Barium and barium compounds
11
exposures to barium sulfate can be controlled to less
than 10 mg/m
3
8-h TWA (total inhalable dust). In some
situations, control will be to levels significantly below
this value. Short-term exposures may be higher than this
for some tasks.
EASE Version 2 predicts that during manual
addition of barite to mixing hoppers, exposure to barium
would be 2–5 mg/m
3
with LEV and 5–50 mg/m
3
without
LEV; during dry crushing and grinding, 2–10 mg/m
3
with
LEV and 50–200 mg/m
3
without LEV; and during dry
manipulation in plastics formulation, in the range
2–5 mg/m
3
with LEV and 5–50 mg/m
3
without LEV. These
predictions are consistent with the data from industry.
Barium sulfate is the major barium compound used
in medicinal diagnostics; it is employed as an opaque
contrast medium for roentgenographic studies of the
gastrointestinal tract, providing another possible source
of human exposure to barium (IPCS, 1990).
7. COMPARATIVE KINETICS AND
METABOLISM IN LABORATORY ANIMALS
AND HUMANS
Information on the gastrointestinal absorption of
barium in humans is limited. Lisk et al. (1988) reported the
results of a mass balance study of one man who
consumed a single dose of 179.2 mg barium (species not
reported) in 92 g of Brazil nuts; it was estimated that at
least 91% of the dose was absorbed. Barium excreted in
the urine was 1.8 and 5.7% of the total dietary barium in
two subjects studied by Tipton et al. (1969). Thirty-
seven people were each administered a single dose
(between 88 and 195 µg of barium) of one of five barium
sulfate X-ray contrast media (Clavel et al., 1987). In 24 h,
the total amount of barium collected in the urine ranged
from 18 to 35 µg and showed a positive correlation with
the amount of barium ingested. The eliminated barium
was stated to be in the range 0.16–0.26 µg/g of barium
administered. Another study also indicated that a very
small proportion of barium sulfate was absorbed after
ingestion of barium sulfate as a radiopaque (Mauras et
al., 1983).
A wide range of absorption efficiencies has been
reported in animal studies. The range of reported oral
absorption for all animal studies was 0.7–85.0%. This
large variation may be explained in part by differences in
study duration (length of time that gastrointestinal
absorption was monitored), species, age, and fasting
status of the animals; however, these experimental
parameters did not affect gastrointestinal absorption of
barium consistently among the different studies. The
presence of food in the gastrointestinal tract appears to
decrease barium absorption, and barium absorption
appears to be higher in young animals than in older ones
(US EPA, 1998).
Richmond et al. (1960, 1962a,b) studied the gastro-
intestinal absorption of barium chloride in several animal
species. Gastrointestinal absorption was approximately
50% (barium chloride) in beagle dogs compared with
30% (barium sulfate) in rats and mice. Using the 30-day
retention data from a study by Della Rosa et al. (1967),
Cuddihy & Griffith (1972) estimated gastrointestinal
absorption efficiencies of 0.7–1.5% in adult beagle dogs
and 7% in younger beagle dogs (43–250 days of age).
McCauley & Washington (1983) and Stoewsand et
al. (1988) compared absorption efficiencies of several
barium compounds. Barium sulfate and barium chloride
were absorbed at “nearly equivalent rates” (based on
blood and tissue levels) in rats following a single gavage
dose of similar barium concentrations (McCauley &
Washington, 1983). Similar concentrations of barium
were found in the bones of rats fed diets with equivalent
doses of barium chloride or barium from Brazil nuts.
McCauley & Washington (1983) suggested that the
similarity in absorption efficiency between barium sulfate
and barium chloride may have been due to the ability of
hydrochloric acid in the stomach to solubilize small
quantities of barium sulfate (barium chloride, barium
sulfate, or barium carbonate had been administered to
the rats at a concentration of 10 mg
133
Ba/litre in the
drinking-water at pH 7.0). This is supported by the
finding that barium carbonate in a vehicle containing
sodium bicarbonate was poorly absorbed. The buffering
capacity of sodium bicarbonate may have impaired the
hydrochloric acid-mediated conversion of barium car-
bonate to barium chloride. The results of these studies
suggest that soluble barium compounds and/or barium
compounds that yield a dissociated barium ion in the
acid environment of the upper gastrointestinal tract have
similar absorption efficiencies.
There is no direct evidence in humans that barium
is absorbed by the respiratory tract. However,
Zschiesche et al. (1992) reported increased plasma and
1
(...continued)
model is in use across the European Union for the
occupational exposure assessment of new and existing
substances.
