Selenium is an essential trace element for living beings. Earth’s crust contains 0.05 to 0.09 mg per gm of selenium. Sea water contains about 0.06 to 0.12 mg per litre of this element while soils on an average may contain 0.1 to 2.0 ppm this metalloid.
Selenium finds application in production of alloys, rubber manufacture, photocopiers and ceramic industries. About 1-10 mg of selenium is present in fossil fuels per kg and it is from the combustion of these flue’s in urban localities that almost 90% of this element in ambient air is derived. The selenium content of air on an average basis ranges between 0.1 to 10 ng per cubic metres. Fly ash and rock phosphates also contain significant amount of this element.
The availability and toxicity of selenium to living system are department upon its chemical from and solubility. In nature and biological system selenium occurs as selenates (Sa6+), selenites (Se4+), elemental selenium (Se) and selenide (Se2-). Selenates are rather soluble compounds like sulphates and are readily taken up by a biological system whereas selenites and elemental selenium are virtually insoluble.
Selenium compounds may be bio-transformed in the body by incorporation into amino-acids, proteins or by methylation. Biologic role of selenium has been attributed to its incorporation in Se-cystein which goes into the organization of glutathione peroxidase. The enzyme reduces organic peroxides and protects membrane lipids and probably proteins also from oxidant damages.
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Selenium is also a component of haem-oxidase. Selenium deficient diets cause necrosis of tissues of a number of organs including liver, kidney, skeletal muscles and testes in mice. Liver necrosis and cardiac myopathy occur in young pigs which are kept on selenium deficient diets and are prevented by supplementing selenium to the diet (Underwood, 1977).
The role of selenium as an essential element has not been conclusively established for humans. However, reports of use of Sodium selenite in the prophylaxis and treatment of an endemic cardiomyopathy in China and alleviation of muscle pain and tenderness in New Zealand women on intravenous adminsitration of Se-methionine does suggest that selenium may have some important role to play in human system as well (Chen et al, 1980, Vane Rij et al, 1979). Both these reports come from localities where due to geo-chemical reasons selenium intake in indigenous population is pretty low. Selenium depletion has also been reported to be associated with cardio-vascular disease and other cardiomyopathies.
Industrial exposures to hydrogen selenide produce garlic like breath, dizziness and lassitude Eyes and nasal irritation may also occur. In experimental animals 10 ppm is fatal. In rabbits 0.01 ml of selenium oxy-chloride, applied dermally results in death. Selenium oxychloride is a vasicant.lt causes blister formation and destruction of tissues of skin exposed to it.
Acute selenium poisoning affects central nervous system which includes nervousness, drowsiness and at times convulsions. Symptoms of chronic inhalation exposures include pallor, coated tongue, gastro-intestinal troubles, nervousness, liver and spleen damage, mucosal irritation and lumbar pain.
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Consumption of 100-1000 ppm of selenium with diet causes ‘blind stagger’ in mammals. It is usually in seleniferous areas only that clear cut evidence of chronic selenium poisoning is observ”. Discoloured or decaying teeth, skin eruptions, gastro-intestinal disorders, lassitude, partial loss of hairs and nails, impairment of vision, weakness in limbs and respiratory failure are the common symptoms.
Livestock consuming plants containing about 25 ppm of selenium suffer from ‘alkali’ disease which is characterised by lack of vitality, loss of hairs, sterility, atrophy of hooves, lameness and anaemia. Loss of fertility and congenital defects has also been observed in animal experime and it is on the basis of such observations that selenium has been considered embryo-toxic and terratogenic element (Moxen and Rhian, 1943, Schroeder and Mitchner, 1972).
However, increased selenium content has also been correlated with a decreased cancer death rate in humans and apparently it exerts a protective action against some types of cancer (Shamberger et al, 1976). In addition, selenium is an antidote to the toxic effects of some metals like, arsenic, mercury and cadmium. The mechanism underlying the protective action is not clear.