Editing Selenium

{{About|the chemical element|the software testing framework|Selenium (software)}}
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{{Infobox selenium}}

'''Selenium''' is a [[chemical element]]; it has [[symbol (chemistry)|symbol]] '''Se''' and [[atomic number]] 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elemental state or as pure [[ore]] compounds in [[Earth's crust]]. Selenium ({{lang|grc|σελήνη}} {{gloss|moon}}) was discovered in 1817 by {{lang|sv|italics=unset|[[Jöns Jacob Berzelius]]}}, who noted the similarity of the new element to the previously discovered [[tellurium]] (named for the Earth).

Selenium is found in [[:Category:Sulfide minerals|metal sulfide ores]], where it substitutes for sulfur. Commercially, selenium is produced as a [[byproduct]] in the refining of these ores. Minerals that are pure [[selenide]] or [[selenate]] compounds are rare. The chief commercial uses for selenium today are [[glassmaking]] and [[pigment]]s. Selenium is a [[semiconductor]] and is used in [[photocell]]s. Applications in [[electronics]], once important, have been mostly replaced with [[silicon]] semiconductor devices. Selenium is still used in a few types of [[Direct current|DC power]] [[surge protector]]s and one type of [[Fluorescence|fluorescent]] [[quantum dot]].

Although [[essential trace element|trace]] amounts of selenium are necessary for [[Cell (biology)#Cellular processes|cellular function]] in many animals, including humans, both elemental selenium and (especially) selenium [[salt (chemistry)|salts]] are toxic in even small doses, causing [[Selenium#Toxicity|selenosis]].<ref>{{Cite journal |last1=Fernández-Bautista |first1=Tamara |last2=Gómez-Gómez |first2=Beatriz |last3=Palacín-García |first3=Roberto |last4=Gracia-Lor |first4=Emma |last5=Pérez-Corona |first5=Teresa |last6=Madrid |first6=Yolanda |date=2022-01-15 |title=Analysis of Se and Hg biomolecules distribution and Se speciation in poorly studied protein fractions of muscle tissues of highly consumed fishes by SEC-UV-ICP-MS and HPLC-ESI-MS/MS |url=https://www.sciencedirect.com/science/article/pii/S0039914021008444 |journal=Talanta |volume=237 |pages=122922 |doi=10.1016/j.talanta.2021.122922 |pmid=34736659 |s2cid=243761320 |issn=0039-9140}}</ref> Symptoms include (in decreasing order of frequency): diarrhea, fatigue, hair loss, joint pain, nail brittleness or discoloration, nausea, headache, tingling, vomiting, and fever.<ref name="pmid20142570">{{cite journal | vauthors=MacFarquhar JK, Broussard, DOL, Jones TF | title=Acute selenium toxicity associated with a dietary supplement | journal=[[JAMA Internal Medicine|Archives of Internal Medicine]] | volume=178 | issue=3 | pages=256–261 | year=2010 |  doi = 10.1001/archinternmed.2009.495 | pmc=3225252 | doi-access=free | pmid=20142570 }}</ref>

Selenium is listed as an ingredient in many multivitamins and other dietary supplements, as well as in [[infant formula]], and is a component of the antioxidant enzymes [[glutathione peroxidase]] and [[thioredoxin reductase]] (which indirectly reduce certain [[Redox|oxidized]] molecules in animals and some plants) as well as in three [[deiodinase]] enzymes. Selenium requirements in plants differ by species, with some plants requiring relatively large amounts and others apparently not requiring any.<ref name="Ruyle">{{cite web |url=https://cals.arizona.edu/arec/pubs/rmg/1%20rangelandmanagement/2%20poisonousplants93.pdf |title=Poisonous Plants on Arizona Rangelands |first=George |last=Ruyle |access-date=2009-01-05 |publisher=The University of Arizona |url-status=dead |archive-url=https://web.archive.org/web/20040715194026/http://cals.arizona.edu/AREC/pubs/rmg/1%20rangelandmanagement/2%20poisonousplants93.pdf |archive-date=2004-07-15 |df=dmy-all}}</ref>

==Characteristics==
===Physical properties===
[[File:Selenium trigonal.jpg|thumb|left|upright|Structure of hexagonal (gray) selenium]]
Selenium forms several [[allotrope]]s that interconvert with [[temperature]] changes, depending somewhat on the rate of temperature change. When prepared in chemical reactions, selenium is usually an [[Amorphous solid|amorphous]], brick-red powder. When rapidly melted, it forms the black, vitreous form, usually sold commercially as beads.<ref name="house2008">{{cite book |title=Inorganic chemistry |first=James E. |last=House |publisher=Academic Press |year=2008 |isbn=978-0-12-356786-4 |page=524}}</ref> The structure of black selenium is irregular and complex and consists of [[polymer]]ic rings with up to 1000&nbsp;atoms per ring. Black selenium is a brittle, lustrous solid that is slightly soluble in [[carbon disulfide|CS<sub>2</sub>]]. Upon heating, it softens at 50&nbsp;°C and converts to gray selenium at 180&nbsp;°C; the transformation temperature is reduced by presence of [[halogen]]s and [[amine]]s.<ref name="ge">{{Greenwood&Earnshaw |pages=751–752}}</ref>

The red α, β, and γ forms are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS<sub>2</sub>). They all have a relatively low, [[monoclinic]] crystal symmetry ([[space group]] 14) and contain nearly identical puckered '''cyclooctaselenium''' (Se<sub>8</sub>) rings as in [[sulfur]].<ref>{{cite journal |last1=Olav Foss and Vitalijus Janickis |title=Crystal structure of γ-monoclinic selenium |journal=Journal of the Chemical Society, Dalton Transactions |date=1980 |issue=4 |pages=624–627 |doi=10.1039/DT9800000624}}</ref> The eight atoms of a ring are not equivalent (i.e. they are not mapped one onto another by any symmetry operation), and in fact in the γ-monoclinic form, half the rings are in one configuration (and its mirror image) and half in another.<ref>{{cite web |title=β–Se (Al) Structure: A_mP32_14_8e |url=https://aflowlib.org/prototype-encyclopedia/A_mP32_14_8e.html |website=Encyclopedia of Crystallographic Prototypes |access-date=2024-01-10 |archive-date=2024-02-08 |archive-url=https://web.archive.org/web/20240208074603/https://aflowlib.org/prototype-encyclopedia/A_mP32_14_8e.html |url-status=dead }}</ref><ref>{{cite web |title=<!-- was β –Se (Al) .. using what I saw at target: --> Se (A<sub>k</sub>) Structure: A_mP64_14_16e |url=https://aflowlib.org/prototype-encyclopedia/A_mP64_14_16e.html |website=Encyclopedia of Crystallographic Prototypes |access-date=2024-01-10 |archive-date=2024-02-08 |archive-url=https://web.archive.org/web/20240208073655/https://aflowlib.org/prototype-encyclopedia/A_mP64_14_16e.html |url-status=dead }}</ref> The packing is most dense in the α form. In the Se<sub>8</sub> rings, the Se–Se distance varies depending on where the pair of atoms is in the ring, but the average is 233.5&nbsp;pm, and the Se–Se–Se angle is on average 105.7°. Other selenium allotropes may contain Se<sub>6</sub> or Se<sub>7</sub> rings.<ref name="ge" />

The most stable and dense form of selenium is gray and has a [[chiral]] [[hexagon]]al crystal lattice (space group 152 or 154 depending on the chirality)<ref>{{cite web |title=γ–Se (A8) Structure: A_hP3_152_a |url=https://aflowlib.org/prototype-encyclopedia/A_hP3_152_a.html |website=Encyclopedia of Crystallographic Prototypes |access-date=2023-12-03 |archive-date=2023-12-03 |archive-url=https://web.archive.org/web/20231203011831/https://aflowlib.org/prototype-encyclopedia/A_hP3_152_a.html |url-status=dead }}</ref> consisting of helical polymeric chains, where the Se–Se distance is 237.3&nbsp;pm and Se–Se–Se angle is 103.1°. The minimum distance between chains is 343.6&nbsp;pm. Gray selenium is formed by mild heating of other allotropes, by slow cooling of molten selenium, or by condensing selenium vapor just below the melting point. Whereas other selenium forms are [[Insulator (electricity)|insulators]], gray selenium is a [[semiconductor]] showing appreciable [[photoconductivity]]. Unlike the other allotropes, it is insoluble in CS<sub>2</sub>.<ref name="ge" /> It resists oxidation by air and is not attacked by nonoxidizing [[acid]]s. With strong reducing agents, it forms polyselenides. Selenium does not exhibit the changes in viscosity that sulfur undergoes when gradually heated.<ref name="house2008" /><ref>{{YouTube|nDEfR2Nw50s|Video of selenium heating}}</ref>

===Isotopes===
{{Main|Isotopes of selenium}}

Selenium has seven naturally occurring [[isotope]]s. Five of these, <sup>74</sup>Se, <sup>76</sup>Se, <sup>77</sup>Se, <sup>78</sup>Se, <sup>80</sup>Se, are stable, with <sup>80</sup>Se being the most abundant (49.6% natural abundance). Also naturally occurring is the long-lived [[primordial radionuclide]] <sup>82</sup>Se, with a [[half-life]] of 8.76×10<sup>19</sup> years.{{NUBASE2020|ref}} The non-primordial radioisotope [[Selenium-79|<sup>79</sup>Se]] also occurs in minute quantities in [[uranium]] ores as a product of [[nuclear fission]]. Selenium also has numerous unstable [[synthetic isotope]]s ranging from <sup>64</sup>Se to <sup>95</sup>Se; the most stable are <sup>75</sup>Se with a half-life of 119.78&nbsp;days and <sup>72</sup>Se with a half-life of 8.4&nbsp;days.{{NUBASE2016|ref}} Isotopes lighter than the stable isotopes primarily undergo [[beta plus decay]] to [[isotopes of arsenic]], and isotopes heavier than the stable isotopes undergo [[beta minus decay]] to [[isotopes of bromine]], with some minor [[neutron emission]] branches in the heaviest known isotopes.

{| class="wikitable"
|+Selenium isotopes of greatest stability
! Isotope
! Nature
! Origin
! Half-life
|-
|<sup>74</sup>Se
|Primordial
|
|Stable
|-
|<sup>76</sup>Se
|Primordial
|
|Stable
|-
|<sup>77</sup>Se
|Primordial
|Fission product
|Stable
|-
|<sup>78</sup>Se
|Primordial
|Fission product
|Stable
|-
|<sup>79</sup>Se
|Trace
|Fission product
|{{val|1=327000}} yr<ref>{{cite web |url=https://www.ptb.de/en/org/6/nachrichten6/2010/60710_en.htm |title=The half-life of <sup>79</sup>Se <!-- title=Physikalisch-Technische Bundesanstalt (PTB) --> |date=2010-09-23 |publisher=Physikalisch-Technische Bundesanstalt |access-date=2012-05-29 |df=dmy-all |archive-url=https://web.archive.org/web/20110927042432/http://www.ptb.de/en/org/6/nachrichten6/2010/60710_en.htm |archive-date=2011-09-27}}</ref><ref>{{cite journal |last2=Bühnemann |first2=Rolf |last3=Hollas |first3=Simon |last4=Kivel |first4=Niko |last5=Kossert |first5=Karsten |last6=Van Winckel |first6=Stefaan |last7=Gostomski |first7=Christoph Lierse v. |date=2010 |title=Preparation of radiochemically pure <sup>79</sup>Se and highly precise determination of its half-life |journal=Applied Radiation and Isotopes |volume=68 |issue=12 |pages=2339–2351 |doi=10.1016/j.apradiso.2010.05.006 |pmid=20627600 |last1=Jörg |first1=Gerhard |bibcode=2010AppRI..68.2339J |display-authors=3}}</ref>
|-
|<sup>80</sup>Se
|Primordial
|Fission product
|Stable
|-
|<sup>82</sup>Se
|Primordial
|Fission product*
|8.76{{e|19}} yr{{NUBASE2020|ref}}{{efn|For all practical purposes, <sup>82</sup>Se is stable.}}
|}

