[Determination of the importance of environmental factors in human cancer: the role of epidemiology].

The evidence in support of the assertion that 80% of human cancers are of environmental origin is reviewed. The environment is used in its broadest sense and is taken to include the widespread general exposures of air and water pollution, the work environment, exposures resulting from personal choice such as smoking and drinking, and the diet. Several examples are chosen to show that a wide range of environmental agents can cause cancer. Consideration of descriptive epidemiological data relating to migrants, geographical variation in incidence, changes in risk over time, correlation studies, clusters and case reports, supports an environmental etiology for many cancers the cause of which is at present unkown. The same data can be used to show that a large proportion of cancer is likely to be due to environmental factors. If the lowest rate observed in a series of populations be regarded as representing a residuum of cancers due to non-environmental causes then it is possible to imagine a hypothetical population with a very low rate indeed. However, it is unlikely that such a level could be achieved even if all causal mechanisms were understood as it is possible that factors leading to one cancer may protect against another. Until evidence to the contrary is aduced it is held that the vast majority of human cancers are due to the environment: the reasons why there has been reluctance to accept this conclusion are discussed.