Selenium (Se), discovered by the Swedish chemist Jakob Berzelius in 1817, is a metalloid and states in group VIA with an atomic weight of 78.96. Selenium has five valence states common in nature, including selenide (2-), elemental Se (0), thioselenate (2+), selenite (4+), and selenate (6+). Selenium is an essential nutrient for humans and animals to form important selenoproteins, including glutathione peroxide, thioredoxin reductase.

The range between the beneficial and harmful concentrations of Se is generally narrow; the minimal Se nutrition levels for animals is about 0.05 to 0.10 mg/kg dry forage feed, while the toxic exposure levels is 2 to 5 mg/kg dry forage. The World Health Organization (WHO) recommends the required dietary intake of Se to be 50-200 μg/day for human adults. Two well-known endemic diseases, Keshan Disease (a degenerative heart disease) and Kaschin-Beck Disease (an osteoarthropathy which causes deformity of the affected joints), were linked to low Se daily intake in Se-deficiency regions. However, when exposed to high levels of Se in diet, Se toxic symptoms of hair and nail loss or nervous system disorders can be observed in Se-enriched areas, such as Enshi, China.