==Chemical compounds==
{{Main|Selenium compounds}}
Selenium compounds commonly exist in the [[oxidation state]]s −2, +2, +4, and +6. 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]].<ref name="ge"/>

===Chalcogen compounds===
[[File:Selenium-dioxide-chain-3D-balls.png|thumb|Structure of the polymer SeO<sub>2</sub>: The (pyramidal) selenium atoms are yellow.|left]]
Selenium forms two [[oxide]]s: [[selenium dioxide]] (SeO<sub>2</sub>) and [[selenium trioxide]] (SeO<sub>3</sub>). Selenium dioxide is formed by combustion of elemental selenium:<ref name="house2008" />
{{block indent|{{chem2|Se + O2 -> SeO2}}}}
It is a [[polymer]]ic solid that forms monomeric SeO<sub>2</sub> molecules in the gas phase. It dissolves in water to form [[selenous acid]], H<sub>2</sub>SeO<sub>3</sub>. Selenous acid can also be made directly by oxidizing elemental selenium with [[nitric acid]]:<ref name="wiberg_holleman">{{cite book |title=Inorganic chemistry |last1=Wiberg |first1=Egon |last2=Wiberg |first2=Nils |last3=Holleman |first3=Arnold Frederick |publisher=Academic Press |year=2001 |location=San Diego |isbn=978-0-12-352651-9 |page=583}}</ref>

{{block indent|{{chem2|3 Se + 4 HNO3 + H2O -> 3 H2SeO3 + 4 NO}}}}
Unlike sulfur, which forms a stable [[sulfur trioxide|trioxide]], selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185&nbsp;°C:<ref name="house2008" /><ref name="wiberg_holleman" />

{{block indent|{{chem2| 2 SeO3 -> 2 SeO2 + O2}} (ΔH {{=}} −54&nbsp;kJ/mol)}}
Selenium trioxide is produced in the laboratory by the reaction of [[anhydrous]] [[potassium selenate]] (K<sub>2</sub>SeO<sub>4</sub>) and sulfur trioxide (SO<sub>3</sub>).<ref>{{Greenwood&Earnshaw|page=780}}</ref>

[[Salt (chemistry)|Salts]] of selenous acid are called selenites. These include [[silver selenite]] (Ag<sub>2</sub>SeO<sub>3</sub>) and [[sodium selenite]] (Na<sub>2</sub>SeO<sub>3</sub>).

[[Hydrogen sulfide]] reacts with aqueous selenous acid to produce [[selenium disulfide]]:
{{block indent|{{chem2|H2SeO3 + 2 H2S -> SeS2 + 3 H2O}}}}

Selenium disulfide consists of 8-membered rings. It has an approximate composition of SeS<sub>2</sub>, with individual rings varying in composition, such as Se<sub>4</sub>S<sub>4</sub> and Se<sub>2</sub>S<sub>6</sub>. Selenium disulfide has been used in shampoo as an anti[[dandruff]] agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in [[fireworks]].<ref name="wiberg_holleman" />

Selenium trioxide may be synthesized by dehydrating [[selenic acid]], H<sub>2</sub>SeO<sub>4</sub>, which is itself produced by the oxidation of selenium dioxide with [[hydrogen peroxide]]:<ref>{{Cite book |doi=10.1002/9780470132517.ch9 |last1=Seppelt |first1=K. |last2=Desmarteau |first2=Darryl D. |chapter=Selenonyl Difluoride |title=Inorganic Syntheses |year=1980 |volume=20 |pages=36–38 |isbn=978-0-471-07715-2 }} The report describes the synthesis of selenic acid.</ref>
{{block indent|{{chem2|SeO2 + H2O2 -> H2SeO4}}}}

Hot, concentrated selenic acid reacts with gold to form gold(III) selenate.<ref>{{Cite journal |last1=Lenher |first1=V. |date=April 1902 |title=Action of selenic acid on gold |journal=Journal of the American Chemical Society |volume=24 |issue=4 |pages=354–355 |doi=10.1021/ja02018a005 |bibcode=1902JAChS..24..354L |url=https://zenodo.org/record/1428902}}</ref>

===Halogen compounds===
Selenium reacts with [[fluorine]] to form [[selenium hexafluoride]]:

{{block indent|{{chem2|Se8 + 24 F2 -> 8 SeF6}}}}

In comparison with its sulfur counterpart ([[sulfur hexafluoride]]), [[selenium hexafluoride]] (SeF<sub>6</sub>) is more reactive and is a toxic [[pulmonary]] irritant.<ref>{{cite book |last1=Proctor |first1=Nick H. |title=Proctor and Hughes' chemical hazards of the workplace |last2=Hathaway |first2=Gloria J. |publisher=Wiley-IEEE |year=2004 |isbn=978-0-471-26883-3 |editor=Hughes, James P. |edition=5th |page=625}}</ref> [[Selenium tetrafluoride]] is a laboratory-scale [[fluorinating agent]].

The only stable [[chloride]]s are [[selenium tetrachloride]] (SeCl<sub>4</sub>) and [[selenium monochloride]] (Se<sub>2</sub>Cl<sub>2</sub>), which might be better known as selenium(I) chloride and is structurally analogous to [[disulfur dichloride]]. Metastable solutions of [[selenium dichloride]] can be prepared from [[sulfuryl chloride]] and selenium (reaction of the elements generates the [[Selenium tetrachloride|tetrachloride]] instead), and constitute an important reagent in the preparation of selenium compounds (e.g. Se<sub>7</sub>). The corresponding [[bromide]]s are all known, and recapitulate the same stability and structure as the chlorides.<ref>{{cite book
|title=Handbook of chalcogen chemistry: new perspectives in sulfur, selenium and tellurium |author=Xu, Zhengtao |editor=Devillanova, Francesco A. |publisher=Royal Society of Chemistry |year=2007 |isbn=978-0-85404-366-8 |page=460}}</ref>

The [[iodide]]s of selenium are not well known, and for a long time were believed not to exist.<ref name=":1">{{Cite journal |last1=Gopal |first1=Madhuban |last2=Milne |first2=John |date=October 1992 |title=Spectroscopic evidence for selenium iodides in carbon disulfide solution: Se3I2, Se2I2, and SeI2 |url=https://pubs.acs.org/doi/abs/10.1021/ic00048a017 |journal=Inorganic Chemistry |language=en |volume=31 |issue=22 |pages=4530–4533 |doi=10.1021/ic00048a017 |issn=0020-1669}}</ref> There is limited [[Spectroscopy|spectroscopic]] evidence that the lower iodides may form in bi-elemental solutions with nonpolar solvents, such as [[carbon disulfide]]<ref>{{Cite journal |last=McCullough |first=James D. |date=December 1939 |title=Evidence for Existence of a Selenium Iodide |url=https://pubs.acs.org/doi/abs/10.1021/ja01267a052 |journal=Journal of the American Chemical Society |language=en |volume=61 |issue=12 |pages=3401–3402 |doi=10.1021/ja01267a052 |bibcode=1939JAChS..61.3401M |issn=0002-7863}}</ref> and [[carbon tetrachloride]];<ref name=":1" /> but even these appear to [[Photosensitivity|decompose under illumination]].<ref>Rao, M.&nbsp;R.&nbsp;Aswatha&nbsp;Narayana. [https://www.ias.ac.in/public/Volumes/seca/012/04/0410-0415.pdf "Selenium iodide"]. In ''Proceedings of the Indian Academy of Sciences-Section&nbsp;A'', vol.&nbsp;12, pp.&nbsp;410-415. Springer India, 1940.</ref>

Some selenium oxyhalides—[[seleninyl fluoride]] (SeOF<sub>2</sub>) and [[selenium oxychloride]] (SeOCl<sub>2</sub>)—have been used as specialty solvents.<ref name="house2008" />

===Metal selenides===
[[File:Polyselenide.svg|thumb|Structures of two polyselenide anions<ref>{{Greenwood&Earnshaw2nd|page=763-765}}</ref>]]
Analogous to the behavior of other chalcogens, selenium forms [[hydrogen selenide]], H<sub>2</sub>Se. It is a strongly [[wikt:odiferous|odiferous]], toxic, and colorless gas. It is more acidic than H<sub>2</sub>S. In solution it ionizes to HSe<sup>−</sup>. The selenide dianion Se<sup>2−</sup> forms a variety of compounds, including the minerals from which selenium is obtained commercially. Illustrative selenides include [[mercury selenide]] (HgSe), [[lead selenide]] (PbSe), [[zinc selenide]] (ZnSe), and [[copper indium gallium diselenide]] (Cu(Ga,In)Se<sub>2</sub>). These materials are [[semiconductor]]s. With highly electropositive metals, such as [[aluminium]], these selenides are prone to hydrolysis, which may be described by this idealized equation:<ref name="house2008" />

:{{chem2 | Al2Se3 + 6 H2O -> 2 Al(OH)3 + 3 H2Se }}

Alkali metal selenides react with selenium to form polyselenides, {{chem|Se|n|2-}}, which exist as chains and rings.

===Other compounds===
Tetraselenium tetranitride, Se<sub>4</sub>N<sub>4</sub>, is an explosive orange compound analogous to [[tetrasulfur tetranitride]] (S<sub>4</sub>N<sub>4</sub>).<ref name="house2008" /><ref>{{cite journal |last1=Woollins |first1=Derek |title=The Reactivity of Se<sub>4</sub>N<sub>4</sub> in Liquid Ammonia |journal=[[Polyhedron (journal)|Polyhedron]] |year=1993 |volume=12 |pages=1129–1133 |doi=10.1016/S0277-5387(00)88201-7 |issue=10 |last2=Kelly |first2=Paul F.}}</ref><ref>{{cite journal |last1=Kelly |first1=P.F. |last2=Slawin |first2=A.M.Z. |last3=Soriano-Rama |first3=A. |title=Use of Se<sub>4</sub>N<sub>4</sub> and Se(NSO)<sub>2</sub> in the preparation of palladium adducts of diselenium dinitride, Se<sub>2</sub>N<sub>2</sub>; crystal structure of {{chem|[PPh|4|]|2|[Pd|2|Br|6|(Se|2|N|2|)|]}} |journal=[[Dalton Transactions]] |date=1997 |pages=559–562 |doi=10.1039/a606311j |issue=4}}</ref> It can be synthesized by the reaction of [[selenium tetrachloride]] (SeCl<sub>4</sub>) with [[Metal bis(trimethylsilyl)amides|{{chem|[((CH|3|)|3|Si)|2|N]|2|Se}}]].<ref>{{cite journal |doi=10.1021/ic00060a031 |title=A simple, efficient synthesis of tetraselenium tetranitride |year=1993 |last1=Siivari |first1=Jari |last2=Chivers |first2=Tristram |last3=Laitinen |first3=Risto S. |journal=Inorganic Chemistry |volume=32 |issue=8 |pages=1519–1520}}</ref>

Selenium reacts with [[cyanide]]s to yield [[Potassium selenocyanate|selenocyanates]]:<ref name="house2008" />

