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On January 27, 1951, the United States began a program of atmospheric testing of nuclear weapons-related devices in Nevada that continued intermittently until August 5, 1963, when the Limited Test Ban Treaty was signed. For nearly fifty years now, the extent and effects of fallout from those tests have generated much political debate and stimulated numerous scientific investigations. The findings of one extended, Congressionally mandated investigation were reported in October 1997 when the National Cancer Institute (NCI) published the two-volume report Estimated Exposures and Thyroid Doses Received by the American People from Iodine-131 in Fallout Following Nevada Atmospheric Nuclear Bomb Tests.

As the NCI was preparing to release its report, the Secretary of the Department of Health and Human Services (DHHS) asked the Institute of Medicine (IOM) and the National Research Council (NRC) to initiate an independent assessment of the public health and medical implications of the iodine-131 (sometimes referred to here as I-131) doses received by the American people and to identify steps that might be taken in response. After consultation, the IOM and the NRC agreed to establish an expert panel to undertake this assessment beginning in October 1997.

This report presents the results of that study. The rest of this chapter offers a brief historical overview, describes the study approach, and provides preliminary technical context for the chapters that follow. To help readers who are not expert in radiation dose reconstruction and related topics, this report includes a glossary of terms. It also includes an extensive set of references to the relevant scientific literature and supplementary appendixes.


During the 1951-1963 period of testing at the Nevada Test Site, nearly 100 nuclear weapons tests were conducted above ground (DOE 1994), and another dozen or so were conducted underground at depths from which some atmospheric release of radioactive material was possible. Well before the Nevada tests began, the United States had the world's first test of a nuclear weapon at Alamogordo, New Mexico in July 1945. Soon thereafter, U.S. bombers dropped atomic bombs on Hiroshima and Nagasaki as part of a strategy to end World War II.

Following the war's end in 1945, the United States began testing nuclear weapons in the Pacific. From 1946 to 1958, 66 nuclear tests were conducted in the Marshall Islands, 12 near Johnson Atoll and 24 near Christmas Island (Simon and Robison 1997). All were atmospheric tests, except for four that were detonated underwater. After the first five Pacific tests, the United States began nuclear testing in Nevada.

Because testing in the Pacific was logistically complex and expensive, the government eventually moved all tests to the Nevada Test Site (NTS), which is located northwest of Las Vegas. This location was selected on the basis of such factors as safety, climate, geology, security, population density, and transportation. A few tests were also conducted at other locations in Nevada, New Mexico, Colorado, Mississippi, and Alaska. In addition to the U.S. testing during the 1950s, the Soviet Union and the United Kingdom also conducted atmospheric and other tests that produced a global fallout of radioactive materials. Subsequently, other nations including France, China, India, and, most recently, Pakistan conducted tests (the latter two involved underground tests only). This later testing has not added greatly to the exposure of the American public, but the additional testing in the 1950s (including a large program of American tests in the Pacific) has complicated efforts to estimate both the fallout of radioactive materials from the Nevada tests and the health consequences of that fallout. In addition to nuclear weapons tests, the backdrop for current concerns about exposure to I- 131 includes the 1986 nuclear accident at Chernobyl and its aftermath, which includes many cases of thyroid cancer in exposed children.

Concern about radioactive fallout north and east of the Nevada Test Site began to emerge soon after weapons testing began. During the earliest years—before 1954—surveillance offsite of the NTS was performed by the Los Alamos Scientific Laboratory and the U.S. Army. The U.S. Public Health Service continued the offsite monitoring during the years 1954 through 1970. After 1970, monitoring activities offsite of the NTS became the responsibility of the Environmental Protection Agency under interagency agreements. Monitoring techniques included exposure-rate measurements along the roads in the counties surrounding the NTS, a fallout collection program using gummed-film devices in nearly every state, aerial monitoring by aircraft (begun in 1963), and other types of air and precipitation measurement (EPA 1984). Systematic studies of fallout patterns were conducted from the earliest test. By the late 1950s, national programs to monitor milk and food contamination had been established because it had become clear that exposure to radioactive iodine represented the major health threat from weapons testing fallout. Such exposure affected primarily the thyroid gland and resulted mostly from the consumption of milk contaminated with iodine-131.

