242 Health Physics reported data on acute mtake from other mgested forms and Sr absorption from simulated distant fallout particles Forlocal fallout of Sr mcorporated m contmental soil, an f, value of 015 1s selected based on few data on the absorption of its silicate form (Hamilton 1944) and simulated local fallout m the oxide form (LeRoy et al 1966) The same f, value (0 15) 1s also chosen for direct soil mgestion from chrome mtakes Literature mformation suggests that overmght fasting of human subjects resulted m mcreased Sr absorption of about 30%, on average (ICRP 1993) The Bravo detonation occurred early m the mormmg when the nearby populations, having had thetr last meal about 12 h earher, were likely m the equivalent of a fastmg condition If fasting conditions can beassessed, f, values may be mcreased by the same ratio (30%) for acute intakes For barium intakesfrom local fallout, an f, value of 0 03 was estimated from the Marshallese urme bioassay data (Table 4) Although “Ba measurements used to derive this f,; value may not be reliable, 1t 1s similar to that estimated for Sr from the bioassay data Given the reported chemical and metabolic similarities between Ba and Sr, we selected an f, value for Ba equalto that for Sr (0.05) for acute intakes from local fallout For chromc intakes, the f, value of 02 adopted by ICRP (1996) appears to be appropriate For regional fallout, the f, value of 01 adopted by ICRP for acute mtakes was selected Theactinide elements (Pu, Np, Am, U) Under high temperatures m nuclear explosions, the actimde elements are expected to form metal oxides If contnental-type soil 1s mcorporated imthefireball, fallout particles will mclude significant amounts ofsilicate and oxide species In nuclear tests on coral islands, the actimde elements are expected to exist in oxide forms, mcorporated in calcium oxide particles The metal oxide forms for most actrmdes are refractory and typically imsoluble under normal environmental conditions In water tests (sub or surface), some of the actimide ele- ments mayfractionate slightly In general, most actimdes are characterized by high soil adsorption (K4), low plant and amma] uptake and low environmental mobility Inhalation 1s generally the major mtake pathway to humans since alimentary tract absorption 1s very low for most actinides However, alimentary tract uptake can contribute to exposure when food with surficial contammation 1s ingested or direct soil mgestion 1s appreciable Direct soil mtake could be a significant modeof exposure im developing countries (Simon 1998) Plutonium (Pu). Typical literature values for Pu f, values are presented in Appendix Reported values August 2010, Volume 99, Number 2 ranged from 2 X 107! to 107° dependmg on the Pu form imgested Thelevel of alimentary tract absorption for Pu 1s influenced by several factors cludmg concentration (mass), oxidation states and bmdmg to complexing agents m water, soil, plant, and anrmaltissues (Harrison etal 1981) Durbin (1975) stated that Pu absorption from the almentary tract 1s affected by the chemical form ingested and that f, values are higher for soluble compounds relative to the hydrolysable salts and msoluble compounds At low mgested mass of Pu, as may be encountered under environmental conditions, absorption may be significantly greater than at high mass mtake as m some ammal studies (ICRP 1989) However, Harrison et al (1981) did not observe such effect m hamsters or rabbits Additionally, when the mgested mass is small, the mfluence of chemical form on Pu absorption seems largely to dimmish (ICRP 1989) Somestudies mdicated that mcorporation of the actrmdes mto plant or animal tissues, as m dietary sources, resulted in a two to four fold mcrease m mgested dose as compared with absorp- tion of Pu salt solutions (Sullivan 1981) In all species studied (rats, mice, gumea pigs, dogs and swine), neo- nates absorbed more actinides than did the adults by two to three orders of magnitude (Sullivan 1981) Also, ammal studies suggest that fastmg can increase Pu absorption by as much as an order of magmtude (ICRP 1986, 2006) The solubility and absorption of Pu oxides also vary with the temperature at which the oxide species was produced For example, Pu systemic absorption, followimg inhalation, 1s higher for low-temperature fired oxides than for high-temperature oxides (ICRP 1986) and it would not be unreasonable to assume that the same 1s valid for mgested Pu oxides On the other hand, a sigmficant part of the “*Pu deposited on the Earth’s surface from fallout was produced from volatilized **U via a(n, y) reaction and subsequent decay of the °U through “*Np and “*Pu (Watters et al 1980) This material 1s formed as smgle atoms and was not subjected to the high temperaturesof nuclear explosions This type of Pu oxide appears to be more soluble than would be expected 1f the oxide had been produced at ngh temperatures Additionally, Sullivan (1981) concluded that the imgestion of smaller particle sizes resulted m higher f, values due to Ingher solubility m the alimentary tract In general, the range of f, values m various ammal species that ingested soluble Pu forms (citrate, chloride, etc ) he between 107% to 1074, and for the oxide forms, the average f, value 1s about 107? (ICRP 1989) The majority of f, values for low mass intake of Pu from global fallout are greater than 10~* (ICRP 1996) Based on a urme bioassay study, Sun and Memhold (1997) estimated an f, value of 4 X 10~ for Pu mcorporated m