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2 Introduction OVER THE PAST HALF CENTURY, the United States government has built many facilities for the design and construction of nuclear weapons and for processing radioactive materials. Particularly in the early years of operation, the radioactive discharges from some of these installations could have been high enough to pose a potential hazard to nearby residents. Concern over the possible exposure has prompted efforts to reconstruct doses because the potential hazard can be evaluated only through documentation of the dose. In this document, dose reconstruction is defined as the process of estimating doses to the public from past releases to the environment of radionuclides or chemicals. These doses form the basis for estimating health risks and for determining whether epidemiologic studies are warranted. Past exposures, not current ones, are the focus of this report. It is important to recognize that not all releases have led to public exposure. Moreover, in some cases, the releases have already occurred, but exposure will happen only in the future—for example, in the case of contaminated groundwater that has not migrated off-site. The methods used to estimate risk from a hypothetical release and to estimate the resulting rate and pathway of off-site transport of contaminants can be quite different from those used for retrospective assessments. Although terms such as "low" and "high" are used often to describe exposures or doses, they are subjective, and when they are applied to doses or dose rates they offer little quantitative guidance. To avoid ambiguity, the committee uses "low" and "high'' to describe the magnitude of
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the dose; the terms do not connote a judgment about the significance of the radiation exposure. The definitions recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 1993; see pp. 680–682 and Table 8, Annex F) are used here: A dose is "low" if it does not exceed 0.2 gray1 (Gy, the international unit for absorbed dose) or 20 rad (rad is a unit of absorbed dose), whatever the dose rate. Similarly, a dose rate is "low" if it does not exceed 0.006 Gy hr-1 (6 mGy hr-1 or 0.6 rad hr-1), whatever the accumulated dose. A "high" dose rate, at least in experimental studies, usually is in the range of 6 to 48 Gy hr-1 (600 to 4,800 rad hr-1). Doses above 0.2 Gy (20 rad) generally are subdivided into those that are "intermediate,'' 0.2–2.0 Gy (20–200 rad), and those that are "high," >2 Gy (>200 rad). As a point of reference, the lifetime (70-year) dose from natural background and medical radiation for the average U.S. resident is about 0.3 Gy (30 rad). It is important to note that serious health effects of exposure to ionizing radiation, such as an increase in cancer, have not been observed directly at doses below 0.2 Gy (20 rad) among the survivors of the atomic bombing of Hiroshima and Nagasaki. The risks assumed to occur at doses below 0.2 Gy (20 rad) are, therefore, extrapolations from the risks seen at intermediate and high doses to doses above natural background radiation. ELEMENTS OF DOSE RECONSTRUCTION Dose reconstruction studies typically strive to estimate representative doses, doses to specific persons, or both. Representative doses are doses to people who have received an average dose in a particular region and not doses to specific individuals who lived near a site. Generally, they can be derived from historic records or from data from the scientific literature that describe diet, lifestyle, and history for people in typical age categories in the area. Representative doses illustrate the magnitude of the dose and the importance of specific pathways and contaminants in general situations. Such doses can be used to determine the potential statistical power of a proposed epidemiologic study. Individual doses are estimated for specific real people, and the studies use demographic, residential, or dietary data that these persons provide for the dose calculation. Depending on the circumstances and types of exposure, an epidemiologic study can use individual doses or representative doses. However, epidemiologic studies that use individual doses generally are considered more accurate than are studies that use representative doses. Individual doses also best address public concerns. The criteria for the design of a dose reconstruction project must be expressed in terms of specific questions. These criteria differ depending
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on whether the project is intended primarily to answer questions posed by the public or by researchers who want to learn more about the health effects of radiation. Statistically detectable effects that are of great scientific interest might have little public health significance. That is, a particular health effect can be too small or uncommon to warrant any change in public health policy (for example, offering cancer screening to all the residents of a given area). Moreover, it is probably better public health policy to use limited public health resources to prevent future exposures to radiation (for example, with a readily available home radon abatement program) than it is to detect and treat the consequences of past exposures. TECHNICAL ASPECTS OF DOSE RECONSTRUCTION The analysis of historic data from a site or operation to determine off-site radiation doses involves several steps: Source term