Gunnar Nordberg
Iridium (Ir) belongs to the platinum family. Its name derives from the colours of its salt, which are reminiscent of a rainbow (iris). Although it is very hard and the most corrosion-resistant metal known, it is attacked by some salts.
Occurrence and Uses
Iridium occurs in nature in the metallic state, usually alloyed with osmium (osmiridium), platinum or gold, and it is produced from these minerals. The metal is used to manufacture crucibles for chemical laboratories and to harden platinum. Recent in vitro studies indicate the possible effects of iridium on Leishmania donovani and the trypanocidal activity of iridium against Trypanosoma brucei. Ir is used in industrial radiology and is a gamma emitter (0.31 MeV at 82.7%) and beta emitter (0.67 MeV at 47.2%). 192Ir is a radioisotope which has also been used for clinical treatment, particularly cancer therapy. It is one of the most frequently used isotopes in interstitial brain irradiation.
Hazards
Very little is known about the toxicity of iridium and its compounds. There has been little opportunity to note any adverse human effects since it is used only in small amounts. All radioisotopes are potentially harmful and must be treated with appropriate safeguards required for handling radioactive sources. Soluble iridium compounds such as iridium tribromide and tetrabromide and iridium trichloride could present both toxic effects of the iridium or the halogen, but data as to its chronic toxicity are unavailable. Iridium trichloride has been reported to be a mild irritant to the skin and is positive in eye irritation test. Inhaled aerosol of metallic iridium is deposited in the upper respiratory ways of rats; the metal is then quickly removed via the gastrointestinal tract, and approximately 95% can be found in the faeces. In humans the only reports are those concerning radiation injuries due to accidental exposure to 192Ir.
Safety and Health Measures
A radiation safety and medical surveillance programme should be in place for persons responsible for nursing care during interstitial brachytherapy. Radiation safety principles include exposure reduction by time, distance and shielding. Nurses who care for brachytherapy patients must wear radiation monitoring devices to record the amount of exposure. To avoid industrial radiography accidents, only trained industrial radiographers should be allowed to handle radionuclides.