Alimentary tract absorption @ S. A. IBRAHIM ET AL. estimate is low and may not be reliable for the reasons stated earlier. The ICRP (1980, 1989, 1996) adopted an f, value of 5 X 10* for both acute and chronic intakes based on the absorption for soluble compounds from animal studies. The f, value for the oxide form from one animal study is about 1 X 10°* (Runkle et al. 1980) and is appropriate for local fallout. The limited data on the absorption of Ru incorporated in diet suggest an f, value of 1 x 10* (Yamagataet al. 1969), and is chosenforall other intakes (Table 9). Other radionuclides Molybdenum (Mo). ”’Mois a moderately fraction- ating radionuclide in fallout debris, particularly in water- surface bursts (Freiling 1961; Crocker 1966). In ICRP (1979), an f, value of 0.8 was adopted for workersforall compoundsother than the sulfide form, for which anf, value of 0.05 was adopted (Appendix). For the public, the f, value was increased to 1.0 in the recent ICRP (1993, 1996) recommendation for chronic intakes with- out much justification. Based on the above information, webelieve that complete absorption (f,; = 1.0) for Mo is unrealistically high when applied to fallout particulates. Furthermore, it is possible that a fraction of the Mo in local fallout debris exposed to seawater may be present as sulfate or sulfide. Due to the lack of fallout-relevant data on Mo, weselected a middle range value between the sulfide and all other forms (f, = 0.2) for acute intakes. For chronic intakes of Mo incorporated in food, Table 9. Summary of selected f, (adult values) for key fallout radionuclides for various intake categories* used in internal dose assessments for the Marshallese populations. Acute intakes Nuclide Iodine (1) Tellurium (Te) Cesium (Cs), Rubidium (Rb) Strontium (Sr) Barium (Ba) Plutonium (Pu), Neptunium (Np), Americium (Am), Cerium (Ce), Yttrtum (Y), Lanthanum (L), Praseodymium (Pr) Local fallout Regional fallout Chronic intakes All locations 1.0 0.2 0.4 1.0 0.2 0.8 1.0 0.3 0.8 0.05 0.05 1X 10° 0.2 0.1 1x 10+ 0.3 0.2 5x 1074 Uranium (U) 2x 10% 2x 10% 2x 10° Zirconium (Zr) Ruthenium (Ru) 2x 10% 1x 107% 2x 10% 1x 107° 1x 10° 1 x 10° Iron (Fe) Cobalt (Co) Zine (Zn) Molybdenum (Mo) Bismuth (Bi) 0.1 0.05 0.35 0.2 0.05 0.1 0.1 0.35 0.2 0.05 0.1 0.1 0.35 0.5 0.05 “Recommendations for f, values for other exposure conditions are presented in the text and in Tables 5-8. 245 a higher f, (0.5) was selected. Dose estimates for Mo based on the above f, values will, however, have a high degree of associated uncertainty. Bismuth (Bi). Based on a limited amount of exper- imental information, ICRP (1980, 1996) adopted an f, value of 0.05 for Bi for both acute and chronic intakes. This value was thought to be appropriate for most common Bi compounds. Given the limited data available, we have no reason to suggest any other value. SUMMARY The purpose of this review was to evaluate the available data and to determine the most appropriate f, values for key fallout radionuclides for use in realistic reconstructions of internal dose from the intake of fallout from nuclear tests conducted at the Marshall Islands in the 1940’s and 1950’s. Currently used f, values adopted by ICRP andotherradiation protection organizations are sometimes conservative and based on data related to radionuclides present in food or as specific forms; some are soluble compounds. Such generic f, values may not be suitable for the specific conditions of exposure to particulate fallout from nuclear weaponstests. Presented f, values are our “best estimates” and were generally based on the mid-range of the available literature values. Specific circumstances of weaponstests conditions were also considered (e.g., radionuclide fractionation and sol- ubility) in our evaluation and used to propose appropriate modifications to currently adopted f, values. Our estimated f, values derived from the urine bioassay data of the Marshallese population were given considerable weight in the selected f, values for Cs, Sr, and Ba presented here. The opportunity to derive empiricallybased absorption parameter values is unique and proved to be significant in our evaluation of f,;. Multiple f, values for various circumstances where persons are exposedto fallout (e.g., close-in vs. distant locations and acute vs. chronic intakes) were presented when supportive information was available (Tables 5—8). Although special attention was given to tests conducted at the Pacific Proving Ground, the approach taken here provides values that should be useful for assessments of exposure to fallout from other regional nuclear tests or events. In the process of evaluation and selection of f, values for the various circumstances of intake, adequate amounts of objective information were not always available and some judgment based on the authors’ experience was applied. For these and other reasons, some degree of uncertainty associated with the f, values presented here is expected. The above examination led to the selection of a consistent and complete set of f; values