Concise International Chemical Assessment Document 33

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.

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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).

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