Congressional hearings were held in the late 1950s and in 1963 to consider the effects of radioactive fallout on the public. One of the earliest reports to estimate radiation dose to the thyroid from radioactive fallout was issued in 1963 (Knapp 1963). The limited test-ban treaty of 1963 was another reflection of public concern, and litigation was yet another consequence. By the late 1970s, hundreds of damage claims had been filed against the U.S. government alleging that illnesses, primarily cancers, resulted from the nuclear tests.

In the 1980s, continued concerns prompted a number of further studies to reevaluate radiation exposures of the public from fallout. In 1983, Public Law 97-414 directed the Secretary of the Department of Health and Human Services to conduct research and develop estimates of the thyroid doses received by the American people from iodine-131 in fallout from the Nevada atmospheric tests (see Box 1.1). (This legislation addressed other issues unrelated to radiation, one of which gave rise to its common labeling as the Orphan Drug Act.)

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BOX 1.1

Provisions of Public Law 97-414, Sections 7(a) and (b), Enacted January 4, 1983, Related to I-131. The law directed the Secretary of Health and Human Services to Conduct scientific research and prepare analyses necessary to develop valid and credible (more...)

Following enactment of this legislation, the Secretary requested that NCI respond to the mandate. In 1983, NCI established a task group to assist it in a program of technical and scientific work that extended for more than a decade. It was quickly obvious to the NCI investigators and their advisory group that individual- specific doses could not be calculated with the information available. The advisory group, however, “suggested that it might be possible to estimate, for each atmospheric nuclear weapons test, the iodine-131 exposures from fallout for representative individuals and for the population of each county in the contiguous U.S.” (see NCI [1997a], Volume 1, page 1.1). This became the task undertaken by the NCI investigators. The probability of causation tables referred to in P.L. 97-414 were prepared by the NIH Ad Hoc Working Group to Develop Radioepidemiological Tables and were published in 1985 (NIH 1985).

The October 1997 publication of the NCI report was preceded by the appearance of newspaper articles that presented information from the report's summary. The articles noted that the analyses were still not available to the public nearly 15 years after the 1983 Congressional mandate and several years after the NCI had completed much of its work.

For the approximately 160 million people living in the United States during the 1950s, the NCI report estimates the average cumulative thyroid dose from fallout was about 0.02 Gy (2 rad, see Glossary). However, individual-specific doses can vary substantially and to enable individuals to estimate their dose, the NCI constructed the Web site (http://rex.nci.nih.gov) mentioned earlier in this text. In principle, an individual can consult this site to determine his or her thyroid dose (based on age, residential history, and recall of milk consumption) from the Nevada Test Site weapons tests.

On the basis of the average doses calculated by county for different age groups, NCI staff initially estimated that 7,500-75,000 exposed persons might develop fallout-associated thyroid cancers during their lifetime (see Appendix B), but subsequent efforts by Land to take into account uncertainties in these estimates yielded a range of 11,300-212,000 cancers with a central or point estimate of 49,000 excess cases (C. Land, communication to NAS, 12/19/97). NCI staff and others have also undertaken epidemiological analyses to see if they can provide confirmation for these estimates (see Chapter 3 and Appendix D).


In August 1997, even before the NCI published its report, the Secretary of Health and Human Services asked the Institute of Medicine (IOM) and the National Research Council (NRC) to undertake an independent assessment of the public health and medical implications of the findings presented in the report and to consider steps that might be taken in response. The IOM and the NRC agreed, and work began in October 1997. The study was to

  1. Assess the soundness of the radiation-dose reconstruction including the methods and data used by the National Cancer Institute, the estimates of thyroid doses in the population exposed, and the estimates of the number of cancers expected;
  2. Provide a preliminary assessment of the public-health and medical implications of the NCI thyroid-dose and cancer-risk estimates, in absolute terms and relative to other known risks, and evaluate the potential for other health effects on the thyroid and other tissues;
  3. Provide information that will enable DHHS to educate and inform members of the public, especially those likely to have been most heavily exposed at the most vulnerable ages, as to what the NCI estimates and their attendant uncertainties mean for the individual, what the risks are relative to other environmental risks, and what types of appropriate actions they should take;
  4. Develop recommendations for various research strategies (epidemiologic, animal, in vitro, molecular, or others) that could refine the risk estimates and thus reduce the uncertainty of the estimated effect of I-131 exposures on public health;
  5. Provide information that will enable DHHS to educate members of the medical profession about the NCI report and the limits of present knowledge regarding the carcinogenic potential of I-131; and
  6. Develop recommendations that will help DHHS address the implications identified in objective 2 above in terms of

    intervention, surveillance, education, and information strategies for public-health authorities and health-care providers and

    clinical-practice guidelines for evaluating, treating, and counseling exposed persons.

