Chemistry Reference
In-Depth Information
5.1 Absorption
Absorption of uranium is low by all exposure
routes (inhalation, oral, and dermal). Once uranium
is absorbed, its distribution and elimination are con-
sidered to be independent of the route of exposure
and a function of solubility. The solubilities of some
common uranium compounds are given in Table 1.
In addition, the solubility is high for uranium tetra-
chloride and uranyl acetate and low for ammonium
diuranate. Although uranium tetrachloride is highly
soluble, it is easily hydrolyzed to less-soluble uranyl
chloride and oxidized to insoluble uranium dioxide,
which reduces absorption of the uranium (ATSDR,
1999).
assessments indicate that lung absorption factors are
2-4% for 3-month-old children and 0.2-2% for adults,
on the basis of compound absorbability (ICRP, 1996).
The more soluble uranium compounds (acetate, fl uo-
ride, hexafl uoride, nitrate, and tetrachloride) absorb
into blood from the alveoli within days. Less soluble
compounds (tetrafl uoride, trioxide, and some diox-
ide and octaoxide) remain in the lung and associated
lymph node glands for weeks. The insoluble com-
pounds (other dioxide and octaoxide) can remain in
the lungs for years. The dust after a metal penetra-
tor impacted a hard target contained several oxides
with moderate to low solubility in simulated lung
fl uid; up to 36% dissolved with <10 day half-time
and >58% dissolved with >100 day half-time (DOD,
2004). The degree of insolubility for any sample of
uranium dioxide or uranium octaoxide depends on
the temperature and conditions under which it was
formed, with high-fi red, sintered samples being less
soluble than those produced at lower temperatures
and pressures.
Animal data support that uranium absorption is
a function of particle size and solubility, but when
aerosols of purifi ed uranium compounds were tested,
greater absorption was observed. Animal data on
deposition and absorption in the lung indicate large
species differences (Spoor and Hursh, 1973; Voegtlin
and Hodge, 1949). Absorption of uranium hexafl uo-
ride was 18-40% in rats and 20-31% in guinea pigs
(Leach
et al
., 1984), and of uranium trioxide was 23%
in dogs (Morrow
et al
., 1972). These higher absorp-
tion values could be artifi cialities if ingestion also
occurred. Aerosols produced for high-dose uranium
studies have been observed to settle onto the fur of
the animals, and ingestion occurred through groom-
ing (Roberts, 1998).
5.1.1 Inhalation
The deposition of inhalable uranium dust particles
in the lungs depends on the particle size, and absorp-
tion increases with solubility in biological fl uids.
Particles larger than 10-
m atmospheric median aero-
dynamic diameter (AMAD) preferentially deposit in
the upper respiratory tract and are likely to be trans-
ported promptly (with half-times of 10-100 minutes
[ICRP, 1994]) out of the tracheobronchial region by
mucociliary action and swallowed. Smaller particles
and vapors travel more deeply into the lung and can
reach the alveoli. The high density of uranium typi-
cally causes particles of its dust to have large AMAD
values. This is supported by fi ndings that uranium
workers exposed to high levels of uranium dust had
very low lung burdens, so only a small fraction pen-
etrated into the alveolar region (West and Scott, 1966;
1969). That fraction was estimated to be 1-5% (Harris,
1961). The uranium dust produced when uranium
metal penetrators were fi red against hard targets
contained a range of particle sizes that would enable
it to distribute signifi cantly through the pulmonary
system (average, 2.5
µ
5.1.2 Ingestion
µ
m AMAD; GSD, 6.1
µ
m; range,
Human gastrointestinal absorption of uranium has
been shown to vary from <0.1-6%, depending on the
solubility of the uranium compound. Absorption was
reported as 0.5-5% in a group of four males ingesting
10.8 mg uranium in a soft drink (Hursh
et al
., 1969),
<0.25-0.40% in a group of 12 volunteers given drinking
water high in uranium (Wrenn
et al
., 1989), and 0.5-5%
in another drinking water study (Harduin
et al
., 1994).
Similar results were obtained in dietary balance stud-
ies (Leggett and Harrison, 1995; Spencer
et al
., 1990;
Wrenn
et al
., 1989). ICRP (1995) recommends the use
of 0.2% for insoluble compounds and 2% for soluble
hexavalent compounds for modeling the kinetics of
dietary uranium in humans unless specifi c additional
information is known. Limited data on infants suggest
0.2-40
m). The size distribution decreased rapidly
with time as larger particles settled out more quickly
(DOD, 2004).
ICRP (1994) developed a deposition model for aero-
sols and vapors that applies to uranium. It includes
three levels of particle solubility and a wide range of
particle sizes, with selectable parameters (for gender,
age, and level of physical exertion), fi ve compartments
(representing the portions of the respiratory tract), and
clearance to blood.
Estimates of systemic absorption from inhaled
uranium-containing dusts in occupational settings
on the basis of urinary excretion of uranium range
from 0.76% for mill workers (Wrenn
et al
., 1985) to 5%
for crushermen (Fisher
et al
., 1983). Pharmacokinetic
µ
