How can smoking cause lung cancer?

Toxins: Irritation, tar and other hundreds of impurities in the cigarettes.

Directly: The contents of cigarette smoke irritates and damages the cells lining the lungs. When the cells undergo repair, errors in replication can occur, leading to cancer. This does not happen every time, and your body can generally fix the errors, but occasionally something gets through, and cancer starts.

Many ways: Smoking accounts for the vast majority (but not all) cases of lung cancer. Cigarette smoke contains many substances (carcinogens) that predispose to cancer. They do this in different ways, like interfering with normal lung cell functions, making them divide more rapidly or die more slowly, or by suppressing the normal immune response to tumors. Don't smoke.