:{{chem2 | 8 KCN + Se8 -> 8 KSeCN }}

===Organoselenium compounds===
{{Main|Organoselenium chemistry}}
Selenium, especially in the II oxidation state, forms a variety of organic derivatives. They are structurally analogous to the corresponding [[organosulfur compounds]]. Especially common are selenides (R<sub>2</sub>Se, analogues of [[thioether]]s), diselenides (R<sub>2</sub>Se<sub>2</sub>, analogues of [[disulfide]]s), and [[selenol]]s (RSeH, analogues of [[thiol]]s). Representatives of selenides, diselenides, and selenols include respectively [[selenomethionine]], [[diphenyldiselenide]], and [[benzeneselenol]]. The [[sulfoxide]] in sulfur chemistry is represented in selenium chemistry by the selenoxides (formula RSe(O)R), which are intermediates in organic synthesis, as illustrated by the [[selenoxide elimination]] reaction. Consistent with trends indicated by the [[double bond rule]], selenoketones, R(C=Se)R, and selenaldehydes, R(C=Se)H, are rarely observed.<ref>{{cite journal |last1=Erker |first1=G. |last2=Hock |last3=Krüger |last4=Werner |last5=Klärner |last6=Artschwager-Perl |title=Synthesis and Cycloadditions of Monomeric Selenobenzophenone |journal=Angewandte Chemie International Edition in English |volume=29 |issue=9 |pages=1067–1068 |year=1990 |doi=10.1002/anie.199010671 |first2=R. |first3=C. |first4=S. |first5=F.G. |first6=U.}}</ref>

==History==
[[File:Luna statue.jpg|thumb|upright|Selenium is named after the [[Selene]], the Greek Goddess of the [[Moon]]]]
Selenium ([[Greek language|Greek]] σελήνη ''[[selene]]'' meaning "Moon") was discovered in 1817 by [[Jöns Jacob Berzelius]] and [[Johan Gottlieb Gahn]].<ref>{{cite journal |last1=Berzelius |first1=J.J.|title=Lettre de M. Berzelius à M. Berthollet sur deux métaux nouveaux |journal=Annales de Chimie et de Physique |year=1818 |volume=7 |pages=199–206 |url=https://books.google.com/books?id=jBIAAAAAMAAJ&pg=PA199 |series=2nd series |trans-title=Letter from Mr. Berzelius to Mr. Berthollet on two new metals |language=fr}} From p. 203: ''"Cependant, pour rappeler les rapports de cette dernière avec le tellure, je l'ai nommée sélénium."'' (However, in order to recall the relationships of this latter [substance (viz, selenium)] to tellurium, I have named it "selenium".)</ref> Both chemists owned a chemistry plant near [[Gripsholm]], Sweden, producing [[sulfuric acid]] by the [[lead chamber process]]. [[Pyrite]] samples from the [[Falun Mine]] produced a red solid precipitate in the lead chambers, which was presumed to be an arsenic compound, so the use of pyrite to make acid was discontinued. Berzelius and Gahn, who wanted to use the pyrite, observed that the red precipitate gave off an odor like [[horseradish]] when burned. This smell was not typical of arsenic, but a similar odor was known from [[tellurium]] compounds. Hence, Berzelius's first letter to [[Alexander Marcet]] stated that this was a tellurium compound. However, the lack of tellurium compounds in the [[Falun Mine]] minerals eventually led Berzelius to reanalyze the red precipitate, and in 1818 he wrote a second letter to Marcet describing a newly found element similar to [[sulfur]] and tellurium. Because of its similarity to tellurium, named for the Earth, Berzelius named the new element after the [[Moon]].<ref>{{cite journal |doi=10.1021/ed009p474 |title=The Discovery of the Elements. VI. Tellurium and Selenium |date=1932 |last1=Weeks |first1=Mary Elvira |author-link1=Mary Elvira Weeks |journal=Journal of Chemical Education |volume=9 |issue=3 |page=474 |bibcode=1932JChEd...9..474W}}</ref><ref>{{cite journal |title=Berzelius' Discovery of Selenium |first=Jan |last=Trofast |url=http://www.iupac.org/publications/ci/2011/3305/5_trofast.html |journal=Chemistry International |volume=33 |issue=5 |year=2011 |pages=16–19}} [http://www.iupac.org/publications/ci/2011/3305/sept11.pdf#page=18 PDF]</ref>

In 1873, [[Willoughby Smith]] found that the [[electrical conductivity]] of grey selenium was affected by light.<ref>{{cite journal |last1=Smith |first1=Willoughby |title=The action of light on selenium |journal=Journal of the Society of Telegraph Engineers |year=1873 |volume=2 |issue=4 |pages=31–33 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112007449892;view=1up;seq=67 |doi=10.1049/jste-1.1873.0023}}</ref><ref>{{cite journal |last1=Smith |first1=Willoughby |title=Effect of light on selenium during the passage of an electric current |journal=Nature |date=20 February 1873 |volume=7 |issue=173 |page=303 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.c2754884;view=1up;seq=321 |doi=10.1038/007303e0 |bibcode=1873Natur...7R.303.|doi-access=free }}</ref> This led to its use as a cell for sensing light. The first commercial products using selenium were developed by [[Werner Siemens]] in the mid-1870s. The selenium cell was used in the [[photophone]] developed by [[Alexander Graham Bell]] in 1879. Selenium transmits an electric current proportional to the amount of light falling on its surface. This phenomenon was used in the design of [[light meter]]s and similar devices. Selenium's semiconductor properties found numerous other applications in electronics.<ref>{{cite journal |url=https://books.google.com/books?id=diwDAAAAMBAJ&pg=PA116 |title=Action of light on selenium|journal =[[Popular Science]] |year=1876 |volume=10 |issue=1 |page=116 |author1=Bonnier Corporation}}</ref><ref>{{cite book |url= https://books.google.com/books?id=CaxdTFMwQEAC&pg=PA77 |title= Earliest semiconductor device| pages= 77–79 |series= Getting to Know Semiconductors |isbn=978-981-02-3516-1|last1= Levinshtein |first1=M.E. |last2=Simin |first2=G.S. |date=1992-12-01 |df=dmy-all}}</ref><ref>{{cite book |url=https://books.google.com/books?id=IYsOEa_AIjUC&pg=PA89 |page=89 |title= Media Technology and Society: A History: From the Telegraph to the Internet |isbn= 978-0-415-14229-8 |last1=Winston |first1=Brian |date=1998-05-29 |publisher=Psychology Press |df=dmy-all}}</ref> The development of [[selenium rectifier]]s began during the early 1930s, and these replaced [[copper oxide]] rectifiers because they were more efficient.<ref>{{cite book |url=https://books.google.com/books?id=rslXJmYPjGIC&pg=PA18 |page=18 |title=A History of the World Semiconductor Industry |isbn=978-0-86341-227-1 |last1=Morris |first1=Peter Robin |date=1990}}</ref><ref>{{cite journal |last=Bergmann |first=Ludwig |year=1931 |journal=Physikalische Zeitschrift |volume=32 |pages=286–288 |title=Über eine neue Selen-Sperrschicht-Photozelle}}</ref><ref>{{cite journal |doi=10.1021/ie50392a002 |title=Industrial Utilization of Selenium and Tellurium |year=1942 |last1=Waitkins |first1=G.R. |last2=Bearse |first2=A.E. |last3=Shutt |first3=R. |journal=Industrial & Engineering Chemistry |volume=34 |issue=8 |pages=899–910}}</ref> These lasted in commercial applications until the 1970s, following which they were replaced with less expensive and even more efficient [[silicon rectifier]]s.

Selenium came to medical notice later because of its toxicity to industrial workers. Selenium was also recognized as an important veterinary toxin, which is seen in animals that have eaten high-selenium plants. In 1954, the first hints of specific biological functions of selenium were discovered in [[microorganism]]s by biochemist, [[Jane Gibson|Jane Pinsent]].<ref>{{cite journal |pmc=1269698 |title=The need for selenite and molybdate in the formation of formic dehydrogenase by members of the Coli-aerogenes group of bacteria |volume=57 |issue =1 |journal=Biochem. J. |year=1954 |pages=10–16 |author=Pinsent, Jane |pmid=13159942 |doi=10.1042/bj0570010}}</ref><ref>{{cite book |doi=10.1007/0-306-47466-2_267 |volume=10 |year=2002 |last1=Stadtman |first1=Thressa C. |isbn=978-0-306-46378-5 |title=Trace Elements in Man and Animals 10 |pages=831–836 |chapter=Some Functions of the Essential Trace Element, Selenium}}</ref> It was discovered to be essential for mammalian life in 1957.<ref>{{cite journal |doi=10.1021/ja01569a087 |year=1957 |last1=Schwarz |first1=Klaus |last2=Foltz |first2=Calvin M. |journal=Journal of the American Chemical Society |volume=79 |issue=12 |pages=3292–3293 |title=Selenium as an Integral Part of Factor&nbsp;3 Against Dietary Necrotic Liver Degeneration|bibcode=1957JAChS..79.3292S }}</ref><ref>{{cite book |doi=10.1007/0-387-33827-6_1 |year=2006 |last1=Oldfield |first1=James E. |isbn=978-0-387-33826-2 |title=Selenium |pages=1–6 |chapter=Selenium: A historical perspective}}</ref> In the 1970s, it was shown to be present in two independent sets of [[enzyme]]s. This was followed by the discovery of [[selenocysteine]] in proteins. During the 1980s, selenocysteine was shown to be encoded by the [[Genetic code|codon UGA]]. The recoding mechanism was worked out first in [[bacteria]] and then in [[mammal]]s (see [[SECIS element]]).<ref>{{cite journal |doi=10.1128/MCB.22.11.3565-3576.2002 |title=How Selenium Has Altered Our Understanding of the Genetic Code |year=2002 |last1=Hatfield |first1=D. L. |last2=Gladyshev |first2=V.N. |journal=Molecular and Cellular Biology |volume=22 |issue=11 |pages=3565–3576 |pmid=11997494 |pmc=133838}}</ref>

==Occurrence==
{{Category see also|Selenide minerals}}
[[File:Selenium in sandstone Westwater Canyon Section 23 Mine Grants, New Mexico.jpg|thumb|Native selenium in sandstone, from a uranium mine near [[Grants, New Mexico]]]]

Native (i.e., elemental) selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedra or tiny acicular (hair-like) crystals.<ref>{{cite web |publisher=Webminerals |url=http://www.galleries.com/minerals/elements/selenium/selenium.htm |title=Native Selenium |access-date=2009-06-06 |df=dmy-all}}</ref> Isolation of selenium is often complicated by the presence of other compounds and elements.

Selenium occurs naturally in a number of inorganic forms, including [[selenide]], [[selenate]], and [[Selenite (ion)|selenite]], but these minerals are rare. The common mineral [[selenite (mineral)|selenite]] is not a selenium mineral, and contains no [[Selenite (ion)|selenite ion]], but is rather a type of [[gypsum]] (calcium sulfate hydrate) named like selenium for the moon well before the discovery of selenium. Selenium is most commonly found as an impurity, replacing a small part of the sulfur in sulfide ores of many metals.<ref name="geosel1">{{cite journal |pmid=9726787 |year=1998 |last=Kabata-Pendias |first=A. |author-link=Alina Kabata-Pendias |title=Geochemistry of selenium |volume=17 |issue=3–4 |pages=173–177 |journal= Journal of Environmental Pathology, Toxicology and Oncology}}</ref><ref name="geosel2">{{cite journal |doi=10.1579/0044-7447(2007)36[94:SGAH]2.0.CO;2 |volume=36 |pages=94–97 |title=Selenium Geochemistry and Health |last1=Fordyce |first1=Fiona |journal=Ambio: A Journal of the Human Environment |year=2007|issue=1 |pmid=17408199 |s2cid=18925825 |url=http://nora.nerc.ac.uk/id/eprint/19045/1/AMBIO_Fordycefinal.pdf }}</ref>

In living systems, selenium is found in the amino acids [[selenomethionine]], [[selenocysteine]], and [[methylselenocysteine]]. In these compounds, selenium plays a role analogous to that of sulfur. Another naturally occurring [[organoselenium compound]] is [[dimethyl selenide]].<ref name="biol1">{{cite journal |doi=10.1515/BC.2007.138 |title=Selenium in chemistry and biochemistry in comparison to sulfur |year=2007 |last1=Wessjohann |first1=Ludger A. |last2=Schneider |first2=Alex |last3=Abbas |first3=Muhammad |last4=Brandt |first4=Wolfgang |journal=Biological Chemistry |volume=388 |issue=10 |pages=997–1006 |pmid=17937613|s2cid=34918691 }}</ref><ref name="biol1a">{{cite journal |doi=10.1039/B205802M |title=Trends in selenium biochemistry |date=2002 |last1=Birringer |first1=Marc |last2=Pilawa |first2=Sandra |last3=Flohé |first3=Leopold |journal=Natural Product Reports |volume=19 |issue=6 |pages=693–718 |pmid=12521265}}</ref>

