areas (such as Nagasaki and Hiroshima in Japan). (2) Sunlight and ultraviolet radiation: Exposure to bare skin for a long time. (3) Thermal radiation: heating with a furnace, sitting on a hot bed and drinking overheated water or soup. (4) Asbestos, glass wire and alumina: inhaled from the respiratory tract by production workers for a long time in large quantities. Biological carcinogenic factors (1) Viruses: Human infection with certain types of viruses, such as herpes virus, may cause cancer. (2) Parasites: Infection with Schistosoma egypti, Schistosoma japonicum and Schistosoma chinensis may cause cancer. (4) Chronic stimulation Long-term mechanical or inflammatory stimulation, such as cervical erosion, chronic calf ulcer, chronic gastritis, etc., may cause cancer. The majority of human tumor heredity is not the direct heredity of tumor itself, but only the susceptibility to tumor, on the basis of which other external factors are needed for tumor to occur. 2.Racial factors: The incidence of breast cancer in European and American countries is very high, the incidence of stomach cancer in Japan and Baltic countries is significantly higher than other countries, liver cancer is one of the most common cancers in African and Southeast Asian countries, and nasopharyngeal cancer is quite common among Cantonese people. The above information shows that cancer may be related to race, but of course, it is also related to living habits, environmental factors or genetic and other conditions. Except for cancer of reproductive organs and breast cancer, which are obviously more common in women, cancer of bladder, gallbladder and thyroid gland are also more common in women than men, while lung cancer, liver cancer, stomach cancer and nasopharyngeal cancer are more common in men. The age factor also has some significance in the occurrence of cancer tumors. There are a few special tumors, such as retinoblastoma and medulloblastoma, which are more common in children, and osteogenic sarcoma, which is more common in young people. 4.Hormonal factors: Endocrine disorders are closely related to the occurrence of certain organ tumors, such as the occurrence of breast cancer may be related to excessive estrogen. 5.Immune function: Many clinical and experimental studies show that the immune status of the body is closely related to the occurrence of tumor. For example, the incidence of leukemia and lymphoma in people with low immune function is significantly higher than the general population.