To undertake the study, the IOM and the NRC established two committees with partially overlapping membership (see rosters at the front of this report). Although the IOM and NRC supported both committees, for convenience this report refers to one committee as the IOM committee and the other as the NRC committee. The IOM committee developed the analyses and guidance on clinical and public health policies and communication presented in Chapter 4. The NRC committee developed the other analyses and guidance and reviewed and approved all the findings of the report. Both committees approved the conclusions and recommendations presented in the Executive Summary. Appendix A describes the committees' activities.


The tasks facing the NCI in 1983—and, subsequently, the IOM and the NRC—were complex. That complexity is reflected in the chapters of the NCI document as listed in Chapter 2 of this report.

As described in Chapter 2, the first technical task for the NRC committee was to review the methods and findings presented in the NCI report. In capsule form, this involved examination of the validity of the use of data from gummed-film collectors as a basis for estimating fallout deposition and related iodine-131 doses, the thoroughness of the identification of the possible pathways of iodine-131 exposure, the reliability and completeness of the measurements available to assess average exposures at the county level, the credibility of the data on milk production and average consumption, the data and soundness of the model to estimate the transfer of iodine into cows' milk, and the other procedures used in the estimation of the doses to representative individuals.

In addition to reviewing the NCI dose reconstruction, the NRC committee considered the estimates of cancer risk that had been presented in a memorandum separate from the main NCI report (see Appendix B). As discussed in Chapter 3, the task here involved reviewing evidence about the link between radiation exposure and thyroid cancer and other thyroid disease; assessing assumptions used in estimating cancer risk; and considering the epidemiological or experimental support for the estimates of cancer risk and the usefulness of further analysis of epidemiological data such as those available in cancer registries supported by NCI and several states.

Chapter 4 examines the criteria for making recommendations about screening for thyroid cancer and reviews evidence about the potential benefits and harms of screening portions or all of the exposed population for thyroid cancer. In developing advice for DHHS about thyroid screening and other possible clinical and public health steps, including communication with the public and with clinicians, the IOM committee relied on the evidence-based approaches to clinical practice that have been developing over more than a decade. The central elements of these approaches stress systematic processes for analyzing scientific evidence about the benefits and harms of clinical practices and developing guidelines for clinician and patient decisions that make clear the assumptions, evidence, and rationale for practice recommendations. Appendix F provides the literature review that provided much of the basis for the analysis in this chapter.

In Chapter 5, the focus is communicating with the public about the information and conclusions presented in both the NCI and the IOM/NRC report. The key conceptual and technical issues examined include the characteristics of risk communication, the matter of credibility of sources of information, the effectiveness of different strategies of risk communication, the need to involve the affected parties in the communication process, and the need to assess the effectiveness of different ways of formulating risk and communicating it to the various interested publics. An example of a method to assist individuals in estimating their personal thyroid cancer risk is provided in Addendum 5. As historical context relevant to understanding government actions and policies during the period of atmospheric weapons testing, a summary of dose limits for exposures of the thyroid for members of the public during this period is included in Appendix E.

Chapter 6 briefly considers directions for future research that might refine the risk estimates and reduce their inherent uncertainties.


An important determinant of the public's reaction to the information about iodine-131 exposure and risks associated with that exposure is that the above-ground nuclear tests were purposive, man-made phenomena that left behind a toxic residue. Since the tests ended, governments and residents of areas adjacent to the test sites have engaged in intermittent, often acrimonious debate about the possible health effects. The historical and political contexts surrounding the Nevada testing program and its aftermath combined with the technical nature of the analyses of iodine-131 exposure, health risks, and screening strategies will make it difficult to communicate information in ways that will be perceived as equally credible and understandable by all those concerned about the consequences of the testing program. The rest of this report offers guidance to the Department of Health and Human Services, but it is only one step in the process of developing credible and helpful communication with the public.