Alimentary tract absorption @ S. A. IBRAHIM ET AL. a typical Marshallese diet (chronic intakes). In that study, direct soil ingestion was assumed to be 500 mg d|, though the value was not based on empirical evidence and, hence, the predicted f, value is uncertain. The earlier ICRP (1979) recommended f, value of 1 X 10° for Pu in the oxide and hydroxide forms and 1 X 10% for all other compounds for occupational exposure. The recent ICRP (1996) adopted an f, value of 5 X 10* for adults and 5 X 10° for children during the first year of life for chronic intakes from environmental exposures. Selected f, values for plutonium. Based onliterature values (Appendix) and information presented earlier on Pu characteristics in fallout, we chose the f, values presented in Table 7. For acute intakes from local fallout for both coral and continental tests, we select an f, value of 1 X 10>. This value is supported by ICRP (1979) and U.S. EPA (1988) recommendations for acute intakes of the oxide and hydroxide forms of Pu and reflects the expected low solubility of Pu associated with large particles. Our selected f, value of 5 X 10* for chronic intakes represents the mid-rangeof literature values for Pu incorporated in foodstuffs and is supported by rough estimates for the Marshallese population (Sun and Meinhold 1997) and is equal to the recent ICRP (1996) recommendation for members of the public. For acute intakes of regional fallout we chose f,; = 1 xX 104, which is assumed to be an order of magnitude greater than for local fallout to reflect the increased Pu solubility and bioavailability with distance as discussed earlier. Although there is no direct evidence to show whether fasting would increase Pu absorption in humans, the fact that a marked increase in Pu absorption was observed after fasting in some animal species suggests that it would be prudent to assume that similar effects may occur in man (ICRP 1986). Therefore, f, values for acute intakes may be increased by an order of magnitude, if fasting conditions can be assessed. Safety tests (non-nuclear detonations of nuclear fuel material) typically produce large particles dominated by the refractory Pu oxide forms. The resulting debris is much more insoluble Acute intakes Coral tests, all types Landtests, continental soil Direct soil ingestion than in some fallout. For this reason, the lower f, (10 °) value is selected in this case. The same f, value (10 °) is also appropriate for direct soil ingestion. Neptunium (Np). Neptunium and plutonium are much alike in their chemical and metabolic characteristics. In ICRP (1980), an f, of 0.01 was recommendedfor acute intakes by workers based on the absorption of Np nitrate in rats (Appendix). In ICRP (1986, 1989), an f, value of 1 X 10* was recommended for environmental exposures to low Np mass. The most recent ICRP recommendation (1996) reduced the Np f, value to 5 X 10* for chronic intakes by members of the public, equal to that for Pu. Based on strong similarities, we chose the same f, values for Np as for Pu, for all types of intakes (Table 7). Americium (Am). The chemical and biological behavior of Am is similar to the lanthanide rare earths (Hamilton 1948). Reported f, values for Am in various ingested forms are within similar range to Pu (Appen- dix). In ICRP (1979), an f, value of 5 X 10* was recommendedfor all Am compoundsfor acute intakes in the work place. The most recent recommendation for the Am f, value is 5 X 10* (ICRP 1996) for chronic exposure of the general public, equal to that for Pu. Based on the state of knowledge at this time, we chose the same f, values for Am as for Pu for all types of intakes (Table 7). Uranium (U). The metabolic behavior of U is different from most actinides. In the oxide form, U is slightly more soluble in water than Pu and Am.Its absorption from the alimentary tract is also higher than most otheractinides. Thisis likely dueto its smaller ionic size in the +6 (UO,"’) oxidation state, the most com- monly encountered form in the environment. Uranium uptake and retention in the skeleton is less than most actinides and exhibits higher accumulation in the kidney. Some f, literature values for uranium are presented in the Appendix. In general, U absorption from the alimentary tract increases with increasing solubility of the ingested compound. Literature values indicate frac- tional absorption in the range of 5 X 102 xX 10°. Uranium uptake was found to be substantially greater in animals under fasting condition (Bhattacharyya et al. 1989). Table 7. Selected f, values for plutonium. Exposure conditions 243 Local fallout Regional fallout Chronic intakes All locations 1x 10° 1x 10> 1x 104 1x 107+ 5x 1074 5 x 107+ — — 1x 10° Selected f, values for uranium. Based on the preceding information, our chosen f, values for uranium are presented in Table 8. For acute intakes from local fallout (coral and continental tests), we chose an f, value of 2 X 10 *, consistent with that adopted for workers