analysis consists of estimating the magnitude of releases to the environment of radionuclides and the periods over which they were released, including episodic releases from nonroutine events. Pathway analysis examines the transport of released radionuclides through environmental pathways to determine their concentrations in environmental media to which people were exposed. These media include air, surface and groundwater, and soil, among others. Assessment of radiation doses and risks brings together all of the data on releases, transport, lifestyle and dietary habits, analysis of agricultural and food-distribution practices, and biologic factors, including the use of biologic dosimetry, to determine doses or to corroborate evidence of doses and to estimate the likelihood of disease in the exposed persons. Examination of epidemiologic considerations takes into account the size and demographic structure of the potentially affected population, the availability and quality of information needed to estimate the dose, the medical information needed, and the feasibility of conducting an investigation that is sufficiently informative and free of bias. Uncertainty and sensitivity analysis identifies the importance of changes in the parameters and values used to estimate confidence intervals in the overall analysis of the dose reconstruction (see Glossary for distinction between uncertainty and sensitivity analysis). As mentioned previously, a limited number of dose reconstruction studies have been done and there are lessons to be learned from these efforts. Seven studies, their approaches to dose estimation, and their pitfalls are summarized in Appendix A. Often, as the studies set out in Appendix A attest, a simple approach, which we term a "scoping study," can determine whether a more com-
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prehensive dose reconstruction study is warranted or even possible, either for its own purpose or as the basis for an epidemiologic study. If a full dose reconstruction study is justified, all relevant processes should be identified and quantified for the dose-determining radionuclides emitted from the source or the manufacturing practice under study. The source term characteristics (the chemical and physical form, time dependence, etc., of the release) and the specific geographic, agricultural, and meteorologic conditions associated with the site, also should be determined. Scoping estimates can be made with bounding calculations and relatively simple models. They are intended merely to provide guidance about the amount of investigation required for subsequent stages in the dose reconstruction study. Scoping estimates are difficult to make, however, unless there is an established set of starting and stopping rules. The stopping rules could be based on exposure levels associated with epidemiologic feasibility, exposure levels that are commensurate with health risk, or exposure levels that are clearly a negligibly small fraction of the estimated total exposure. Scoping estimates are most useful for identifying episodes that require more detailed investigation. Scoping studies are discussed further in the following section. Historic records are commonly the foundation of a dose reconstruction project, and it is always preferable to use measured data (historic records) rather than models in the reconstruction of doses. The use of previous summaries of data predicting radiation exposures or environmental concentrations should be avoided if possible, especially for the years being reconstructed. Instead, emphasis should be given to the basic data, such as the records made by workers involved in actual operations and to quality control of the data. If it can be shown that the basic data consistently match the information given in summary reports, then the use of summary documents can be defended (Till 1993). There are two common types of searches of the historic records. In a selective search, documents are sought that are clearly needed to support the research, and specific pieces of information are sought as the study progresses. Its advantage is that finite resources are saved by not having to review all or even a large fraction of the accumulated records. The disadvantage is that important data can be overlooked, possibly leading to deficiencies in the dose reconstruction, and conceivably to a loss of credibility (Till 1993). In a comprehensive search, all records of potential importance are reviewed and catalogued before the dose reconstruction starts. This approach is likely to strengthen the dose reconstruction effort. Whichever approach is used, it is important that the representativeness of the data that are used in the dose reconstruction is carefully evaluated. Epidemiologic study of populations requires the use of quantitative dose reconstruction data. Because different levels of epidemiologic inves-