Concise International Chemical Assessment Document 33
12
urine levels of barium compounds in workers exposed to
barium during welding, thus indicating that airborne
barium is absorbed either by the respiratory system or
by the gastrointestinal tract following mucociliary clear-
ance. Following termination of barite exposure, Doig
(1976) showed a clearing of lung opacities in workers.
A suspension of 23, 233, or 2330 mg
133
Ba in
isotonic saline was instilled into the trachea and then
blown into the “deep respiratory tract” of rats (Cember et
al., 1961). Four rats from each group were sacrificed at
intervals up to 20 days after administration, and lungs,
kidneys, spleen, and tracheobronchial lymph nodes were
extracted and examined radiologically for the presence of
the
133
Ba. The clearance half-time of the
133
Ba from the
deep respiratory tract for all dose levels was determined
to be between 8 and 10 days and was not influenced by
the dose administered. Less than 0.1% of the instilled
dose of
133
Ba was detected in the tissues analysed
(excluding lungs).
Animal studies provide evidence that barium com-
pounds, including poorly water soluble barium sulfate,
are cleared from the respiratory tract. Collectively, these
studies suggest that barium is absorbed following inhal-
ation exposure. Morrow et al. (1964) estimated that the
biological half-time of
131
BaSO
4
in the lower respiratory
tract was 8 days in dogs inhaling 1.1 mg barium sulfate/
litre for 30–90 min. Twenty-four hours after an intra-
tracheal injection of
133
BaSO
4
, 15.3% of the radioactivity
was cleared from the lungs.
The barium sulfate was
cleared via mucociliary clearance mechanisms (7.9% of
initial radioactive burden) and via lung-to-blood transfer
(7.4% of radioactivity) (Spritzer & Watson, 1964).
Clearance half-times of 66 and 88 days were calculated
for the cranial and caudal regions of the trachea in rats
intratracheally administered 2 mg
133
BaSO
4
(Takahashi &
Patrick, 1987). Cuddihy et al. (1974) showed uptake of
barium in the bone following inhalation exposure in rats.
Differences in water solubility appear to account
for observed differences in respiratory tract clearance
rates for barium compounds. The clearance half-times
were proportional to solubility in dogs exposed to
aerosols of barium chloride, barium sulfate, heat-treated
barium sulfate (likely oxidized), or barium incorporated in
fused montmorillonite clay particles (Cuddihy et al.,
1974).
No data are available on dermal absorption of
barium compounds.
The highest concentrations of barium (approxi-
mately 91% of the total body burden) are found in the
bone (IPCS, 1990). Reeves (1986) noted that osseous
uptake of barium was 1.5–5 times higher than that of
calcium or strontium. In the bone, barium is primarily
deposited in areas of active bone growth (IPCS, 1990).
The uptake of barium into the bone appears to be rapid.
One day after rats were exposed to barium chloride
aerosols, 78% of the total barium body burden was
found in the skeleton; by 11 days post-exposure, more
than 95% of the total body burden was found in the
skeleton (Cuddihy et al., 1974).
The remainder of the barium in the body is found
in soft tissues, particularly aorta, brain, heart, kidney,
spleen, pancreas, and lung (IPCS, 1990). High concen-
trations of barium are sometimes found in the eye,
primarily in the pigmented structures (Reeves, 1986).
McCauley & Washington (1983) found that 24 h after
administration of an oral dose of
133
BaCl
2
to dogs,
133
Ba
levels in the heart were 3 times higher than in the eye,
skeletal muscle, and kidneys, which had similar concen-
trations. Levels in these tissues were higher than the
whole-blood concentration, suggesting that they con-
centrated barium.
Barium is excreted primarily in the faeces following
oral, inhalation, and parenteral exposure, but it is also
excreted in the urine. At a normal intake level of 1.33 mg
barium/day (1.24, 0.086, and 0.001 mg/day from food,
water, and air, respectively), humans eliminated
approximately 90% of the barium in the faeces and 2% in
the urine (Schroeder et al., 1972). Tipton et al. (1969)
found similar results; in two men studied, 95–98% and
2–5% of the daily barium intake were excreted in the
faeces and urine, respectively. In the tracheal instillation
study of Cember et al. (1961), urine and faeces were
collected for 21 days in two high-dose animals. Faecal
elimination accounted for around two-thirds of the
radioactivity administered, and the urine for around 10%.
Overall, this study indicated that very little of the
administered barium is absorbed, with the majority of the
compound being eliminated in the faeces.
The biological half-times of barium of 3.6, 34.2, and
1033 days were estimated in humans using a three-
component exponential function (Rundo, 1967).
Following inhalation exposure to
140
BaCl
2
–
140
LaCl
2
, a
half-time of 12.8 days was estimated in beagle dogs
(Cuddihy & Griffith, 1972).