How smoking causes lung cancer. Doctors believe smoking causes lung cancer by damaging the cells that line the lungs. When you inhale cigarette smoke, which is full of cancer-causing substances (carcinogens), changes in the lung tissue begin almost immediately. At first your body may be able to repair this damage.But with each repeated exposure, normal cells that line your lungs are increasingly damaged.ronchioles and alveoli. Smoking causes the majority of lung cancers â both in smokers and in people exposed to secondhand smoke. But lung cancer also occurs in people who never smoked and in those who never had prolonged exposure to secondhand smoke.