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tigation demand different amounts of detail and precision in dose estimates, dosimetry and epidemiologic screening efforts will be most informative if they are conducted interactively and in parallel. Delaying an epidemiologic investigation until the dose reconstruction is complete could diminish the usefulness of the reconstruction and jeopardize the epidemiologic study. For example, the farther one gets from the time period of interest the more difficult it becomes to assemble the complete population and to reconstruct the information needed for exposure assessment. Moreover, the likelihood of a variety of biases increases as the publicity surrounding a dose reconstruction broadens. Finally, negative public perception brought about by delays in directly addressing the issues of concern could require additional efforts that cannot be justified scientifically. STRUCTURE OF A SCOPING STUDY Several steps can be identified in the conduct of a scoping study as outlined in Figure 2–1. The call for a study often begins with public input or an expression of strong public concern about observed or suspected health effects experienced by the population near a site. Any data and evidence that supports public concerns are evaluated, and the accessible data are identified. The feasibility and plausibility of a potential study are assessed based on evidence of probable effects. A preliminary epidemiologic assessment is made to estimate the affected population size, identify the demographic composition of the population, review available medical data, estimate the study's statistical power, and assess the ability to conduct a rigorous study in which biases are minimized. A parallel, preliminary dose assessment is done to describe major release streams and environmental pathways, the nature of effluents of concern, and the target population and to give a best estimate of annual organ doses to representative exposed individuals. A qualitative severity assessment, based on conservative and nonconservative ("realistic") assumptions, is done separately for the dosimetric and epidemiologic studies. These could be high-versus-low assessments or they could be based on a point rating scale to be compared against the decision criteria discussed below. Finally, a site is given a priority ranking by comparison with information from similar scoping studies conducted for other sites using the same rules. This process is necessarily iterative, and it is subject to the emergence of additional information including that resulting from any ongoing public involvement. To summarize briefly, For dose reconstruction, a scoping study provides preliminary
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FIGURE 2–1 Proposed structure of a dose reconstruction scoping study. estimates of source term (if needed), environmental transport (if needed), radiation and radionuclide exposures, and radiation dose. A comprehensive, detailed study provides more refined estimates of the same quantities. For epidemiology, the first stage is more of a feasibility study—it
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identifies the population at risk and the available data and predicts the sampling sizes needed to proceed with a comprehensive, detailed study to look for health effects and relationships between health effects and dose. PUBLIC INVOLVEMENT If dose reconstruction studies are to receive public acceptance, members of the public must be involved in their design and implementation. In the past, however, dose reconstruction and epidemiologic studies often were conceived and conducted by individual scientists or by groups of scientists pursuing new information in relative isolation from the lay community. The results of such studies were nevertheless viewed as credible. Times have changed. The public demands that exposed populations be involved in the development and conduct of dose reconstruction studies. The studies frequently are conducted in an atmosphere of suspicion and distrust, with many in the study community believing that their concerns are inadequately addressed. This need to involve concerned parties from the beginning planning and throughout the whole process when risk assessment is involved was recently emphasized by another National Research Council Committee (NRC 1994c). There are at least three important reasons for public involvement: First, public involvement will help ensure that the public's concerns will be addressed; second, it is the only way the public will have confidence in the results of the study; and third, members of the public sometimes have information that is otherwise unavailable to the investigators. For example, local citizens might be aware of data sources, historic documents, or local traditions unknown to the investigators. As soon as possible after a decision is made to study a contaminated area, an advisory or steering committee—an oversight and decision-making body—should be constituted. Members of the public should be appointed, should have access to all information, and should participate equally in decision-making. The deliberations of this advisory committee should be public—the public must be involved through its representatives on the advisory committee. Researchers must make regular progress reports and seek advice on public concerns, making certain that the ideas of the public are considered seriously. Although the public must be involved in oversight and decision-making, the scientific aspects of the study should remain the responsibility of the scientists. To maximize public participation, a public announcement of the proposed study should be made and there should be an open meeting at which the need for and conduct of the study are explained and an opportunity for questions is provided. Public meetings are only one means of