Certain soils are selenium-rich, and selenium can be [[bioaccumulation|bioconcentrated]] by some plants. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff.<ref name="geosel1" /><ref name="geosel2" /> Ocean water contains significant amounts of selenium.<ref>{{cite journal |doi=10.1016/S0012-821X(01)00370-3 |title=Role of oceans as biogenic sources of selenium |year=2001 |last1=Amouroux |first1=David |last2=Liss |first2=Peter S. |last3=Tessier |first3=Emmanuel |last4=Hamren-Larsson |first4=Marie |last5=Donard |first5=Olivier F.X. |journal=Earth and Planetary Science Letters |volume=189 |issue=3–4 |pages=277–283 |bibcode=2001E&PSL.189..277A |display-authors=3}}</ref><ref>{{cite journal |doi=10.1080/08910600701698986 |title=How to use the world's scarce selenium resources efficiently to increase the selenium concentration in food |year=2007 |last1=Haug |first1=Anna |last2=Graham |first2=Robin D. |last3=Christophersen |first3=Olav A. |last4=Lyons |first4=Graham H. |journal=Microbial Ecology in Health and Disease |volume=19 |issue=4 |pages=209–228 |pmid=18833333 |pmc=2556185}}</ref>

Typical background concentrations of selenium do not exceed 1&nbsp;ng/m<sup>3</sup> in the atmosphere; 1&nbsp;mg/kg in soil and vegetation and 0.5&nbsp;μg/L in freshwater and seawater.<ref>{{Cite book|last=Rieuwerts|first=John|url=https://www.worldcat.org/oclc/886492996|title=The Elements of Environmental Pollution|publisher=Earthscan Routledge|year=2015|isbn=978-0-415-85919-6|location=London and New York|pages=262|oclc=886492996}}</ref>

Anthropogenic sources of selenium include coal burning, and the mining and smelting of sulfide ores.<ref>{{cite web |url=http://www.atsdr.cdc.gov/toxprofiles/tp92-c1.pdf |publisher=Agency for Toxic Substances and Disease Registry |title=Public Health Statement: Selenium |access-date=2009-01-05 |df=dmy-all}}</ref>

==Production==
Selenium is most commonly produced from [[selenide]] in many [[sulfide]] [[ore]]s, such as those of [[copper]], [[nickel]], or [[lead]]. Electrolytic metal refining is particularly productive of selenium as a byproduct, obtained from the [[anode]] mud of copper refineries. Another source was the mud from the [[Lead chamber process|lead chambers]] of [[sulfuric acid]] plants, a process that is no longer used. Selenium can be refined from these muds by a number of methods. However, most elemental selenium comes as a byproduct of [[Refining (metallurgy)|refining]] copper or producing [[sulfuric acid]].<ref>{{cite web |url=http://www.atsdr.cdc.gov/toxprofiles/tp92-c5.pdf |publisher=Agency for Toxic Substances and Disease Registry |title=Public Health Statement: Selenium – Production, Import/Export, Use, and Disposal |access-date=2009-01-05 |df=dmy-all}}</ref><ref>{{cite web |url=http://www.webelements.com/webelements/elements/text/Se/key.html |title=Chemistry: Periodic Table: selenium: key information |access-date= 2009-01-06 |publisher= webelements}}</ref> Since its invention, [[solvent extraction and electrowinning]] (SX/EW) production of copper produces an increasing share of the worldwide copper supply.<ref>{{cite journal |doi=10.1016/S0301-4207(03)00025-4 |title=SX-EW copper and the technology cycle |year=2002 |last1=Bartos |first1=P.J. |journal=Resources Policy |volume=28 |issue=3–4 |pages=85–94|bibcode=2002RePol..28...85B }}</ref> This changes the availability of selenium because only a comparably small part of the selenium in the ore is leached with the copper.<ref name="Naumov">{{cite journal |last1=Naumov |first1=A. V. |date=2010 |title=Selenium and tellurium: State of the markets, the crisis, and its consequences |journal=Metallurgist |volume=54 |issue=3–4 |pages=197–200 |doi=10.1007/s11015-010-9280-7 |s2cid=137066827}}</ref>

Industrial production of selenium usually involves the extraction of [[selenium dioxide]] from residues obtained during the purification of copper. Common production from the residue then begins by oxidation with [[sodium carbonate]] to produce selenium dioxide, which is mixed with water and [[acid]]ified to form [[selenous acid]] ([[oxidation]] step). Selenous acid is bubbled with [[sulfur dioxide]] ([[Redox|reduction]] step) to give elemental selenium.<ref>{{cite journal |doi=10.1007/BF03220269 |title=Recovering selenium and tellurium from copper refinery slimes |year=1989 |last1=Hoffmann |first1=James E. |journal=JOM |volume=41 |issue=7 |pages=33–38 |bibcode=1989JOM....41g..33H|s2cid=138253358 }}</ref><ref>{{cite journal |doi=10.1007/BF03220271|title= Recovering selenium from copper refinery slimes |date=1989 |last1=Hyvärinen |first1=Olli |last2=Lindroos |first2=Leo |last3=Yllö |first3=Erkki |journal=JOM |volume=41 |issue=7 |pages=42–43 |bibcode=1989JOM....41g..42H|s2cid= 138555797 }}</ref><!-- http://eprints.nmlindia.org/2071/-->

About 2,000&nbsp;tonnes of selenium were produced in 2011 worldwide, mostly in Germany (650&nbsp;t), Japan (630&nbsp;t), Belgium (200&nbsp;t), and Russia (140&nbsp;t), and the total reserves were estimated at 93,000&nbsp;tonnes. These data exclude two major producers: the United States and China. A previous sharp increase was observed in 2004 from $4–$5 to $27/lb. The price was relatively stable during 2004–2010 at about US$30&nbsp;per pound (in 100&nbsp;pound lots) but increased to $65/lb in 2011. The consumption in 2010 was divided as follows: metallurgy – 30%, glass manufacturing – 30%, agriculture – 10%, chemicals and pigments – 10%, and electronics – 10%. China is the dominant consumer of selenium at 1,500–2,000&nbsp;tonnes/year.<ref name="usgs" />

==Applications==

===Manganese electrolysis===
During the [[electrowinning]] of manganese, the addition of [[selenium dioxide]] decreases the power necessary to operate the [[electrolytic cell|electrolysis cells]]. China is the largest consumer of selenium dioxide for this purpose. For every tonne of manganese, an average 2&nbsp;kg selenium oxide is used.<ref name="usgs">{{cite web|title= Selenium and Tellurium: Statistics and Information|url= http://minerals.usgs.gov/minerals/pubs/commodity/selenium/|publisher= United States Geological Survey|access-date= 2012-05-30|archive-date= 2012-05-08|archive-url= https://web.archive.org/web/20120508085217/http://minerals.usgs.gov/minerals/pubs/commodity/selenium/|url-status= dead}}</ref><ref>{{cite journal|doi= 10.1016/j.electacta.2011.06.111|title= Studies of the reduction mechanism of selenium dioxide and its impact on the microstructure of manganese electrodeposit|date= 2011|last1= Sun|first1= Yan|last2= Tian|first2= Xike|last3= He|first3= Binbin|last4= Yang|first4= Chao|last5= Pi|first5= Zhenbang|last6= Wang|first6= Yanxin|last7= Zhang|first7= Suxin|journal= Electrochimica Acta|volume= 56|issue= 24|pages= 8305–8310 |display-authors=3}}</ref> <!--http://www.asianmetal.com/report/en/2008mn_en.pdf-->

===Glass production===
The largest commercial use of selenium, accounting for about 50% of consumption, is for the production of glass. Selenium compounds confer a red color to glass. This color cancels out the green or yellow tints that arise from iron impurities typical for most glass. For this purpose, various selenite and selenate salts are added. For other applications, a red color may be desired, produced by mixtures of CdSe and CdS.<ref name="Ullmann">Bernd E. Langner (2005), "Selenium and Selenium Compounds", ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a23_525}}.</ref>

===Alloys===
Selenium is used with [[bismuth]] in [[brass]]es to replace more toxic [[lead]]. The regulation of lead in drinking water applications such as in the US with the [[Safe Drinking Water Act]] of 1974, made a reduction of lead in brass necessary. The new brass is marketed under the name EnviroBrass.<ref>{{cite book|url= https://books.google.com/books?id=sxkPJzmkhnUC&pg=PA91|page= 91|title= Copper and Copper Alloys|isbn= 978-0-87170-726-0|last1= Davis|first1=Joseph R.|date= 2001 |publisher= ASM Int. }}</ref><!--https://books.google.com/books?id=SZ1RAAAAMAAJ&pg=PA378--> Like lead and sulfur, selenium improves the machinability of steel at concentrations around 0.15%.<ref>{{cite book|url= https://books.google.com/books?id=QahG1Ou1cyEC&pg=PA67|page= 67|title= Cutting Data for Turning of Steel|isbn= 978-0-8311-3314-6|last1= Isakov|first1= Edmund|date= 2008-10-31|publisher= Industrial Press}}</ref><ref>{{cite journal|doi= 10.1007/BF00708374|title= Effect of selenium on the structure and properties of structural steel|date= 1979|last1= Gol'Dshtein|first1=Ya. E.|last2= Mushtakova|first2=T. L.|last3= Komissarova|first3=T. A.|journal= Metal Science and Heat Treatment|volume= 21|issue= 10|pages= 741–746|bibcode= 1979MSHT...21..741G|s2cid= 135853965}}</ref> Selenium produces the same machinability improvement in copper alloys.<ref>{{cite book|url= https://books.google.com/books?id=sxkPJzmkhnUC&pg=PA278 |page=278 |title= Copper and Copper Alloys|isbn= 978-0-87170-726-0|last= Davis |first=Joseph R.|publisher=[[ASM International (society)|ASM International]]|date= 2001}}</ref>

===Lithium–selenium batteries===
The lithium–selenium (Li–Se) battery was considered for energy storage in the family of lithium batteries in the 2010s.<ref>{{cite journal|doi= 10.1039/C6SE00094K|title= The rise of lithium–selenium batteries|date= 2017|last1= Eftekhari|first1=Ali|journal= Sustainable Energy & Fuels|volume= 1|pages= 14–29}}</ref>

