Fallout from Nuclear Weapons Tests and Cancer Risks

Fallout—What We've Learned

Over the more than five decades since radioactive fallout was first recognized as a potential public-health risk, it has stimulated interdisciplinary research in areas of science as diverse as nuclear and radiation physics, chemistry, statistics, ecology, meteorology, genetics, cell biology, physiology, exposure and risk assessment, and epidemiology.

Individual radionuclides in fallout were recognized early on as opportune tracers by which the kinetic behavior of elements could be studied, both among components of ecosystems and in their transport to people. The phenomenon of fallout, while contributing only modestly to our overall understanding of radiation risks, has taught us much about pathways of exposure and about cancer risks to the public in settings outside the medical and occupational arenas. And in particular, fallout studies helped increase our understanding of health risks from specific radionuclides, for example, I-131. This has made possible the development of the National Cancer Institute's thyroid dose and risk calculator (see "Estimating Your Thyroid Cancer Risk," below).

In the U.S., it took a number of years for the differences in dose and cancer risk from regional and global fallout to be understood. We have learned that the internal doses from global fallout were considerably smaller for the thyroid, but greater for the red bone marrow, than those from Nevada fallout, whereas the doses from external irradiation were similar for Nevada and for global fallout.

We estimate that in the U.S. the primary cancer risks from past exposure to radioactive fallout are thyroid cancer and leukemia, whereas in a very few cases—for example, the Marshall Islands—large internal doses as a result of ingestion of radionuclides have led to significant risks of cancers in the stomach and colon. Our research has quantified the likely number of cancer cases to be expected in the U.S. from Nevada exposures and has contributed to the assessment of risk at other worldwide locations.

Nuclear testing in the atmosphere began 60 years ago. It ended in 1980, in part because of public concerns about involuntary exposure to fallout. By that time, increased cancer risk had been established as the principal late health effect of radiation exposure, based primarily on studies of populations exposed to medical x rays, to radium and radon decay products from the manufacture of luminescent (radium) watch dials and in uranium mining, and to direct radiation from the atomic bombings of Hiroshima and Nagasaki. Since then, organ-specific dose-response relationships for radiation-related risks of malignant and more recently benign disease (for example, cardiovascular disease and benign neoplasms of various organs) have been increasingly well quantified with further follow up of these and other populations, and it is increasingly clear that radiation-related risk may persist throughout life. Fallout studies have substantially clarified the consequences of exposure to specific organs from internal contamination with radioactive materials—for example, I-131 in the thyroid gland—and there is every reason to believe that, on a dose-specific basis, increased risks from fallout should be similar to those from other radiation sources. Our improved understanding of individual radionuclides, radiation dose and related health risk is due in part to decades of study of fallout from nuclear testing; that same understanding today makes us better prepared to respond to nuclear terrorism, accidents or other events that could disperse radioactive materials in the atmosphere.

Estimating Your Thyroid Cancer Risk

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A Web-based calculator developed by the National Cancer Institute is available to anyone wishing to estimate individual thyroid cancer risks associated with exposure to I-131 radiation in fallout from the Nevada Test Site, for persons who lived in the U.S. during the 1950s. The calculator can be accessed through the Internet at its stand-alone Web page (http://ntsi131.nci.nih.gov/) or through the main NCI Web site (http://www.cancer.gov/i131), which provides more general information about the NTS, I-131 and radioactive fallout. Information required for the calculation includes gender, age at exposure, places of residence during the years 1951–71, and sources and approximate amounts of milk consumed.

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