Magnesium Calcium Strontium Barium and Radium Mary Beth Genter PhD DABT

4.0 Barium

4.0.3 Trade Names: UN 1399, UN 1400, UN 1854 4.0.4 Molecular Weight: 137.33

4.0.5 Molecular Formula: Ba

4.1 Chemical and Physical Properties

4.1.1 General Melting point: 725°C; boiling point: 1640°C; specific gravity: 3.51 at 20°C. Barium exists as multiple isotopes: 138 (71.66%); 137 (11.32%); 136 (7.81%); 135 (6.59%); 134 (2.42%); 132 (0.097%); 130 (0.101%) (4). Barium participates in chemical reactions typical of alkaline-earth metals, that is, it reacts violently with acids, water, and carbon tetrachloride. Pure barium metal exists as yellowish-white, slightly lustrous lumps that are somewhat malleable and very easily oxidized (must be kept under petroleum or other oxygen-free liquid to exclude air). Typical of other alkaline-earth metals, barium decomposes in water, evolving hydrogen gas. There are approximately 40 different barium salts. Of these barium salts, approximately half are freely soluble in water, whereas others are practically insoluble (notably barium sulfate and carbonate). Solutions of soluble barium salts give a white precipitate with sulfuric acid or soluble sulfates, and they color nonluminous flame green. Water or acid soluble barium salts should be regarded as poisonous.

4.1.2 Odor and Warning Properties Most barium salts are odorless or have an odor and characteristic of the associated anion.

4.2 Production and Use

Barium carbonate occurs in nature as the mineral witherite; barium sulfate occurs in nature as the mineral barite; also as barytes, heavy spar. Barium is used as a carrier for radium. Alloys of barium with Al or Mg are used as getters in electronic tubes. Barium carbonate is used as rodenticide (4, 60) and in paints, enamels, and marble substitutes (4). Barium sulfate (multiple trade names, including Bakontal, Esophotrast, Micropaque, Raybar) is used as an X-ray-contrast material (4) and as a weighting substance for golf balls (61). Barium nitrate is used in the manufacture of pyrotechnics and green signal lights. Barium sulfide is used as a depilatory and in luminous paints.

4.3 Exposure Assessment

4.3.1 Air Barium can be released into the air during mining and in various industrial processes.

4.3.2 Background Levels Barium's abundance in earth's crust is approximately 0.05%. The background concentration of Ba in groundwater is approximately 0.1 mg/L, although significant regional excursions from this value have been documented (62). Seawater reportedly contains approximately 13 mg/L (5). Various measurements have revealed soil concentrations to range from 15 to 3000 ppm, and the average atmospheric concentration in North America is reported to be 0.12 mg/m (5). Brazil nuts are exceptionally high in barium, with a concentration of 3000-4000 ppm (5). Depending on the geographic site, daily barium intake is estimated to be 3001700 mg/person (63, 64).

4.3.3 Workplace Methods Barium-containing fluxes used in welding can result in significant airborne barium fumes and elevated urinary barium concentrations in exposed workers (65).

4.3.4 Community Methods In addition to workplace exposures, consumer products can be a source of barium exposure. For example, about half of a sample of crayons was demonstrated to contain barium capable of migrating, thus representing a potential source of exposure for children (66). It has also been predicted that some lipsticks can represent a significant source of barium exposure (64).

4.3.5 Biomonitoring/Biomarkers No biomarkers of exposure to barium have been recognized, although methods to measure barium in various physical media have been described (67).

4.3.5.1 Blood Inductively coupled plasma-atomic absorption spectrometry (ICP-AES) has been used for measurement of barium compounds (as Ba) in biological materials, including blood. The detetion limit for Ba in blood by this method is reported to be 0.6 mg/L of blood (68). Neutron activation analysis has also been used for determining levels of Ba in human blood, with detection limits of

7 mg Ba/L of red blood cells and 66 mg Ba/L of plasma (7).

4.3.5.2 Urine Barium concentrations in urine of workers exposed to barium welding fumes reportedly increased, but the method used was not described (65). Subsequently, methods for detecting barium in urine have been published (6, 68, 69). The detection limit for Ba in urine measured by ICP-AES is reported to be 0.26 mg/L of urine (68). A proposed reference value for Ba in urine for biological monitoring purposes is <15 mg/g creatinine (70).

