[11] Symptoms include (in decreasing order of frequency): diarrhea, fatigue, hair loss, joint pain, nail brittleness or discoloration, nausea, headache, tingling, vomiting, and fever.

[15] The red α, β, and γ forms are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS2).

It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, and also has similarities to arsenic.

[37] Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jacob Berzelius and Johan Gottlieb Gahn.

[38] Both chemists owned a chemistry plant near Gripsholm, Sweden, producing sulfuric acid by the lead chamber process.

Selenium occurs naturally in a number of inorganic forms, including selenide, selenate, and selenite, but these minerals are rare.

In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff.

[76] Selenium was used as the photoabsorbing layer in the first solid-state solar cell, which was demonstrated by the English physicist William Grylls Adams and his student Richard Evans Day in 1876.

[81] However, a significant deficit in the open-circuit voltage is currently the main limiting factor to further improve the efficiency, necessitating defect-engineering strategies for selenium thin-films to enhance the carrier lifetime.

One notable exception is in power DC surge protection, where the superior energy capabilities of selenium suppressors make them more desirable than metal-oxide varistors.

[91] Ionized selenium (Se+24, where 24 of the outer D, S and P orbitals are stripped away due to high input energies[clarification needed]) is one of the active mediums used in X-ray lasers.

Selenium pollution might impact some aquatic systems and may be caused by anthropogenic factors such as farming runoff and industrial processes.

[103] High selenium levels in waterways were found to cause congenital disorders in oviparous species, including wetland birds[104] and fish.

Other problems include degeneration of liver tissue, swelling around the heart, damaged egg follicles in ovaries, cataracts, and accumulation of fluid in the body cavity and head.

[110] At the Kesterson National Wildlife Refuge in California, thousands of fish and waterbirds were poisoned by selenium in agricultural irrigation drainage.

[115] The glutathione peroxidase family of enzymes (GSH-Px) catalyze reactions that remove reactive oxygen species such as hydrogen peroxide and organic hydroperoxides.

[121] Evidence suggests that the molecular mechanisms of mercury toxicity include the irreversible inhibition of selenoenzymes that are required to prevent and reverse oxidative damage in brain and endocrine tissues.

The main selenium indicator plants are Astragalus species (including some locoweeds), prince's plume (Stanleya sp.

The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchins, fish, and chickens.

[128] Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e., selenium, vanadium, magnesium, copper, and zinc, may have been lacking in some terrestrial mineral-deficient areas.

These findings suggest that, with the exception of vertebrates, aquatic life supports selenium use, whereas terrestrial habitats lead to lowered use of this trace element.

From about 500 million years ago, freshwater and terrestrial plants slowly optimized the production of "new" endogenous antioxidants such as ascorbic acid (vitamin C), polyphenols (including flavonoids), tocopherols, etc.

In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late Jurassic period.

[137] In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous shale) have suffered from selenium toxicity.

[138] Signs and symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage.

[141] Selenium also occurs in organic compounds, such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses.

[146] Selenium may be measured in blood, plasma, serum, or urine to monitor excessive environmental or occupational exposure, to confirm a diagnosis of poisoning in hospitalized victims, or to investigate a suspected case of fatal overdose.

Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations.

[147] Selenium deficiency can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition, and[148] in those of advanced age (over 90).

[citation needed] In June 2015, the US Food and Drug Administration (FDA) published its final rule establishing a requirement for minimum and maximum levels of selenium in infant formula.