How do we know that cigarette smoking causes lung cancer?

The short answer is - from personal experience.

In twenty five years, involved in the care of patients with lung cancer, more than 3000 cases, taking a careful history of smoking in all patients, I have never seen a case of small cell lung cancer in a non-smoker; nor have I ever seen a case of large cell undifferentiated lung cancer in a non-smoker. I have seen one case of squamous cancer in a non-smoker.

The long answer is that the association of lung cancer with smoking has been proven by large epidemiological studies.

Doll and Hill in England (1)and Hammond and Horn in the U.S. (2) proved beyond any shadow of doubt that cigarette smoking markedly increased the chances that a person would develop lung cancer.

Auerbach described the changes in the bronchial epithelium in relation to smoking and in relation to lung cancer and reproduced all of the pathologic features leading up to lung cancer in dogs with induced cigarette smoking. (3)

Since that time there have been literally thousands of other studies which have consistently reproduced these findings. There is absolutely no reasonable doubt.

CIGARETTE SMOKING CAUSES LUNG CANCER!

In addition, there is now also direct molecular evidence of the causation of cancer by carcinogens found in tobacco. One of the best examples of this type of evidence can be found (full text or abstract) at Science web site.

References:

1. Doll R, Hill AB. A Study of the Aetiology of Carcinoma of the Lung. British Medical Journal 1952;2:1271-1286.
2. Hammond EC, Horn D. Smoking and Death Rates- Report on Forty-Four Months of Follow-up of 187,783 Men. Journal of the American Medical .Association1958;166:1159-72, 1294-1308.
3. Auerbach O , Stout AP, Hammond EC, Garfinkle L. Changes in bronchial epithelium in relation to cigarette smoking and in relation to lung cancer. New Enland J Med August 10, 1961;265:253-67.
4. Hammond EC, Auerbach O, Garfinkle L, et al. Effects of cigarette smoking on dogs. Arch Environ Health 1970;21:740-68.

For information on the etiology of lung cancer try-Etiology