Male smokers who started smoking before age 20, have smoked for >20 years, smoke >20 cigarettes per day, and are >45 years old are at high risk for lung cancer. The “lung cancer risk index” = number of cigarettes smoked per day x number of years of smoking, if this index is ≥ 400, it indicates that the smoker is at serious risk of lung cancer and must be extra careful. In addition, smoking and other cancer-causing factors often have a synergistic effect, and smokers with a family history of tumors or a history of cancer-causing occupations should also be on high alert. There are data showing the benefits of quitting at all ages, and quitting before age 30 can reduce the risk of lung cancer by 90%. The benefits of quitting slowly become apparent after 5 years of quitting, when the incidence of lung cancer decreases; it is only after 15 years of quitting that the incidence of lung cancer is similar to that of nonsmokers. A 50-year prospective study of British male physicians found that smokers died on average about 10 years earlier compared to nonsmokers; quitting at age 60, 50, 40, or 30 years gained about 3, 6, 9, or 10 years of life expectancy, respectively.