===Solar cells===
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.<ref>{{cite journal |last1=Adams |first1=William Grylls |last2=Day |first2=Richard Evans |title=The Action of Light on Selenium |journal=Philosophical Transactions of the Royal Society of London |date=1877 |volume=167 |pages=313–349|bibcode=1877RSPT..167..313A }}</ref> Only a few years later, [[Charles Fritts]] fabricated the first thin-film solar cell, also using selenium as the photoabsorber. However, with the emergence of silicon solar cells in the 1950s, research on selenium thin-film solar cells declined. As a result, the record efficiency of 5.0% demonstrated by Tokio Nakada and Akio Kunioka in 1985 remained unchanged for more than 30 years.<ref>{{cite journal |last1=Nakada |first1=Tokio |last2=Kunioka |first2=Akio |title=Polycrystalline Thin-Film TiO 2 /Se Solar Cells |journal=Japanese Journal of Applied Physics |date=1 July 1985 |volume=24 |issue=7A |pages=L536 |doi=10.1143/JJAP.24.L536|bibcode=1985JaJAP..24L.536N |s2cid=118838432 }}</ref> In 2017, researchers from [[IBM]] achieved a new record efficiency of 6.5% by redesigning the device structure.<ref>{{cite journal |last1=Todorov |first1=Teodor K. |last2=Singh |first2=Saurabh |last3=Bishop |first3=Douglas M. |last4=Gunawan |first4=Oki |last5=Lee |first5=Yun Seog |last6=Gershon |first6=Talia S. |last7=Brew |first7=Kevin W. |last8=Antunez |first8=Priscilla D. |last9=Haight |first9=Richard |title=Ultrathin high band gap solar cells with improved efficiencies from the world's oldest photovoltaic material |journal=Nature Communications |date=25 September 2017 |volume=8 |issue=1 |page=682 |doi=10.1038/s41467-017-00582-9|pmid=28947765 |pmc=5613033 |bibcode=2017NatCo...8..682T }}</ref> Following this achievement, selenium has gained renewed interest as a wide bandgap photoabsorber with the potential of being integrated in [[Multi-junction solar cell|tandem]] with lower bandgap photoabsorbers.<ref>{{cite journal |last1=Youngman |first1=Tomas H. |last2=Nielsen |first2=Rasmus |last3=Crovetto |first3=Andrea |last4=Seger |first4=Brian |last5=Hansen |first5=Ole |last6=Chorkendorff |first6=Ib |last7=Vesborg |first7=Peter C. K. |title=Semitransparent Selenium Solar Cells as a Top Cell for Tandem Photovoltaics |journal=Solar RRL |date=July 2021 |volume=5 |issue=7 |doi=10.1002/solr.202100111|s2cid=235575161 }}</ref> In 2024, the first selenium-based tandem solar cell was demonstrated, showcasing a selenium top cell monolithically integrated with a silicon bottom cell.<ref>{{cite journal |last1=Nielsen |first1=Rasmus |last2=Crovetto |first2=Andrea |last3=Assar |first3=Alireza |last4=Hansen |first4=Ole |last5=Chorkendorff |first5=Ib |last6=Vesborg |first6=Peter C.K. |title=Monolithic Selenium/Silicon Tandem Solar Cells |journal=PRX Energy |date=12 March 2024 |volume=3 |issue=1 |page=013013 |doi=10.1103/PRXEnergy.3.013013|arxiv=2307.05996 |bibcode=2024PRXE....3a3013N }}</ref> 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]].<ref>{{cite journal |last1=Nielsen |first1=Rasmus |last2=Youngman |first2=Tomas H. |last3=Moustafa |first3=Hadeel |last4=Levcenco |first4=Sergiu |last5=Hempel |first5=Hannes |last6=Crovetto |first6=Andrea |last7=Olsen |first7=Thomas |last8=Hansen |first8=Ole |last9=Chorkendorff |first9=Ib |last10=Unold |first10=Thomas |last11=Vesborg |first11=Peter C. K. |title=Origin of photovoltaic losses in selenium solar cells with open-circuit voltages approaching 1 V |journal=Journal of Materials Chemistry A |date=2022 |volume=10 |issue=45 |pages=24199–24207 |doi=10.1039/D2TA07729A|s2cid=253315416 }}</ref><ref>{{Cite journal |last1=Nielsen |first1=Rasmus S. |last2=Gunawan |first2=Oki |last3=Todorov |first3=Teodor |last4=Møller |first4=Clara B. |last5=Hansen |first5=Ole |last6=Vesborg |first6=Peter C. K. |date=3 April 2025 |title=Variable-temperature and carrier-resolved photo-Hall measurements of high-performance selenium thin-film solar cells |journal=Physical Review B |volume=111 |issue=16 |pages=165202 |doi=10.1103/PhysRevB.111.165202 |arxiv=2409.12804 |bibcode=2025PhRvB.111p5202N |issn=2469-9950}}</ref> As of now, the only defect-engineering strategy that has been investigated for selenium thin-film solar cells involves [[Laser-heated pedestal growth|crystallizing selenium using a laser]].<ref>{{cite journal |last1=Nielsen |first1=Rasmus |last2=Hemmingsen |first2=Tobias H. |last3=Bonczyk |first3=Tobias G. |last4=Hansen |first4=Ole |last5=Chorkendorff |first5=Ib |last6=Vesborg |first6=Peter C. K. |title=Laser-Annealing and Solid-Phase Epitaxy of Selenium Thin-Film Solar Cells |journal=ACS Applied Energy Materials |date=11 September 2023 |volume=6 |issue=17 |pages=8849–8856 |doi=10.1021/acsaem.3c01464|arxiv=2306.11311 |s2cid=259203956 }}</ref>

===Photoconductors===
Amorphous selenium (α-Se) thin films have found application as photoconductors in [[flat-panel detector|flat-panel X-ray detectors]]. These detectors use amorphous selenium to capture and convert incident X-ray photons directly into electric charge. Selenium has been chosen for this application among other semiconductors owing to a combination of its favorable technological and physical properties:<ref name=r1>{{cite journal|doi=10.1109/JSEN.2019.2950319|title=Recent Developments of Amorphous Selenium-Based X-Ray Detectors: A Review |year=2020 |last1=Huang |first1=Heyuan |last2=Abbaszadeh |first2=Shiva |journal=IEEE Sensors Journal |volume=20 |issue=4 |pages=1694–1704 |bibcode=2020ISenJ..20.1694H |s2cid=208833373 |doi-access=free }}</ref><ref name=r2>{{cite journal|doi=10.1002/pssb.200982007|title=Amorphous selenium and its alloys from early xeroradiography to high resolution X-ray image detectors and ultrasensitive imaging tubes |year=2009 |last1=Kasap |first1=Safa |last2=Frey |first2=Joel B. |last3=Belev |first3=George |last4=Tousignant |first4=Olivier |last5=Mani |first5=Habib |last6=Laperriere |first6=Luc |last7=Reznik |first7=Alla|author7-link=Alla Reznik |last8=Rowlands |first8=John A. |journal=Physica Status Solidi B |volume=246 |issue=8 |pages=1794–1805 |bibcode=2009PSSBR.246.1794K |s2cid=122848842 }}</ref>
# Amorphous selenium has a low melting point, high vapor pressure, and uniform structure. These three properties allow quick and easy deposition of large-area uniform films with a thickness up to 1&nbsp;mm at a rate of 1–5&nbsp;μm/min. Their uniformity and lack of grain boundaries, which are intrinsic to polycrystalline materials, improve the X-ray image quality. Meanwhile the large area is essential for scanning the human body or luggage items. 
# Selenium is less toxic than many compound semiconductors that contain arsenic or heavy metals such as mercury or lead.
# The mobility in applied electric field is sufficiently high both for electrons and holes, so that in a typical 0.2&nbsp;mm thick device, c. 98% of electrons and holes produced by X-rays are collected at the electrodes without being trapped by various defects. Consequently, device sensitivity is high, and its behavior is easy to describe by simple transport equations.

===Rectifiers===
[[Selenium rectifier]]s were first used in 1933. They have mostly been replaced by silicon-based devices. One notable exception is in power DC [[surge protection]], where the superior energy capabilities of selenium suppressors make them more desirable than [[metal-oxide varistor]]s.{{citation needed|date=July 2023}}

===Other uses===
The demand for selenium by the electronics industry is declining.<ref name="usgs" /> Its [[photovoltaics|photovoltaic]] and [[photoconductivity|photoconductive]] properties are still useful in [[photocopying]],<ref>{{cite journal|doi =10.1080/03086648808079729|title =Application of Selenium-Tellurium Photoconductors to the Xerographic Copying and Printing Processes|date =1988|last1 =Springett|first1 = B. E.|journal =Phosphorus and Sulfur and the Related Elements|volume =38|issue =3–4|pages =341–350}}</ref><ref>{{cite book|url =https://books.google.com/books?id=y1BuoXpPX3kC&pg=PA547| pages =547–548|title =Computer Systems Architecture: A Networking Approach|isbn =978-0-321-34079-5|last=Williams |first=Rob|date =2006| publisher= Prentice Hall}}</ref><ref>{{cite book|chapter-url =https://books.google.com/books?id=y8U4HGZP_O0C&pg=PA81| pages= 81–83| chapter= The Laser Printer|publisher =Wiley-VCH|title =Lasers|isbn =978-3-527-64005-8|last1=Diels |first1=Jean-Claude|last2=Arissian |first2=Ladan|date =2011}}</ref><ref>{{cite book|url =https://books.google.com/books?id=BiOxDxNMeyoC&pg=PA3| pages=3–5| publisher = Springer|title =Organic Electronics|isbn =978-3-642-04537-0|author =Meller, Gregor|author2 =Grasser, Tibor|name-list-style =amp|date =2009}}</ref><!--The use of tellurium-doped selenium was first displaced by amorphous silicon and now organic photosensitive polymers took over making the selenium drums obsolete technology.--> [[photocell]]s, [[light meter]]s and [[solar cell]]s. Its use as a photoconductor in plain-paper copiers once was a leading application, but in the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers switched to organic photoconductors.{{Citation needed|date=June 2024}}

[[Zinc selenide]] was the first material for blue [[LED]]s, but [[gallium nitride]] dominates that market.<ref>{{cite book |last=Normile |first=Dennis |title=Popular Science |date=2000 |page=57 |chapter=The birth of the Blues |chapter-url=https://books.google.com/books?id=D2zyNlMu7kkC&pg=PA57}}</ref> [[Cadmium selenide]] can be used to make [[quantum dot]]s.<ref>{{cite journal|doi=10.1021/ed300568e |title=Simple Syntheses of CdSe Quantum Dots |date=2014 |last1=Landry |first1=Matthew L. |last2=Morrell |first2=Thomas E. |last3=Karagounis |first3=Theodora K. |last4=Hsia |first4=Chih-Hao |last5=Wang |first5=Chia-Ying |journal=Journal of Chemical Education |volume=91 |issue=2 |pages=274–279 |bibcode=2014JChEd..91..274L }}</ref> Sheets of amorphous selenium convert [[X-ray]] images to patterns of charge in [[xeroradiography]] and in solid-state, flat-panel X-ray cameras.<ref>{{cite journal |last1=Kasap |first1=Safa |last2=Frey |first2=Joel B. |last3=Belev |first3=George |last4=Tousignant |first4=Olivier |last5=Mani |first5=Habib |last6=Laperriere |first6=Luc |last7=Reznik |first7=Alla |last8=Rowlands |first8=John A. |display-authors=3 |date=2009 |title=Amorphous selenium and its alloys from early xeroradiography to high resolution X-ray image detectors and ultrasensitive imaging tubes |journal=Physica Status Solidi B |volume=246 |issue=8 |pages=1794–1805 |bibcode=2009PSSBR.246.1794K |doi=10.1002/pssb.200982007 |s2cid=122848842}}</ref> Ionized selenium (Se+24, where 24 of the outer D, S and P orbitals are stripped away due to high input energies{{clarify|date=April 2023}}) is one of the active mediums used in X-ray lasers.<ref>{{Cite book |last=Svelto |first=Orazio |title=Principles of LASERs fourth ed |publisher=Plenum |year=1998 |isbn=978-0-306-45748-7 |pages=457}}</ref> <sup>75</sup>Se is used as a gamma source in industrial radiography.<ref>{{cite news |last1=Hayward |first1=Peter |last2=Currie |first2=Dean |title=Radiography of Welds Using Selenium 75, Ir 192 and X-rays |url=http://www.ndt.net/article/apcndt2006/papers/12.pdf}}</ref>

