Importance of environmental factors in cancer.

The present paper reviews the nature of epidemiological studies and their importance as a background for determining the hazard of cancer. An attempt is made to explain how risks in the cancer area may be compared by the use of cumulative data over a lifespan as compared to use of age-specific rates. In addition, the classification of the aetiological factors so far identified is summarized and it is pointed out that to date, the most important environmental risks relate to those of cultural habits. Of the 85% of cancers for which the causes have not yet been established, there is circumstantial evidence that the greater part of these are dependent directly or indirectly on environmental factors.