Gallium, Indium and Thallium
Galllium, indium and to a lesser extent thallium are listed as semiconductor metals. Gallium and indium are relatively non-toxic elements whereas thallium is cytotoxic. Gallium is known to inhibit certain intracellular proteins, including δ-aminolaevulinic acid, involved in haem synthesis. It binds ferritin and can disturb iron and zinc metabolism but it is not mutagenic and no evidence is seen that its main semiconductor complex, gallium arsenide, is carcinogenic in animals or humans. Indium is also not mutagenic but does induce reactive oxygen radicals. No evidence is seen that it is carcinogenic in epidemiological studies of indium-related industry, but it is irritant and can evoke pulmonary distress following prolonged inhalation. Limited experimental evidence shows that indium phosphide is carcinogenic in long-term rodent studies but results are inconsistent. It is concluded that indium should not be identified as a carcinogen on available evidence. Chemically gallium arsenide and indium phosphide are very stable compounds with a strong lattice-like configuration. Thallium is also non-carcinogenic but is identified as a neurotoxin on account of its profound action in displacing K(I) from dependent enzymes and through impairment of neuromuscular conduction. Epidemiological evidence provided by competent studies in the UK and USA have failed to establish a carcinogenic risk in workers employed in semiconductor-related professions. Tumour numbers are low and, although in excess of expected levels, fail to provide evidence that exposure to semiconductor metals is a cause of increased cancer mortality in any organ system.