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.Occupational and environmental exposure Lung cancer is the most important type 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. 3.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 the gas and dust of these mines 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. 4, 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, reduced 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 the population and/or individuals who are susceptible to environmental carcinogens. 6.Atmospheric pollution The high incidence of lung cancer in developed countries is mainly due to the pollution of the atmosphere with harmful substances such as benzo(a)pyrene carcinogenic hydrocarbons from the combustion of oil, coal and internal combustion engines and asphalt road dust in industrial and transportation developed areas. Atmospheric pollution and smoking may contribute to each other and play a synergistic role in the incidence of lung cancer.