Although the theme of this chapter emphasizes the genetic basis of cancer, there is no contradiction in considering the role of environment in carcinogenesis. By environment, we include exposure to a wide variety of different types of agents—food, natural and artificial radiation, chemicals, even which viruses and bacteria are colonizing the gut. The risk for cancer shows significant variation among different populations and even within the same population in different environments. For example, gastric cancer is almost three times as common among Japanese in Japan as among Japanese living in Hawaii or Los Angeles. In some cases, environmental agents act as mutagens that cause somatic mutations; the somatic mutations, in turn, are responsible for carcinogenesis. According to some estimates based chiefly on data from the aftermath of the atomic bombings of Hiroshima and Nagasaki, as much as 75% of the risk for cancer may be environmental in origin. In other cases, there appears to be a correlation between certain exposures and risk for cancer, such as the benefits of dietary fiber or low-dose aspirin therapy in lowering colon cancer risks. The nature of environmental agents that increase or reduce the risk for cancer, the assessment of the additional risk associated with exposure, and ways of protecting the population from such hazards are matters of strong public concern. Ionizing radiation is known to increase the risk for cancer. Everyone is exposed to some degree of ionizing radiation through background radiation (which varies greatly from place to place) and medical exposure. The risk is dependent on the age at exposure, being greatest for children younger than 10 years and for older adults. Although there are still large areas of uncertainty about the magnitude of the effects of radiation (especially low-level radiation) on cancer risk, some information can be gleaned from events involving large-scale release of radiation into the environment. The data for survivors of the Hiroshima and Nagasaki atomic bombings, for example, show a long latency period, in the 5-year range for leukemia but up to 40 years for some tumors. In contrast, there has been little increase in cancer detectable among populations exposed to ionizing radiation by the more recent nuclear accident at Chernobyl, with the exception of a significant fivefold to sixfold increase in thyroid cancer among the most heavily exposed children living in Belarus. The increase in thyroid cancer is almost certainly caused by the radioactive iodine 131I that was present in the nuclear material released from the damaged reactor and was taken up and concentrated within the thyroid gland.
Cancer and the Environment
Radiation
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