As the years passed, some of these doctors died; Doll and Bradford Hill investigated the cause of death in each ease. Some of the deaths were from lung cancer and two facts emerged quite clearly. First, there was a very clear relationship between cigarette smoking and lung cancer. Second, the chances of dying from the disease increased with: increasing cigarette consumption. Since then a mass of evidence confirming these results has poured in from all over the world. There have been Attempts to finds other explanations of the connection between lung cancer and smoking but none fits the facts as well as the simple explanation that there is something in the cigarette which is it a cause of lung cancer.

As the investigation into British doctors proceeded and .the results of other investigations became available, it became clear that cigarette smoking was an important factor in causing other diseases of which the most important are chronic bronchitis (see Bronchitis, pp 238 39) and coronary heart disease (see Heart disease, pp 674—76). The end result was the demonstration of two important facts: first, cigarette smokers live, on average, shorter lives than non-smokers, and second, that giving up smoking removes this excess risk in proportion to the time elapsed since giving up. British doctors gave up cigarette smoking in large numbers after the facts became known. The evidence shows that in males aged between 35 and 64 in the period from 1953 to 1965, lung cancer deaths rose by 7 per cent in the general population but fell by 38 per cent in doctors. It also revealed that deaths from chronic bronchitis fell by 4 per cent in the general population and by 24 per cent in doctors; heart and blood-vessel diseases rose by 10 per cent in the general population and fell by 6 per cent in doctors.