Selenium
Selenium is now firmly established as an essential trace element, although historically it was associated only with toxicity as a result of occupational or environmental exposure (Glover 1970; Yang et al. 1983). Selenium is present in the earth’s crust in relatively small quantities, but some rocks may contain levels of up to 1.5 ppm. Selenium is also found in coal, and the combustion of fossil fuels is an important source of its occurrence as pollution in the environ ment. Most of the earliest reports of selenium toxicity are associated with poisoning in grazing livestock and the cause of ‘alkali disease’ and ‘blind staggers’. The sources of selenium in such cases are particular plants that concentrate selenium when growing in selenium-rich soils. There are many industrial uses of selenium, such as the manufacture of semiconductors, glass and ceramics. Selenium compounds are also used as anti-dandruff agents in shampoos and as gun-blueing compounds, which contain sele-nous acid. Many nutritional supplements now contain selenium compounds, sometimes in relatively high concentrations, which may cause toxicity if ingested in excess (Clark et al. 1996). Selenium is a metalloid and is in group 6 of the periodic table. Common oxidation states of selenium are2 (selenide),4 (selenite)and6 (selen ate). There are also a number of important organ selenium compounds in which selenium is able to substitute for sulfur (e.g. selenocysteine and selenomethionine).
Recent years have seen a large increase in our understanding of the essential biological role of selenium, and important selenoproteins and glutathione peroxidases (GSHPx) are found in most tissues of the body(Thompson1998).Selenium has been shown to have an important role in thyroid function, fertility (particularly sperm motility), mood regulation, immunity to infectious disease and as a cellular ‘antioxidant’, acting in association with vitamins E and C (Reilly 1993; Rayman 2000). Deficiency of selenium is associated with a number of disorders, such as Keshan disease (China), and, more recently with an increased risk of cancer. Selenium can also have a protective role in ameliorating human exposure to mercury (Hansen 1988). In cases of occupational exposure, selenium is associated with signs and symptoms such as skin irritation, garlic breath, metallic taste, painful nail beds and pulmonary oedema.
Acute poisoning has occurred after ingestion or inhalation of selenium compounds. A number of fatalities that involved selenium are well described (Köppel et al. 1986; Matoba et al. 1986; Schellmann et al. 1986; Quadrani et al. 2000). Reference ranges for selenium in whole blood and plasma vary from country to country because of differences in dietary sources of selenium. In the UK, adult plasma reference ranges are generally between 70 and 130 lg/L. In cases of acute poisoning, very high concentrations of selenium may be detected in whole blood, plasma and urine. Blood selenium values <1000 μg/L indicate minimal toxicity, whereas values >2000 lg/L predict serious complications (Gasmi et al. 1997).
