The number of cigarettes smoked per day x the number of years of smoking history > 400 is a high risk of lung cancer There are more than 600 kinds of harmful substances in the smoke produced by smoking, including more than 40 kinds of carcinogenic substances. Studies have shown that smokers are 8-12 times more likely to develop lung cancer, 8 times more likely to develop laryngeal cancer, 6 times more likely to develop esophageal cancer, and 4 times more likely to develop bladder cancer than non-smokers. In addition to smokers, passive smokers are at greater risk. According to statistics, when a cigarette is puffed hard, it can burn up some harmful substances. But most of the time, a cigarette rests on the hand, more spontaneous combustion, it will produce many toxins. Statistically, if one person in a family smokes, their family members have twice the chance of getting lung cancer than the family members of non-smokers, and if two people smoke, their family members are twice as likely to get lung cancer than the family members of non-smokers. There is a medical rule of thumb for smoking, if a person smokes > 400 cigarettes per day x number of years of smoking history, then they are at high risk for lung cancer. If a young person starts smoking at the age of 20 and has 20 cigarettes a day, he is likely to get lung cancer by the time he is 40. So teenagers should never smoke.