Alimentary tract absorption @ S. A. IBRAHIM ET AL. increase strontium absorption in humans (Hamilton 1944), A sampleofliterature values of f, for Sr is provided in the Appendix. In general, Sr is absorbed moreefficiently from the alimentary tract when incorporated in dietary components (chronic intakes) compared to other ingested forms. Humandata on the absorption fraction of Sr in soluble forms, dietary sources, and from simulated fallout particles give values ranging from about 0.10 to about 0.50. An f, value of 0.3 was reported by Hardyet al. (1965) for °’Sr absorption. Their calculated value was based on a single U.S. human subject who consumed food products obtained from the Marshall Islands and may be relevant to chronic intakes. The ICRP (1979, 1989, 1993, 1996) and Nuclear Energy Agency (Kendall et al. 1988) adopted an f, value of 0.3 for both acute and chronic intakes based on the upperlimit of absorption for various ingested forms. For occupational exposure, the U.S. Environmental Protection Agency (U.S. EPA 1988) and ICRP (1979) list two different ingestion solubility classes for strontium with f, values of 0.3 and 0.01 for classes D and Y,respectively, or Type M and Type S according to ICRP Publication 66 (1994). In general, isotopes of strontium generated from nuclear tests on coral islands are expected to be in the oxide or the hydroxide form in fresh fallout debris and in the carbonate form in aged material. However, in most tests conducted at the Marshall Islands (land and water), fallout debris was exposed to seawater to various degrees. Chemical analysis of fallout particles from the Mike test indicated the presence of hydrated calcium sulfate (gypsum). Apparently, when calcium hydroxide formed in the fireball is exposed to the sea-water environment, which is rich in sulfate and magnesium ions, calcium can precipitate as sulfate on particle surface to form a shell of gypsum and magnesium hydroxide around its inner core (Heidt et al. 1953; Adamset al. 1958). Strontium and Ba are likely to react with the sea-water sulfate in the same manner. It is well known that Sr and Ba sulfates are very poorly soluble in water; therefore, their biological availability and absorption by humans may also be diminished. Additionally, Libby (1957) hypothesized that in high-yield (megaton) weapons, the fireball remains sufficiently hot, so that ”’Sr will have been produced before the condensation of particles causing “Sr to incorporate in the center of fallout particles. In that case, *’Sr would be less biologically Ba from tests conducted at the Marshall Islands might have been trapped in insoluble particles at close-in locations and, thus, less biologically available. No infor- mation on the alimentary tract absorption of Sr sulfate could be found in theliterature, however. Asfor Ba, the alimentary tract absorption fraction of 'SBa tracer in the oxide form in simulated fallout particles averaged 0.01 and 0.06 for close-in and distant locations, respectively (LeRoy et al. 1966). Barium sulfate is also very poorly absorbed (less than 1%) from the alimentary tract of adult humans (Boender and Verloop 1969; Figueroa et al. 1968). For workers, the ICRP recommendation (1980) for f,; was 0.10 based on literature values for all compoundsof Ba. In more recent ICRP reports (1993, 1996), an f, value of 0.20 was adopted for members of the public equal to Ra (in view of the chemical similarities of Ba and Ra). Several metabolic similarities between Sr, Ba, and Ca were also noted by Leggett (1992). Hamilton (1948) reported that the metabolic behavior of Sr and Ba are essentially indistinguishable in therat. Selected f, values for strontium and barium. Our “best estimates” for f, for Sr under various intake conditions are presented in Table 6. For acute intakes from local fallout, we estimated an f, of about 0.02 for Sr and “Sr from the Marshallese urine bioassay data (Table 4). However, analytical methods and counting technology that were available in the 1950’s for the measurement of beta-emitting Sr isotopes were minimal. Therefore, several sources of uncertainties associated with the urine bioassay data and the derivation of f, for Sr (presented earlier) should be expected. For the above reasons, it was prudent to choose a higher f, value of 0.05 for the acute intakes of Sr isotopes from local fallout. This f, value reflects the likely chemical forms and associated solubility of Sr in coral fallout exposed to sea water (discussed earlier). For chronic intakes, an f, value of 0.3 is selected based on reported literature values for Sr incorporated in food and milk (Appendix). This value is currently adopted by the ICRP (1996)for Sr intakes by membersof the public. Our selected f, value of 0.2 for regional fallout represents the mid-range of Table 6. Selected f, values for strontium. Acute intakes available than in low magnitude (kiloton) weapons where *°Sr would be condensed more on fallout particle sur- faces. As discussed earlier, the solubility of Sr and Ba from close-in fallout simulated particles was much lower than for distant locations (Table 3). Therefore, it is possible that a significant proportion of the total Sr and 241 Exposure conditions Coral tests, all types Landtests, continental soil Direct soil ingestion Local fallout Regional fallout Chronic intakes All locations 0.05 0.15 0.20 0.20 0.30 0.30 — — 0.15