Gunnar Nordberg
Occurrence and Uses
Barium (Ba) is abundant in nature and accounts for approximately 0.04% of the earth’s crust. The chief sources are the minerals barite (barium sulphate, BaSO4) and witherite (barium carbonate, BaCO3). Barium metal is produced in only limited quantities, by aluminium reduction of barium oxide in a retort.
Barium is used extensively in the manufacture of alloys for nickel barium parts found in ignition equipment for automobiles and in the manufacture of glass, ceramics and television picture tubes. Barite (BaSO4), or barium sulphate, is primarily used in the manufacture of lithopone, a white powder containing 20% barium sulphate, 30% zinc sulphide and less than 8% zinc oxide. Lithopone is widely employed as a pigment in white paints. Chemically precipitated barium sulphate—blanc fixe—is used in high-quality paints, in x-ray diagnostic work and in the glass and paper industries. It is also used in the manufacture of photographic papers, artificial ivory and cellophane. Crude barite is used as a thixotropic mud in oil-well drilling.
Barium hydroxide (Ba(OH)2) is found in lubricants, pesticides, the sugar industry, corrosion inhibitors, drilling fluids and water softeners. It is also used in glass manufacture, synthetic rubber vulcanization, animal and vegetable oil refining, and fresco painting. Barium carbonate (BaCO3) is obtained as a precipitate of barite and is used in the brick, ceramics, paint, rubber, oil-well drilling and paper industries. It also finds use in enamels, marble substitutes, optical glass and electrodes.
Barium oxide (BaO) is a white alkaline powder which is used to dry gases and solvents. At 450°C it combines with oxygen to produce barium peroxide (BaO2), an oxidizing agent in organic synthesis and a bleaching material for animal substances and vegetable fibres. Barium peroxide is used in the textile industry for dyeing and printing, in powder aluminium for welding and in pyrotechnics.
Barium chloride (BaCl2) is obtained by roasting barite with coal and calcium chloride, and is used in the manufacture of pigments, colour lakes and glass, and as a mordant for acid dyes. It is also useful for weighting and dyeing textile fabrics and in aluminium refining. Barium chloride is a pesticide, a compound added to boilers for softening water, and a tanning and finishing agent for leather. Barium nitrate (Ba(NO3)2) is used in pyrotechnics and the electronics industries.
Hazards
Barium metal has only limited use and presents an explosion hazard. The soluble compounds of barium (chloride, nitrate, hydroxide) are highly toxic; the inhalation of the insoluble compounds (sulphate) may give rise to pneumoconiosis. Many of the compounds, including the sulphide, oxide and carbonate, may cause local irritation to the eyes, nose, throat and skin. Certain compounds, particularly the peroxide, nitrate and chlorate, present fire hazards in use and storage.
Toxicity
When the soluble compounds enter by the oral route they are highly toxic, with a fatal dose of the chloride thought to be 0.8 to 0.9 g. However, although poisoning due to the ingestion of these compounds does occasionally occur, very few cases of industrial poisoning have been reported. Poisoning may result when workers are exposed to atmospheric concentrations of the dust of soluble compounds such as may occur during grinding. These compounds exert a strong and prolonged stimulant action on all forms of muscle, markedly increasing contractility. In the heart, irregular contractions may be followed by fibrillation, and there is evidence of a coronary constrictor action. Other effects include intestinal peristalsis, vascular constriction, bladder contraction and an increase in voluntary muscle tension. Barium compounds also have irritant effects on mucous membranes and the eye.
Barium carbonate, an insoluble compound, does not appear to have pathological effects from inhalation; however, it can cause severe poisoning from oral intake, and in rats it impairs the function of the male and female gonads; the foetus is sensitive to barium carbonate during the first half of pregnancy.
Pneumoconiosis
Barium sulphate is characterized by its extreme insolubility, a property which makes it non-toxic to humans. For this reason and due to its high radio-opacity, barium sulphate is used as an opaque medium in x-ray examination of the gastrointestinal, respiratory and urinary systems. It is also inert in the human lung, as has been demonstrated by its lack of adverse effects following deliberate introduction into the bronchial tract as a contrast medium in bronchography and by industrial exposure to high concentrations of fine dust.
Inhalation, however, may lead to deposition in the lungs in sufficient quantities to produce baritosis (a benign pneumoconiosis, which principally occurs in the mining, grinding and bagging of barite, but has been reported in the manufacture of lithopone). The first reported case of baritosis was accompanied by symptoms and disability, but these were associated later with other lung disease. Subsequent studies have contrasted the unimpressive nature of the clinical picture and the total absence of symptoms and abnormal physical signs with the well marked x-ray changes, which show disseminated nodular opacities throughout both lungs. The opacities are discrete but sometimes so numerous as to overlap and appear confluent. No massive shadows have been reported. The outstanding feature of the radiographs is the marked radio-opacity of the nodules, which is understandable in view of the substance’s use as a radio-opaque medium. The size of the individual elements may vary between 1 and 5 mm in diameter, although the average is about 3 mm or less, and the shape has been described variously as “rounded” and “dendritic”. In some cases, a number of very dense points have been found to lie in a matrix of lower density.
In one series of cases, dust concentrations of up to 11,000 particles/cm3 were measured at the workplace, and chemical analysis showed that the total silica content lay between 0.07 and 1.96%, quartz not being detectable by x-ray diffraction. Men exposed for up to 20 years and exhibiting x-ray changes were symptomless, had excellent lung function and were capable of carrying out strenuous work. Years after the exposure has ceased, follow-up examinations show a marked clearing of x-ray abnormalities.
Reports of post-mortem findings in pure baritosis are practically non-existent. However, baritosis may be associated with silicosis in mining due to contamination of barite ore by siliceous rock, and, in grinding, if siliceous millstones are used.
Safety and Health Measures
Adequate washing and other sanitary facilities should be provided for workers exposed to toxic soluble barium compounds, and rigorous personal hygiene measures should be encouraged. Smoking and consumption of food and beverages in workshops should be prohibited. Floors in workshops should be made of impermeable materials and frequently washed down. Employees working on such processes as barite leaching with sulphuric acid should be supplied with acid-resistant clothing and suitable hand and face protection. Although baritosis is benign, efforts should still be made to reduce atmospheric concentrations of barite dust to a minimum. In addition, particular attention should be paid to the presence of free silica in the airborne dust.