Occurrence and Uses
In nature, Indium (In) is widely distributed and occurs most frequently together with zinc minerals (sphalerite, marmatite, christophite), its chief commercial source. It is also found in the ores of tin, manganese, tungsten, copper, iron, lead, cobalt and bismuth, but generally in amounts of less than 0.1%.
Indium is generally used in industry for surface protection or in alloys. A thin coat of indium increases the resistance of metals to corrosion and wear. It prolongs the life of moving parts in bearings and finds wide use in the aircraft and automobile industries. It is used in dental alloys, and its “wettability” makes it ideal for plating glass. Because of its resistance to corrosion, indium is utilized extensively in making motion picture screens, cathode ray oscilloscopes and mirrors. When joined with antimony and germanium in an extremely pure combination, it is widely used in the manufacture of transistors and other sensitive electronic components. Radioisotopes of indium in compounds such as indium trichloride and colloidal indium hydroxide are used in organic scanning and in the treatment of tumours.
In addition to the metal, the most common industrial compounds of indium are the trichloride, used in electroplating; the sesquioxide, used in glass manufacture; the sulphate; and the antimonide and the arsenide used as semiconductor material.
No cases have been reported of systemic effects in humans exposed to indium. Probably the greatest current potential hazard comes from the use of indium together with arsenic, antimony and germanium in the electronics industry. This is due primarily to the fumes given off during welding and soldering processes in the manufacture of electronic components. Any hazard arising from the purification of indium is probably attributable to the presence of other metals, such as lead, or chemicals, such as cyanide, used in the electroplating process. Exposure of the skin to indium does not seem to present a serious hazard. The tissue distribution of indium in various chemical forms has been studied by administration to laboratory animals.
The sites of highest concentration were kidney, spleen, liver and salivary glands. After inhalation, widespread lung changes were observed, such as interstitial and desquamative pneumonitis with consequent respiratory insufficiency.
The results of animal studies showed that the more soluble salts of indium were very toxic, with lethality occurring after administration of less than 5 mg/kg by way of parenteral routes of injection. However, after gavage, indium was poorly absorbed and essentially non-toxic. Histophathological studies indicated that death was due primarily to degenerative lesions in the liver and kidney. Minor changes in the blood have also been noted. In chronic poisoning by indium chloride the main change is a chronic interstitial nephritis with proteinuria. The toxicity from the more insoluble form, indium sesquioxide, was only moderate to slight, requiring up to several hundred mg/kg for lethal effect. After administration of indium arsenide to hamsters, the uptake in various organs differed from the distribution of ionic indium or arsenic compounds.
Safety and Health Measures
Preventing the inhalation of indium fumes by the use of correct ventilation appears to be the most practical safety measure. When handling indium arsenide, safety precautions such as those applied for arsenic should be observed. In the field of nuclear medicine, correct radiation safety measures must be followed when handling radioactive indium isotopes. Intoxication in rats from indium-induced hepatic necrosis has been reduced considerably by administration of ferric dextran, the action of which is apparently very specific. The use of ferric dextran as a prophylactic treatment in humans has not been possible owing to a lack of serious cases of industrial exposures to indium.