Smoking is currently considered the most important risk factor for lung cancer. There are more than 3,000 chemicals in tobacco, including multi-chain aromatic hydrocarbons (e.g. benzo(a)pyrene) and nitrosamines, which have strong carcinogenic activity. Multi-chain aromatic hydrocarbons and nitrosamines can cause DNA damage to bronchial epithelial cells through various mechanisms, resulting in activation of oncogenes (e.g. Ras gene) and inactivation of oncogenes (e.g. p53, FHIT gene, etc.), thus causing cell transformation and eventually cancer. 2.Atmospheric pollution With the progress of industrialization, the production and use of many industrial raw materials and products with carcinogenic effects are rapidly increasing, and the carcinogenic effects of these substances not only cause damage to direct producers and consumers, but also cause serious pollution to the atmosphere. The smoke and exhaust from modern vehicles such as automobiles and the asphalt widely used in the construction industry can cause serious pollution of the atmosphere. Scientific studies have shown that carcinogenic substances do exist in these pollutants, the most important of which is 3,4-benzopyrene, which has a strong carcinogenic effect and can cause squamous lung cancer in mice in small doses. Lung cancer mortality in many industrial cities is correlated with the amount of benzo(a)pyrene in the air. Residents of cities with high air pollution inhale more benzopyrene in the air than 20 paper cigarettes per day. Another indisputable fact is that the incidence of lung cancer is significantly higher in cities with many factories and heavy air pollution and in rural areas with fresh air and light pollution. A study has found that the concentration of aromatic polycyclic hydrocarbons in the atmosphere is closely related to lung cancer mortality. Surveys in both China and other countries around the world have found that the incidence and mortality rates of lung cancer in urban residents are higher than those in rural areas, and the mortality rates of lung cancer in suburban areas are higher than those in remote suburban areas. These findings clearly tell us that air pollution plays a role in the occurrence of lung cancer. 3.Occupational and environmental exposure Lung cancer is the most important kind of occupational cancer. It is estimated that about 10% of lung cancer patients have a history of environmental and occupational exposure. The following nine occupational environmental carcinogens have been shown to increase the incidence of lung cancer: by-products of aluminum products, arsenic, asbestos, bis-chloromethylether, chromium compounds, coke ovens, mustard gas, nickel-containing impurities, and vinyl chloride. Long-term exposure to beryllium, cadmium, silicon, formalin and other substances can also increase the incidence of lung cancer. Air pollution, especially industrial waste gas, can all trigger lung cancer. 4.Ionizing radiation The lung is a more sensitive organ to radiation. The initial evidence of lung cancer caused by ionizing radiation comes from the information of Schneeberg-joakimov mine. The high concentration of radon and its daughter in the air of this mine induces mostly small cell carcinoma of bronchus. In the United States, it was reported that 70% to 80% of miners mining radioactive ores died of radiation-induced occupational lung cancer, mainly squamous carcinoma. The incidence rate starts to increase when the exposure accumulation of radon and its daughters exceeds 120 working level days (WLM), and increases more significantly up to 20-30 times over 1800 WLM. Exposing mice to these mine gases and dusts can induce lung tumors. Beebe found in the lifelong follow-up of Hiroshima A-bomb survivors that survivors less than 1400m from the blast center had significantly more deaths from lung cancer than survivors 1400-1900m and 2000m away from the blast center. 5, previous chronic lung infections such as tuberculosis, bronchiectasis and other patients, bronchial epithelium in the process of chronic infection may metastasize into squamous epithelium resulting in cancer, but less common. Genetic factors such as family reunion, genetic susceptibility, lowered immune function, metabolic and endocrine dysfunction may also play an important role in the occurrence of lung cancer. Many studies have proved that genetic factors may play an important role in people and/or individuals who are susceptible to environmental carcinogens. For normal people, they should normally take an active role in preventing and avoiding