4.3.5.3 Other In the workplace soluble barium compounds can be monitored in air by flame atomic absorption. Samples can be collected on 0.8-mm cellulose ester membranes at a flow rate of 1 to

4.4 Toxic Effects

4.4.1 Experimental Studies 4.4.1.1 Acute Toxicity LD50 values were derived following intravenous administration of soluble barium salts in two strains of mice: Swiss Webster and ICR. The Swiss Webster mice were the more sensitive strain, with LD50 values of 8.1, 8.5, and 11.3 mg/kg for barium chloride, nitrate, and acetate, respectively. The corresponding values for the ICR mice were 19.2, 20.1, and 23.3 mg/kg for barium chloride, nitrate, and acetate, respectively (8). In rats, the oral

LD50 of barium zirconate was reported to be >1.98 g/kg and the inhalation LD50 of barium zirconate was calculated to be 0.42 g/kg (71). A soluble form of barium, barium chloride, was considerably more acutely toxic in rats, with the LD50 calculated to be 220 mg/kg (as Ba) in weanling rats, and

132 mg/kg (as Ba) for adult rats (72). Elevated blood pressure, bronchoconstriction, ECG abnormalities, and myocardial hyperexcitability were demonstrated in guinea pigs administered barium-containing fume extract. These effects could be modified by nifedipine and propranolol (73). Similarly, intravenous administration of barium chloride to rabbits resulted in severe ventricular dysrhythmias, which were relieved by treatment with doxepin or verapamil (74).

4.4.1.2 Chronic and Subchronic Toxicity Barite dust inhaled by guinea pigs was reportedly associated with nodular pulmonary granulation, characteristic of human baritosis (1). A more recent study revealed no pulmonary granulomas following inhalation exposure to barium zirconate, although thickening of alveolar walls, as well as the medial layer of arteriole walls, was noted, with the general picture of a chronic interstitial pneumonitis (71).

The results of at least three subchronic barium chloride drinking-water studies have been published. In the first of these, groups of young adult Charles River rats of both sexes were exposed at concentrations of barium chloride of <250 mg/L (ppm) for 4, 8, or 13 weeks. No significant toxic effects were noted. A tissue distribution study revealed that barium concentration in several tissues increased with dose, but not duration of exposure, and that the highest concentrations were found in bone (72). A subsequent subchronic study in which Fischer 344/N rats and B6C3F1 mice were administered barium chloride in the drinking water at levels up to 4000 ppm revealed renal toxicity as the major toxicological finding (75). Barium-treated male and female rats exhibited higher serum phosphorous than did controls, but other electrolytes and hematological values were within normal ranges. In both species, the animals in the 4000-ppm treatment group exhibited alterations in motor activity, grip strength, and thermal sensitivity. Central nervous system effects were also reported after subchronic-chronic subcutaneous injection of barium chloride (43).

A third barium chloride drinking water study was designed to evaluate the potential in vivo cardiovascular effects of long-term barium exposure. Long-term administration of barium chloride (100 ppm) resulted in hypertension, hypersensitivity to barbiturate anesthesia, disturbances in myocardial energy metabolism, and depressed cardiac excitability, preferentially in the arterioventricular nodal region of the heart (76).

4.4.1.3 Pharmacokinetics, Metabolism, and Mechanisms The barium ion is a chemical antagonist to

potassium, and it appears that symptoms of barium poisoning are attributable to Ba -induced hypokalemia (77). a-Adrenergic responsiveness was found to be enhanced in barium-treated Purkinje fibers (78). Barium mimics calcium in its effects, but their kinetics in subcellular stores may be

different (79). Barium also inhibits Ca -activated K channels, which prolongs excitation and can cause potentially lethal neuroexcitatory and spasmogenic effects (30).

4.4.1.3.1 Absorption Barium, in food sources, is absorbed from the GI tract into the bloodstream at approximately 6% of the administered dose (5). Approximately 75% of inhaled barium is absorbed (5). In beagle dogs, respiratory tract absorption of inhaled particles (1.2-2.1 mm median aerodynamic diameter) contributed significantly to the absorption of BaC^. Approximately 75% of the body burden was absorbed into the circulatory system within hours of inhalation exposure; 55% was attributed to absorption deposited in the pulmonary regions of the lung and 2% to GI absorption (50).

4.4.1.3.2 Distribution Studies in rats showed that barium is cleared from the serum by deposition in bone and teeth (5, 80). Barium is also deposited in muscles, and is stored in lung, with little retention in liver, kidneys, spleen, brain, heart, or hair (81).

4.4.1.3.3 Excretion A study of the metabolism of 140Ba in rats showed the urinary and fecal excretions to be 7 and 20%, respectively (1). The half life in the human body is estimated to be 3.6 days (5).

4.4.1.4 Reproductive and Developmental A subchronic study in which Fischer 344/N rats and B6C3F1 mice were administered barium chloride in the drinking water at levels of ^4000 ppm revealed no anatomical effects in either species, but the offspring of the high dose rats exhibited a marginal reduction in pup weight. No effects were seen on reproductive indices (75). An earlier study in which BaC^ (20 mg) was injected into chick yolk sacs revealed curled toes in 50% of the surviving chicks (Ridgeway and Karnofsky, cited in Ref. 14).

4.4.1.5 Carcinogenesis A National Toxicology Program (NTP) 2-year study with male and female rats and mice, in which barium chloride hydrate was administered in drinking water, reported no evidence of carcinogenicity in either species (82). Bronchogenic carcinomas reportedly developed in

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