Selenium catalyzes some chemical reactions, but it is not widely used because of issues with toxicity.<ref>{{Cite journal|url=https://pubs.rsc.org/en/content/articlehtml/2019/cy/c8cy02274g|doi = 10.1039/C8CY02274G|title = Selenium reagents as catalysts|year = 2019|last1 = Singh|first1 = Fateh V.|last2 = Wirth|first2 = Thomas|journal = Catalysis Science & Technology|volume = 9|issue = 5|pages = 1073–1091|s2cid = 104468775}}</ref> In [[X-ray crystallography]], incorporation of one or more selenium atoms in place of sulfur helps with multiple-wavelength anomalous dispersion and [[single wavelength anomalous dispersion]] phasing.<ref>{{cite journal|doi= 10.1098/rspa.1993.0087|title= New Techniques of Applying Multi-Wavelength Anomalous Scattering Data|date= 1993|last1= Hai-Fu|first1= F.|last2= Woolfson|first2=M. M.|last3= Jia-Xing|first3= Y.|journal= Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume= 442|issue= 1914|pages= 13–32|bibcode= 1993RSPSA.442...13H |s2cid= 122722520}}</ref>

Selenium is used in the [[photographic print toning|toning of photographic prints]], and it is sold as a toner by numerous photographic manufacturers. Selenium intensifies and extends the tonal range of black-and-white photographic images and improves the permanence of prints.<ref>{{cite journal |last=MacLean |first=Marion E. |date=1937 |title=A project for general chemistry students: Color toning of photographic prints |journal=Journal of Chemical Education |volume=14 |issue=1 |page=31 |bibcode=1937JChEd..14...31M |doi=10.1021/ed014p31}}</ref><ref>{{cite journal |last=Penichon |first=Sylvie |date=1999 |title=Differences in Image Tonality Produced by Different Toning Protocols for Matte Collodion Photographs |journal=Journal of the American Institute for Conservation |volume=38 |issue=2 |pages=124–143 |doi=10.2307/3180042 |jstor=3180042}}</ref><ref>{{cite book |last=McKenzie |first=Joy |url=https://archive.org/details/exploringbasicbl0000mcke |title=Exploring Basic Black & White Photography |date=2003 |publisher=Delmar |isbn=978-1-4018-1556-1 |page=[https://archive.org/details/exploringbasicbl0000mcke/page/176 176] |url-access=registration}}</ref> Small amounts of organoselenium compounds have been used to modify the catalysts used for the [[sulfur vulcanization|vulcanization]] for the production of rubber.<ref name="Naumov" /> Selenium is used in some anti-dandruff shampoos in the form of [[selenium disulfide]] such as Selsun and Vichy Dereos<ref>{{Cite web |title=What is Dandruff? |url=https://www.vichy.co.uk/on/demandware.static/-/Sites-vic-master-catalog/default/dw41dcc5e7/VIC/ProductImages/Blog-Imagery-Vichy/Vichy_Customer_Leaflet_Dandruff.pdf |access-date=3 October 2023 |website=Vichy UK}}</ref> brands.

==Pollution==
Selenium pollution might impact some aquatic systems and may be caused by anthropogenic factors such as farming runoff and industrial processes.<ref>{{cite journal|last=Lemly|first=A. Dennis|date=2004-09-01|title=Aquatic selenium pollution is a global environmental safety issue|url=http://www.sciencedirect.com/science/article/pii/S0147651303000952|journal=Ecotoxicology and Environmental Safety|language=en|volume=59|issue=1|pages=44–56|doi=10.1016/S0147-6513(03)00095-2|pmid=15261722|bibcode=2004EcoES..59...44L |issn=0147-6513}}</ref> People who eat more fish are generally healthier than those who eat less,<ref>{{cite journal|last1=Estruch |first1=Ramon |last2=Sacanella |first2=Emilio |last3=Ros |first3=Emilio |date=4 January 2021 |title=Should we all go pesco-vegetarian? |journal=European Heart Journal |volume=42|issue=12 |pages=1144–1146 |doi=10.1093/eurheartj/ehaa1088|pmid=33393612|issn=0195-668X|doi-access=free}}</ref> which suggests no major human health concern from selenium pollution, although selenium has a potential effect on humans.<ref>{{cite journal|last1=Gribble |first1=Matthew| last2=Karimi |first2=Roxanne |last3=Feingold |first3=Beth |last4=Nyland |first4=Jennifer |last5=O'Hara |first5=Todd |last6=Gladyshev |first6=Michail |last7=Chen |first7=Celia | date=September 8, 2015 | title=Mercury, selenium and fish oils in marine food webs and implications for human health |quote=at higher doses, selenium might be toxic to a range of animals including humans |journal=Journal of the Marine Biological Association of the United Kingdom |volume=1 |issue=96 |pages=43–59 |doi=10.1017/S0025315415001356 |pmid=26834292 |pmc=4720108}}</ref>

Selenium poisoning of water systems may result whenever new [[Agricultural pollution|agricultural run-off]] courses through dry lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in wetlands as the water evaporates. Selenium pollution of waterways also occurs when selenium is leached from coal flue ash, mining and [[metal smelting]], crude oil processing, and landfill.<ref>{{cite journal |last=Lemly |first=D. |date=2004 |title=Aquatic selenium pollution is a global environmental safety issue |url=https://zenodo.org/record/1259837 |journal=Ecotoxicology and Environmental Safety |volume=59 |issue=1 |pages=44–56 |bibcode=2004EcoES..59...44L |doi=10.1016/S0147-6513(03)00095-2 |pmid=15261722}}</ref> High selenium levels in waterways were found to cause congenital disorders in oviparous species, including wetland birds<ref>{{cite book |last=Ohlendorf |first=H. M. |url=https://books.google.com/books?id=qN0I3husm50C&pg=PA477 |title=Ecotoxicology of selenium |date=2003 |publisher=Lewis Publishers |isbn=978-1-56670-546-2 |series=Handbook of ecotoxicology |location=Boca Raton |pages=466–491}}</ref> and fish.<ref>{{cite journal |last=Lemly |first=A. D. |date=1997 |title=A teratogenic deformity index for evaluating impacts of selenium on fish populations |url=https://zenodo.org/record/1229572 |journal=Ecotoxicology and Environmental Safety |volume=37 |issue=3 |pages=259–266 |bibcode=1997EcoES..37..259L |doi=10.1006/eesa.1997.1554 |pmid=9378093}}</ref> Elevated dietary [[methylmercury]] levels can amplify the harm of selenium toxicity in oviparous species.<ref>{{cite journal |last1=Penglase |first1=S. |last2=Hamre |first2=K. |last3=Ellingsen |first3=S. |date=2014 |title=Selenium and mercury have a synergistic negative effect on fish reproduction |journal=Aquatic Toxicology |volume=149 |pages=16–24 |bibcode=2014AqTox.149...16P |doi=10.1016/j.aquatox.2014.01.020 |pmid=24555955}}</ref><ref>{{cite journal |last1=Heinz |first1=G. H. |last2=Hoffman |first2=D. J. |date=1998 |title=Methylmercury chloride and selenomethionine interactions on health and reproduction in mallards |journal=Environmental Toxicology and Chemistry |volume=17 |issue=2 |pages=139–145 |doi=10.1002/etc.5620170202 |bibcode=1998EnvTC..17..139H |s2cid=95466655}}</ref>

Selenium is [[bioaccumulation|bioaccumulated]] in aquatic habitats, which results in higher concentrations in organisms than the surrounding water. Organoselenium compounds can be concentrated over 200,000 times by [[zooplankton]] when water concentrations are in the 0.5 to 0.8&nbsp;μg Se/L range. Inorganic selenium bioaccumulates more readily in [[phytoplankton]] than zooplankton. Phytoplankton can concentrate inorganic selenium by a factor of 3000. Further concentration through bioaccumulation occurs along the food chain, as predators consume selenium-rich prey. It is recommended that a water concentration of 2&nbsp;μg Se/L be considered highly hazardous to sensitive fish and [[Water bird|aquatic birds]]. Selenium poisoning can be passed from parents to offspring through the egg, and selenium poisoning may persist for many generations. Reproduction of mallard ducks is impaired at dietary concentrations of 7&nbsp;μg Se/L. Many [[Benthic zone|benthic]] invertebrates can tolerate selenium concentrations up to 300&nbsp;μg/L of selenium in their diet.<ref>{{cite book| last = Lemly| first = Dennis| title = Selenium Assessment in Aquatic Ecosystems: A guide for hazard evaluation and water quality criteria| publisher = Springer| year = 1998| url = https://books.google.com/books?id=qGH37iOW7yMC&q=Selenium+Assessment+in+Aquatic+Ecosystems| isbn = 0-387-95346-9}}</ref>

Bioaccumulation of selenium in aquatic environments causes fish kills depending on the species in the affected area. There are, however, a few species that have been seen to survive these events and tolerate the increased selenium. It has also been suggested that the season could have an impact on the harmful effects of selenium on fish.<ref name="Hamilton 1–31">{{Cite journal|last=Hamilton|first=Steven J.|date=2004-06-29|title=Review of selenium toxicity in the aquatic food chain|url=http://www.sciencedirect.com/science/article/pii/S0048969704000609|journal=Science of the Total Environment|language=en|volume=326|issue=1|pages=1–31|doi=10.1016/j.scitotenv.2004.01.019|pmid=15142762|bibcode=2004ScTEn.326....1H|issn=0048-9697}}</ref> Substantial physiological changes may occur in fish with high tissue concentrations of selenium. Fish affected by selenium may experience swelling of the [[gill lamellae]], which impedes oxygen diffusion across the gills and blood flow within the gills. Respiratory capacity is further reduced due to selenium binding to [[hemoglobin]]. Other problems include degeneration of liver tissue, swelling around the heart, damaged egg follicles in ovaries, [[cataract]]s, and accumulation of fluid in the body cavity and head. Selenium often causes a malformed fish fetus which may have problems feeding or respiring; distortion of the fins or spine is also common. Adult fish may appear healthy despite their inability to produce viable offspring.{{Citation needed|date=June 2024}}

===Examples===
In [[Belews Lake]] North Carolina, 19 species of fish were eliminated from the lake due to 150–200&nbsp;μg Se/L wastewater discharged from 1974 to 1986 from a [[Duke Energy]] coal-fired power plant.<ref>{{Cite journal |last=Lemly |first=A.Dennis |date=2002–2004 |title=Symptoms and implications of selenium toxicity in fish: the Belews Lake case example |url=http://dx.doi.org/10.1016/s0166-445x(01)00264-8 |journal=Aquatic Toxicology |volume=57 |issue=1–2 |pages=39–49 |doi=10.1016/s0166-445x(01)00264-8 |pmid=11879937 |bibcode=2002AqTox..57...39L |issn=0166-445X}}</ref> At the [[Kesterson National Wildlife Refuge]] in California, thousands of fish and waterbirds were poisoned by selenium in agricultural irrigation drainage.{{Citation needed|date=June 2024}}

==Biological role==
{{Main|Selenium in biology}}
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Although it is toxic in large doses, selenium is an essential [[micronutrient]] for animals. In plants, it occurs as a bystander mineral,<ref>{{Cite book |last=Atroshi |first=Faik |url=https://books.google.com/books?id=UCShDwAAQBAJ&dq=info:AK393JUK2tEJ:scholar.google.com/&pg=PR11 |title=Pharmacology and Nutritional Intervention in the Treatment of Disease |date=2014-05-28 |publisher=BoD – Books on Demand |isbn=978-953-51-1383-6 |language=en}}</ref> sometimes in toxic proportions in [[forage]] (some plants may accumulate selenium as a defense against being eaten by animals,<ref>{{Cite journal |last1=Freeman |first1=John L. |last2=Lindblom |first2=Stormy Dawn |last3=Quinn |first3=Colin F. |last4=Fakra |first4=Sirine |last5=Marcus |first5=Matthew A. |last6=Pilon-Smits |first6=Elizabeth A. H. |date=2007 |title=Selenium accumulation protects plants from herbivory by Orthoptera via toxicity and deterrence |journal=The New Phytologist |volume=175 |issue=3 |pages=490–500 |doi=10.1111/j.1469-8137.2007.02119.x |issn=0028-646X |pmid=17635224|s2cid=2330947 |doi-access=free |bibcode=2007NewPh.175..490F }}</ref> but other plants, such as [[locoweed]], require selenium, and their growth indicates the presence of selenium in soil).<ref>{{Cite report |date=1986 |title=Selenium concentrations in leaf material from Astragalus Oxyphysus (diablo locoweed) and Atriplex Lentiformis (quail bush) in the interior Coast Ranges and the western San Joaquin Valley, California |institution=U.S. Geological Survey |id=Water-Resources Investigations Report 86-4066|url=https://www.academia.edu/61786801}}</ref> The selenium content in the human body is believed to be in the range of 13–20&nbsp;mg.<ref>A common reference for this is {{cite journal |last1=Schroeder |first1=H. A. |last2=Frost |first2=D. V. |last3=Balassa |first3=J. J. |date=1970 |title=Essential trace metals in man: Selenium |journal=Journal of Chronic Diseases |volume=23 |issue=4 |pages=227–243 |doi=10.1016/0021-9681(70)90003-2 |osti=6424964 |pmid=4926392}}</ref>

