Epidemiological review of lung cancer in man.

Lung cancer is primarily a 20th century phenomenon; its epidemiology is reviewed and the usefulness of epidemiological studies in providing clues to etiology is evaluated. Internationally, lung cancer accounts for up to 13% of all deaths in age groups above 45 years. It is a disease characterized by large international variations in incidence; in addition, urban-rural differences occur in many countries. Male:female ratios are high, 4 to 5; but after a rise during the first half of the century, these ratios are now declining in a number of countries. Neither racial distribution nor migrant studies indicate that genetic factors play any important role in the etiology of lung cancer. The only possible indication of their playing such a role is the high incidence of lung cancer among Chinese, primarily Cantonese, women. A genetic determination of individual susceptibility has been suggested, but racial differences remain to be demonstrated. The increase in lung cancer that started during the first decades of the 20th century in Europe and North America is consistent with the introduction of a carcinogen into the environment at the turn of the century. Both spatial and temporal trends of lung cancer support the hypothesis that cigarette smoking is the major etiological factor. This is further supported by numerous specific investigations of the problem. A decrease in lung cancer incidence in certain age groups in a number of countries may reflect changing smoking habits and the diminishing tar content of modern cigarettes, thus indicating ways of prevention. Current knowledge of general air pollution seems to ascribe it a secondary role as an etiological factor. The risk of lung cancer is greater in certain occupations, often because of an interaction between industrial air pollution and smoking. It may be concluded that lung cancer is largely a problem of modern society and that the epidemiological results are consistent with an etiological role of preventable environmental factors.