Evidence that the various common types of cancer are largely avoidable diseases is reviewed. Life-style and other environmental factors are divided into a dozen categories, and for each category the evidence relating those particular factors to cancer onset rates is summarized. Where possible, an estimate is made of the percentage of current U.S. cancer mortality that might have been caused or avoided by that category of factors. These estimates are based chiefly on evidence from epidemiology, as the available evidence from animal and other laboratory studies cannot provide reliable human risk assessments. By far the largest reliably known percentage is the 30% of current U.S. cancer deaths that are due to tobacco, although it is possible that some nutritional factor(s) may eventually be found to be of comparable importance. The percentage of U.S. cancer deaths that are due to tobacco is still increasing, and must be expected to continue to increase for some years yet due to the delayed effects of the adoption of cigarettes in earlier decades.
Trends in mortality and in onset rates for many separate types of cancer are studied in detail in appendixes to this paper. Biases in the available data on registration of new cases produce apparent trends in cancer incidence which are spurious. Biases also produce spurious trends in cancer death certification rates, especially among old people. In (and before) middle age, where the biases are smaller, there appear to be a few real increases and a few real decreases in mortality from some particular types of cancer, but there is no evidence of any generalized increase other than that due to tobacco. Moderate increases or decreases due to some new agent(s) or habit(s) might of course be overlooked in such large-scale analyses. But, such analyses do suggest that, apart from cancer of the respiratory tract, the types of cancer that are currently common are not peculiarly modern diseases and are likely to depend chiefly on some longestablished factor(s). (A prospective study utilizing both questionnaires and stored blood and other biological materials might help elucidate these factors.)
environmental exposures, much has been learned about exogenous carcinogens through study of their effects or noneffects in children: 1. Ionizing radiation poses some risk no matter how small the dose. 2. Concepts about the viral etiology of cancer have had to be adapted to fit observations in children concerning candidate viruses. 3. Transplacental chemical carcinogenesis has become a reality and poses an increasing threat as chemical pollution worsens. 4. Questions have been raised about the risk of breast feeding in (at present) rare instances when the mother has been heavily exposed to chemicals that are excreted in the fat of breast milk. 5. A few drugs administered to children induce cancers within the pediatric age-span. The pediatrician must take action not only against exogenous agents that induce cancer while the patient is under his care, but also against exposures that begin in utero and lie latent or accumulate throughout life to give rise to cancers in the years or decades ahead. There is much more to carcinogenesis than the effects of the environment. Important information has been gained about the origins of cancer and about human biology in general through studies of children who are unusually susceptible to certain forms of neoplasia. Knowing the mechanisms involved may lead to new modes of treatment, to screening tests for environmental carcinogens or to methods for detecting cancer early enough for treatment to be life-saving.