The causes of lung cancer are multiple, but the most recognized and important cause is smoking. The risk of lung cancer from smoking is related to the amount of cigarettes smoked, the number of years smoked, the type of cigarettes smoked, and the age at which smoking began. The incidence of lung cancer in smokers is 8 to 20 times higher than in nonsmokers. The relative risk of small cell lung cancer from smoking is 17.5 for non-small cell lung cancer and 6.7 for non-small cell lung cancer. There is a positive relationship between the amount of smoking and the risk of lung cancer; long-term smoking of 10-20 cigarettes per day is associated with a 15% incidence of lung cancer; smoking 21-40 cigarettes per day is associated with a 28% incidence of lung cancer; and smoking more than 40 cigarettes per day is associated with a 68% incidence of lung cancer. The risk of lung cancer is still 94.4% when you stop smoking for 1 year, 85.2% when you stop for 2-3 years, 56.1% when you stop for 4-9 years, 39.3% when you stop for 10-15 years, and 5.3% even when you stop smoking for 16 years. The main carcinogenic chemicals in tobacco are nitrosamines. The International Agency for Research on Cancer (IARC) has conducted a comprehensive study based on recent epidemiological findings from around the world and determined that tobacco is a definite human carcinogen. The increased incidence of lung cancer from passive smoking has received widespread attention. It has been found that although passive smokers inhale the side streams of cigarette combustion, the air passing through the side streams is less, the combustion is incomplete and the temperature is lower, which is suitable for the formation of carcinogenic polycyclic aromatic hydrocarbons, so the content is higher than that of the central streams.