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engaging the public and enlisting its involvement. Electronic and print media should also be used. Members of the press can be allies, not adversaries, in informing the public. Other educational efforts in schools, churches, and service organizations should be considered and undertaken where appropriate. Workshops, which are less formal than public meetings, have proved especially useful in the Fernald Dosimetry Reconstruction Project, permitting the public and the investigators to focus on specific topics and to discuss the scientific aspects of the study. A reading room where relevant materials are gathered is an excellent means of allowing the public to become better informed. In addition to public participation, other means exist for ensuring the credibility of the study. One of these is through periodic review of the study by scientists who are not engaged in its conduct and have no interest, or appearance of interest, in its outcome. In some communities, citizen groups have organized ad hoc epidemiologic studies. Such studies should be discouraged unless they are based on sound scientific principles. After a dose reconstruction study is completed, particular attention should be given to how the results of the study are to be announced. Preliminary analyses or tabulations are often misinterpreted. The procedure for public release of the results should be made clear at the beginning, and whatever that procedure, the results should be presented in ways that will maximize public understanding. The reconstructed external exposures and exposures to ingested or inhaled radionuclides should be given as ranges rather than as point estimates only, and the meaning of such ranges should be explained clearly by comparing the exposures to common exposures such as natural background or common medical radio-diagnostic procedures. SUMMARY AND RECOMMENDATIONS Dose reconstruction studies must rely on solid science, state-of-the-art methods, and careful peer review if they are to be viewed as credible. Ultimately, a dose reconstruction study will be judged by the scientific community primarily on the basis of the technical quality of the study and its contribution to science. However, what the public seeks from a dose reconstruction project is accuracy and candor. Openness, early and continuous public involvement, and clear communication of the study's findings as it progresses can improve the scientific quality of the work. Meaningful public involvement is essential to the success of dose reconstruction studies. To achieve these ends, the committee makes seven general recommendations focusing on the organizational aspects of all dose reconstruction studies:
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In dose reconstruction studies, thorough consideration should be given to the collection of representative data, to an evaluation of their representativeness, to quality control, and to public involvement from the time the decision-making process begins, through the release of any results, and after the study concludes in any follow-up activities. An advisory or steering committee should be established at the outset of a dose reconstruction study. This committee should consist of members of the public and knowledgeable scientists who are not associated with the investigators or their sponsors. The meetings of this committee should be open to the public. Along with its steering duties, this committee should be charged with responsibility for establishing an interactive process to communicate the elements and conclusions of the study to the general public. Dose reconstruction studies should begin with a scoping study—a preliminary analysis—to determine whether a comprehensive dose reconstruction study is needed or even possible, either for its own purposes or as the basis for a comprehensive epidemiologic study. All dose reconstruction studies should be reviewed by groups of scientists and public health officials who are not directly involved in the study, either as participants or as advisors, and time and resources should be allocated for resolving discrepancies in the results. There should be coordination between the dosimetric and epidemiologic efforts, which should begin at the outset of the dosimetric study and continue throughout. Premature dissemination of the results should be avoided. Results should not be disclosed until the dose assessment study is complete, has undergone peer review, and has been published. Dissemination of data being considered during the study is appropriate and desirable. A clear understanding of the public's concern should be gained before the study begins. A dose reconstruction study should not proceed until the design is such that it is likely that the results will address the public's concern. NOTE 1. In this report, units related to ionizing radiation will be given as international units (International System of Units, SI) derived from the seven defined units adopted in 1960 at the eleventh Conférénce Générale des Poids et Mesures (Bureau International des Poids et Mesures 1991). An account of SI units and their application can be found in NCRP Report No. 82 (NCRP 1985). However, values expressed using the SI units of absorbed dose (gray), dose equivalent (sievert), and activity (becquerel) will be followed in parentheses by values expressed using the more traditional units of rad, rem, and curie, respectively. The prefix milli (m) represents one-thousandth, or 0.001, and micro (µ) represents one-millionth, or 10-6.
Representative terms from entire chapter: