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