Over 40 different chemicals found in cigarette smoke cause an increase in cancer when given over several years to laboratory rats. Defenders of the tobacco industry frequently claim that no one has proved that cigarette smoking causes lung cancer. In one sense they are right. Proof from epidemiology differs from proof in a laboratory experiment. What we have seen here is that the more closely we can meet the several criteria linking A and B, the more confident we can be that A causes B. Few epidemiological studies have met these criteria better than those studying the statistical relationship between smoking and health.
Smoking is probably the greatest single cause of preventable illness in the United States.