Selenium is a component of the unusual [[amino acid]]s [[selenocysteine]] and [[selenomethionine]]. In humans, selenium is a [[dietary mineral|trace element]] nutrient that functions as [[Cofactor (biochemistry)|cofactor]] for [[redox|reduction]] of [[antioxidant]] enzymes, such as [[glutathione peroxidase]]s and certain forms of [[thioredoxin reductase]] found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium).<ref>{{cite journal |doi=10.1152/physrev.00039.2013 |title=Selenoproteins: Molecular Pathways and Physiological Roles |date=2014 |last1=Labunskyy |first1=Vyacheslav M. |last2=Hatfield |first2=Dolph L. |last3=Gladyshev |first3=Vadim N. |journal=Physiological Reviews |volume=94 |issue=3 |pages=739–777 |pmid=24987004 |pmc=4101630 }}</ref>
 
The [[glutathione peroxidase]] family of enzymes (GSH-Px) catalyze reactions that remove reactive oxygen species such as [[hydrogen peroxide]] and organic [[hydroperoxide]]s.<ref>{{cite web | title = Entrez Gene: GPX1 glutathione peroxidase 1| url = https://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=2876}}</ref>

The [[Thyroid|thyroid gland]] and every cell that uses thyroid hormone also use selenium,<ref>{{Cite journal|last1=Pakdel|first1=Farzad|last2=Ghazavi|first2=Roghayeh|last3=Heidary|first3=Roghayeh|last4=Nezamabadi|first4=Athena|last5=Parvizi|first5=Maryam|last6=Haji Safar Ali Memar|first6=Mahsa|last7=Gharebaghi|first7=Reza|last8=Heidary|first8=Fatemeh|date=2019|title=Effect of Selenium on Thyroid Disorders: Scientometric Analysis|journal=Iranian Journal of Public Health|volume=48|issue=3|pages=410–420|issn=2251-6085|pmc=6570790|pmid=31223567}}</ref> which is a cofactor for the three of the four known types of [[Deiodinase|thyroid hormone deiodinases]], which activate and then deactivate various [[thyroid hormones]] and their metabolites; the [[iodothyronine deiodinase]]s are the subfamily of deiodinase enzymes that use selenium as the otherwise rare amino acid selenocysteine.

Increased dietary selenium reduces the effects of mercury toxicity,<ref>{{cite journal|last1=Ralston|first1=N. V.|last2=Ralston|first2=C. R.|last3=Blackwell|first3=JL III|last4=Raymond|first4=L. J.|doi=10.1016/j.neuro.2008.07.007|title=Dietary and tissue selenium in relation to methylmercury toxicity|url=http://www.soest.hawaii.edu/oceanography/courses_html/OCN331/Mercury3.pdf|journal=Neurotoxicology|volume=29|issue=5|pages=802–811|pmid=18761370|date=2008|bibcode=2008NeuTx..29..802R |citeseerx=10.1.1.549.3878|access-date=2012-09-28|archive-date=2012-07-24|archive-url=https://web.archive.org/web/20120724193621/http://www.soest.hawaii.edu/oceanography/courses_html/OCN331/Mercury3.pdf|url-status=dead}}</ref><ref>{{cite journal|last1=Penglase|first1=S.|last2=Hamre|first2=K.|last3=Ellingsen|first3=S.|title=Selenium prevents downregulation of antioxidant selenoprotein genes by methylmercury|journal=Free Radical Biology and Medicine|date=2014|volume=75|pages=95–104|doi=10.1016/j.freeradbiomed.2014.07.019|pmid=25064324|hdl=1956/8708|hdl-access=free}}</ref><ref>{{cite journal|last1=Usuki|first1=F.|last2=Yamashita|first2=A.|last3=Fujimura|first3=M.|title=Post-transcriptional defects of antioxidant selenoenzymes cause oxidative stress under methylmercury exposure|journal=The Journal of Biological Chemistry|date=2011|volume=286|issue=8|pages=6641–6649|doi=10.1074/jbc.M110.168872|pmid=21106535|pmc=3057802|doi-access=free}}</ref> although it is effective only at low to modest doses of mercury.<ref>{{cite journal|last1=Ohi|first1=G.|last2=Seki|first2=H.|last3=Maeda|first3=H.|last4=Yagyu|first4=H.|title=Protective effect of selenite against methylmercury toxicity: observations concerning time, dose and route factors in the development of selenium attenuation|journal=Industrial Health|date=1975|volume=13|issue=3|pages=93–99|doi=10.2486/indhealth.13.93|doi-access=free|bibcode=1975IndHe..13...93O }}</ref> 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.<ref>{{cite journal |last1= Ralston |first1= N. V. C. |last2= Raymond |first2=L. J. |date= 2010 |title= Dietary selenium's protective effects against methylmercury toxicity |journal= Toxicology |volume= 278 |issue= 1|pages= 112–123 |doi= 10.1016/j.tox.2010.06.004 |pmid= 20561558 |bibcode= 2010Toxgy.278..112R }}</ref><ref>{{cite journal |last1= Carvalho |first1= C. M. L. |last2= Chew |first2= Hashemy S. I. |last3= Hashemy |first3= J. |last4= Lu |first4= A. |date= 2008 |title= Inhibition of the human thioredoxin system: A molecular mechanism of mercury toxicity |journal= Journal of Biological Chemistry |volume= 283 |issue= 18|pages= 11913–11923 |doi= 10.1074/jbc.M710133200 |pmid= 18321861 |last5= Holmgren |first5=A. |display-authors=3|doi-access= free }}</ref> The selenium-containing compound [[selenoneine]] is present in the blood of [[bluefin tuna]].<ref>Michiaki Yamashita, Shintaro Imamura, Md. Anwar Hossain, Ken Touhata, Takeshi Yabu, and Yumiko Yamashita, [http://www.fasebj.org/content/26/1_Supplement/969.13.short "Strong antioxidant activity of the novel selenium-containing imidazole compound 'selenoneine{{'"}}], ''The FASEB Journal'', vol. 26 no. 1, supplement 969.13, April 2012</ref><ref>{{cite journal | pmid = 21540999 | doi= 10.4331/wjbc.v1.i5.144 | volume=1 | issue= 5 | title=Discovery of the strong antioxidant selenoneine in tuna and selenium redox metabolism | pmc=3083957 | year=2010 | journal=World Journal of Biological Chemistry | pages=144–150 | last1 = Yamashita | first1 = Y. | last2 = Yabu | first2 = T. | last3 = Yamashita | first3 = M. | doi-access= free }}</ref> Certain plants are considered indicators of high selenium content of the soil because they require high levels of selenium to thrive. The main selenium indicator plants are ''[[Astragalus (plant)|Astragalus]]'' species (including some [[locoweed]]s), prince's plume (''[[Stanleya (plant)|Stanleya]]'' sp.), woody asters (''[[Xylorhiza (plant)|Xylorhiza]]'' sp.), and false goldenweed (''[[Oonopsis]]'' sp.).<ref>{{cite web |last=Zane Davis |first=T. |date=2008-03-27 |title=Selenium in Plants |url=http://www.ars.usda.gov/SP2UserFiles/Place/54282000/PPClassPPSlides/3-27-08DavisSelenium.pdf |access-date=2008-12-05 |page=8}}</ref>

===Evolution in biology===
{{Main|Evolution of dietary antioxidants}}

From about three billion years ago, [[prokaryotic]] selenoprotein families drove the evolution of the amino acid selenocysteine. Several selenoproteins are known in bacteria, archaea, and eukaryotes, invariably owing to the presence of selenocysteine,<ref name="glady">{{cite journal |title=Selenocysteine-containing proteins in mammals |journal=Journal of Biomedical Science |volume=6 |issue=3 |pages=151–160 |date=1999 |pmid=10343164 |doi=10.1007/BF02255899 |last1=Gladyshev |first1=Vadim N. |last2=Hatfield |first2=Dolph L.|url=https://digitalcommons.unl.edu/biochemgladyshev/77 }}</ref>  Just as for mammals, selenoprotein protect unicellular organisms against oxidative damage.

Selenoprotein families of GSH-Px and the deiodinases of eukaryotic cells seem to have a bacterial [[Phylogenetics|phylogenetic]] origin. The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchins, fish, and chickens.<ref>{{cite journal |last=Stadtman |first=T. C. |title=Selenocysteine |journal=Annual Review of Biochemistry |volume=65 |pages=83–100 |date=1996 |issue=1 |pmid=8811175 |doi=10.1146/annurev.bi.65.070196.000503}}</ref>

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.<ref name="glady" /> Marine organisms retained and sometimes expanded their selenoproteomes, whereas the selenoproteomes of some terrestrial organisms were lowered or completely lost. These findings suggest that, with the exception of [[vertebrate]]s, aquatic life supports selenium use, whereas terrestrial habitats lead to lowered use of this trace element.<ref>{{cite journal |title=Evolutionary dynamics of eukaryotic selenoproteomes: large selenoproteomes may associate with aquatic life and small with terrestrial life |journal=Genome Biology |volume=8 |issue=9 |pages=R198 |date=2007 |pmid=17880704 |pmc=2375036 |doi=10.1186/gb-2007-8-9-r198 |last1=Lobanov |first1=Alexey V. |last2=Fomenko |first2=Dmitri E. |last3=Zhang |first3=Yan |last4=Sengupta |first4=Aniruddha |last5=Hatfield |first5=Dolph L. |last6=Gladyshev |first6=Vadim N. |display-authors=3 |doi-access=free }}</ref> Marine fishes and vertebrate thyroid glands have the highest concentration of selenium and iodine. From about 500&nbsp;million years ago, freshwater and terrestrial plants slowly optimized the production of "new" endogenous antioxidants such as [[ascorbic acid]] (vitamin C), [[polyphenol]]s (including flavonoids), [[tocopherol]]s, etc. A few of these appeared in the last 50–200 million years in fruits and flowers of [[angiosperm]] plants. In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late [[Jurassic]] period.{{Citation needed|date=June 2016}}

About 200 million years ago, new selenoproteins were developed as mammalian GSH-Px enzymes.<ref>{{cite journal |title=Reconsidering the evolution of eukaryotic selenoproteins: a novel nonmammalian family with scattered phylogenetic distribution |journal=EMBO Reports |volume=5 |issue=1 |pages=71–7 |date=2004 |pmid=14710190 |pmc=1298953 |doi=10.1038/sj.embor.7400036 |last1=Castellano |first1=Sergi |last2=Novoselov |first2=Sergey V. |last3=Kryukov |first3=Gregory V. |last4=Lescure |first4=Alain |last5=Blanco |first5=Enrique |last6=Krol |first6=Alain |last7=Gladyshev |first7=Vadim N. |last8=Guigó |first8=Roderic |display-authors=3}}</ref><ref>{{cite journal |title=The prokaryotic selenoproteome |journal=EMBO Reports |volume=5 |issue=5 |pages=538–43 |date=2004 |pmid=15105824 |pmc=1299047 |doi=10.1038/sj.embor.7400126 |last1=Kryukov |first1=Gregory V. |last2=Gladyshev |first2=Vadim N.}}</ref><ref>{{cite journal |title=Selenoprotein synthesis in archaea: identification of an mRNA element of ''Methanococcus jannaschii'' probably directing selenocysteine insertion |journal=Journal of Molecular Biology |volume=266 |issue=4 |pages=637–41 |date=1997 |pmid=9102456 |doi=10.1006/jmbi.1996.0812 |last1=Wilting |first1=R. |last2=Schorling |first2=S. |last3=Persson |first3=B. C. |last4=Böck |first4=A.}}</ref><ref>{{cite journal |title=The microbial selenoproteome of the Sargasso Sea |journal=Genome Biology |volume=6 |issue=4 |pages=R37 |date=2005 |pmid=15833124 |pmc=1088965 |doi=10.1186/gb-2005-6-4-r37 |last1=Zhang |first1=Yan |last2=Fomenko |first2=Dmitri E. |last3=Gladyshev |first3=Vadim N. |doi-access=free }}</ref>

