Tobacco is by far the single most important cause of cancer in developed countries. When it is chewed it can cause cancers of the mouth and esophagus and when tobacco is smoked it is a major cause of cancers of the mouth, pharynx, esophagus, larynx, lung, pancreas, renal pelvis, and bladder. For these eight type of cancers epidemiological evidence indicates that prolonged smoking of average numbers of cigarettes per day increases the risk 3 to 20 times. It is also now clear that cigarette smoking also causes a proportion of several other types of cancer, increasing the incidence up to twice that in non-smokers: namely, cancers of the lip, nose, nasopharynx, stomach, liver, renal and myeloid leukemia. Although the proportional increases are not large, the consistency of the findings in different countries, the evidence of dose response relationships, the lower mortality in ex-smokers than in continuing smokers confirm the relationship between these type of cancers and tobacco.
In sum, smoking is estimated to have caused 30 per cent of all fatal cancers in the United Kingdom in 2005, down from 34 per cent 20 years earlier. The reduction was substantial in men (down from 52 per cent to 40 per cent) but it was largely counteracted by the increase in women (from 12 per cent to 20 per cent). In men, there have been decreases in some developed countries, but increases in others, particularly in central and eastern Europe. In women, the proportion of cancer deaths attributed to smoking was generally low in 1985, but has subsequently increased in all the developed countries and must be expected to increase further.
Tobacco related cancer cases are however lower in countries such as France, where very few middle-aged or elderly women had been smoking for long enough time for any material effect to be produced. In developing countries, the effects of smoking have only recently begun to be studied systematically and much remains unclear. In general, women in developing countries do not smoke or if they do they smoke very little. In men, however, there has been a very large increase in cigarette consumption, the full effects of which is yet to materialize. China, with 20 per cent of the world’s population, smokes 30 per cent of the world’s cigarettes and by 2000 smoking was already responsible for about 20 per cent of male cancer deaths in China.
In India many men have been smoked ‘bidis’ (small home-manufactured cigarettes) for decades. There the proportion of occurrence of cancer may be even greater. It is chiefly because in India there is a widespread of habit of chewing quids containing betel and tobacco. Smoking can act as an aggravating factor for the cause of cancers of the mouth, esophagus, or stomach in those who habitually chew such quids containing betel and tobacco. In some parts of South America, the male lung cancer rates from smoking are already as high as in developed countries. Overall, tobacco may be causing about as many cancer deaths in developing as in developed countries. On the whole tobacco would be responsible for about 20 per cent of cancer deaths throughout the world.