radiation environment is lacking, is quite uncertain, or is a matter of dispute. In its review of dose reconstructions for individual atomic veterans, the committee encountered both extremes of difficulty in scenario development.

The task of defining exposure scenarios is the most important part of the dose reconstruction process for atomic veterans. An assumed exposure scenario provides the basis of an estimate of dose, so the adequacy of an estimated dose for purposes of dose reconstruction can be no better than the validity of the assumed scenario. The validity of an assumed scenario often cannot be determined by objective means alone, such as film-badge measurements of dose or a complete and indisputable record of a veteran’s activities. Rather, a considerable amount of subjective judgment is often required in defining exposure scenarios, and it is often the case that more than one scenario is plausible. Thus, it is critical that all plausible scenarios be investigated, especially when plausible alternatives would result in higher estimates of dose. Veterans themselves can often provide information that can be used to develop plausible scenarios.

I.C.2.2 Identification of Exposure Pathways

Radiation doses can be received as a result of external or internal exposure. The term external exposure refers to irradiation of the body by sources outside the body. Only radiation that can penetrate the body surface and irradiate radiosensitive tissues of the skin and deeper organs is of concern with respect to external exposure. For atomic veterans, that radiation includes neutrons and photons (gamma rays and X rays) of any energy produced in a detonation or by decay of radionuclides and higher-energy electrons (beta particles) produced in radioactive decay. Nonpenetrating radiation, such as lower-energy electrons and alpha particles produced in radioactive decay, generally is not of concern in estimating dose from external exposure. External exposure to neutrons and photons usually is assumed to result in nearly the same dose to all body organs and tissues, whereas external exposure to higher-energy electrons results in a dose only to tissues near the body surface, including the skin and lens of the eye.

The term internal exposure refers to irradiation of the body by sources inside the body. Internal exposure can occur as a result of intakes of radionuclides by inhalation, ingestion, or absorption through the skin or an open wound. In cases of internal exposure, all radiation (photons, electrons, and alpha particles) emitted by the radionuclides of concern is important and is taken into account in estimating dose. Internal exposure can result in doses that are nearly the same in all organs or tissues, as when a radionuclide is distributed throughout the body (for example, 3H, 14C, and 137Cs), or doses that are highly nonuniform and occur mainly at sites of radionuclide deposition in the body (for example, irradiation of the thyroid due to intakes of 131I, irradiation of bone surfaces and bone marrow due to intakes of 90Sr, and irradiation of the lungs, bone surfaces, bone marrow, and liver due to inhalation of plutonium).

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