===Toxicity===
[[File:Selenium paradox.jpg|thumb|upright=1.3|Selenium at nutritional levels or low concentrations is required for cell [[homeostasis]], playing a role as an [[Antioxidant|anti-oxidant]] through [[selenoprotein]]s, thus, act chemo-preventive against cancer. In contrast, supra-nutritional levels or higher concentrations act as [[pro-oxidant]] in tumour cells, thus can be exploited as chemo-therapeutic against [[cancer]].<ref name=":0">{{Cite journal|last1=Razaghi|first1=Ali|last2=Poorebrahim|first2=Mansour|last3=Sarhan|first3=Dhifaf|last4=Björnstedt|first4=Mikael|date=2021-09-01|title=Selenium stimulates the antitumour immunity: Insights to future research|url=https://www.ejcancer.com/article/S0959-8049(21)00462-7/abstract|journal=European Journal of Cancer|language=English|volume=155|pages=256–267|doi=10.1016/j.ejca.2021.07.013|pmid=34392068|issn=0959-8049|doi-access=free}}</ref>]]
Although selenium is an essential [[dietary mineral|trace element]], it is toxic if taken in excess. Exceeding the [[Dietary Reference Intake|Tolerable Upper Intake Level]] of 400 micrograms per day can lead to selenosis.<ref>{{cite web |url= http://ods.od.nih.gov/factsheets/selenium.asp#h7 |title= Dietary Supplement Fact Sheet: Selenium |publisher= National Institutes of Health; Office of Dietary Supplements |access-date= 2009-01-05}}</ref> This 400&nbsp;μg Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow-up study on the same five people in 1992.<ref>{{cite book |title= Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids |publisher= Institute of Medicine |date= August 15, 2000 |pages= 314–315 |url= http://www.nap.edu/openbook.php?record_id=9810&page=315 |isbn= 978-0-309-06949-6 |author=((Panel on Dietary Antioxidants and Related Compounds, Subcommittees on Upper Reference Levels of Nutrients and Interpretation and Uses of DRIs, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine))|doi= 10.17226/9810 |pmid= 25077263 }}</ref> The 1992 study found the maximum safe dietary selenium intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to avoid creating an imbalance of nutrients in the diet and to accord with data from other countries.<ref>{{cite journal |last1= Yang |first1=G.|last2= Zhou |first2=R.|date= 1994 |title= Further Observations on the Human Maximum Safe Dietary Selenium Intake in a Seleniferous Area of China |journal= Journal of Trace Elements and Electrolytes in Health and Disease |volume= 8 |issue= 3–4 |pages= 159–165 |pmid=7599506
}}</ref> In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous [[shale]]) have suffered from selenium toxicity. This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded.<ref>{{cite journal |last1= Yang |first1=Guang-Qi|last2= Xia |first2=Yi-Ming|date= 1995 |title= Studies on Human Dietary Requirements and Safe Range of Dietary Intakes of Selenium in China and Their Application in the Prevention of Related Endemic Diseases |journal= Biomedical and Environmental Sciences |volume= 8 |issue= 3|pages= 187–201 |pmid= 8561918}}</ref>

Signs and symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, [[sloughing]] of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can exhibit [[cirrhosis]] of the liver, [[pulmonary edema]], or death.<ref>{{cite web |url= http://www.atsdr.cdc.gov/toxprofiles/tp92-c3.pdf |publisher= Agency for Toxic Substances and Disease Registry |title= Public Health Statement: Health Effects |access-date= 2009-01-05}}</ref> Elemental selenium and most metallic [[selenide]]s have relatively low toxicities because of low [[bioavailability]]. By contrast, [[selenate]]s and [[Selenite (ion)|selenites]] have an oxidant mode of action similar to that of [[arsenic trioxide]] and are very toxic. The chronic toxic dose of selenite for humans is about 2400 to 3000 micrograms of selenium per day.<ref>{{cite journal |last= Wilber |first=C. G. |title= Toxicology of selenium |date= 1980 |journal= Clinical Toxicology |volume= 17 |pages= 171–230 |doi= 10.3109/15563658008985076 |pmid= 6998645| issue= 2}}</ref> [[Hydrogen selenide]] is an extremely toxic, corrosive gas.<ref>{{cite journal |doi= 10.3109/10915818609140736 |title= Selenium Toxicity in Animals with Emphasis on Man |date= 1986 |journal= International Journal of Toxicology |volume= 5 |pages= 45–70 |last= Olson |first=O. E.|s2cid= 74619246 |doi-access= free }}</ref> 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.

On 19 April 2009, 21 [[Polo pony|polo ponies]] died shortly before a match in the United States Polo Open. Three days later, a pharmacy released a statement explaining that the horses had received an incorrect dose of one of the ingredients used in a vitamin/mineral supplement compound that had been incorrectly prepared by a [[compounding pharmacy]]. Analysis of blood levels of [[inorganic compound]]s in the supplement indicated the selenium concentrations were 10 to 15 times higher than normal in the [[blood sample]]s and 15 to 20 times higher than normal in the liver samples. Selenium was later confirmed to be the toxic factor.<ref>{{cite web| date= 2009-05-06| url= http://www.horsetalk.co.nz/news/2009/05/033.shtml| title= Polo pony selenium levels up to 20 times higher than normal| access-date= 2009-05-05| archive-date= 2016-08-19| archive-url= https://web.archive.org/web/20160819235242/http://www.horsetalk.co.nz/news/2009/05/033.shtml| url-status= dead}}</ref>

[[File:Se dose-response curve for juvenile salmon mortality - percent scale.jpg|thumb|Relationship between survival of juvenile salmon and concentration of selenium in their tissues after 90 days (Chinook salmon<ref name="Hamilton" />) or 45 days (Atlantic salmon<ref name="Poston" />) exposure to dietary selenium. The 10% lethality level (LC10=1.84 μg/g) was derived by applying the biphasic model of Brain and Cousens<ref>{{cite journal|last1=Brain |first1=P.|title=An equation to describe dose responses where there is stimulation of growth at low doses|journal=Weed Research|last2=Cousens |first2=R.|date=1989|doi=10.1111/j.1365-3180.1989.tb00845.x|volume= 29|pages= 93–96|issue=2|bibcode=1989WeedR..29...93B }}</ref> to only the Chinook salmon data. The Chinook salmon data comprise two series of dietary treatments, combined here because the effects on survival are indistinguishable.]]

In fish and other wildlife, selenium is necessary for life but toxic in high doses. For [[salmon]], the optimal selenium concentration is about 1 microgram selenium per gram of whole body weight. Much below that level, young salmon die from deficiency;<ref name="Poston">{{cite journal| last1= Poston|first1=H. A.| date= 1976| title= Vitamin E and selenium interrelations in the diet of Atlantic salmon (''Salmo salar''): gross, histological and biochemical signs|pmid=932827|journal= Journal of Nutrition|volume= 106|pages= 892–904| last2= Combs| first2=G. F. Jr.| last3= Leibovitz| first3=L.| issue= 7|doi=10.1093/jn/106.7.892}}</ref> much above, they die from toxic excess.<ref name="Hamilton">{{cite journal| last1= Hamilton|first1= Steven J. |date= 1990| title= Toxicity of organic selenium in the diet to chinook salmon|journal= Environ. Toxicol. Chem.|volume= 9|issue = 3| pages= 347–358|doi =10.1002/etc.5620090310| last2= Buhl| first2= Kevin J.| last3= Faerber| first3= Neil L.| last4= Bullard| first4= Fern A.| last5= Wiedmeyer| first5= Raymond H. |bibcode= 1990EnvTC...9..347H |display-authors=3}}</ref>

The [[Occupational Safety and Health Administration]] (OSHA) has set the legal limit ([[permissible exposure limit]]) for selenium in the workplace at 0.2&nbsp;mg/m<sup>3</sup> over an 8-hour workday. The [[National Institute for Occupational Safety and Health]] (NIOSH) has set a [[Recommended exposure limit]] (REL) of 0.2&nbsp;mg/m<sup>3</sup> over an 8-hour workday. At levels of 1&nbsp;mg/m<sup>3</sup>, selenium is [[IDLH|immediately dangerous to life and health]].<ref>{{Cite web|title = NIOSH Pocket Guide to Chemical Hazards – Selenium|url = https://www.cdc.gov/niosh/npg/npgd0550.html|publisher = National Institute for Occupational Safety & Health|location=United States|access-date = 2015-11-21}}</ref>

==== Detection in biological fluids ====
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. Some analytical techniques can distinguish organic from inorganic forms of the element. Both organic and inorganic forms of selenium are largely converted to monosaccharide conjugates (selenosugars) in the body before elimination in the urine. Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations.<ref>{{cite book |last=Baselt |first=R. |title=Disposition of Toxic Drugs and Chemicals in Man |date=2008 |publisher=Biomedical Publications |isbn=978-0-9626523-5-6 |edition=8th |place=Foster City, California |pages=1416–1420}}</ref>

===Deficiency===
{{Main|Selenium deficiency}}
Selenium deficiency can occur in patients with severely compromised [[intestine|intestinal]] function, those undergoing [[total parenteral nutrition]], and<ref>{{cite journal |title= Effect of micronutrient status on natural killer cell immune function in healthy free-living subjects aged ≥90 y |volume= 71 |issue= 2 |pages= 590–598 |journal= American Journal of Clinical Nutrition |pmid= 10648276 |date=2000 |last1= Ravaglia |first1=G.|last2= Forti |first2=P. |last3= Maioli |first3=F. |last4= Bastagli |first4=L. |last5= Facchini |first5=A. |last6= Mariani |first6=E. |last7= Savarino |first7=L. |last8= Sassi |first8=S. |last9= Cucinotta |first9=D. |last10=Lenaz |first10=G. |display-authors=3|doi= 10.1093/ajcn/71.2.590 |doi-access= free }}</ref> in those of advanced age (over 90). Also, people dependent on food grown from selenium-deficient soil are at risk. Although [[New Zealand]] soil has low levels of selenium, the residents have not detected adverse health effects.<ref>{{cite web| url= http://www.medsafe.govt.nz/Profs/PUarticles/Sel.htm |website= Prescriber Update Articles |title= Selenium |access-date= 2009-07-13 |publisher= New Zealand Medicines and Medical Devices Safety Authority |author= MedSafe Editorial Team}}</ref>

Selenium deficiency, defined by low (<60% of normal) selenoenzyme activity levels in brain and endocrine tissues, occurs only when a low selenium level is linked with additional stress, such as high exposures to [[Mercury (element)|mercury]]<ref>{{cite journal|last1=Ralston |first1=N. V. C.|last2=Raymond |first2=L. J.|title=Dietary selenium's protective effects against methylmercury toxicity|journal=Toxicology|date=2010|volume=278|pages=112–123|doi=10.1016/j.tox.2010.06.004|pmid=20561558|issue=1|bibcode=2010Toxgy.278..112R }}</ref> or increased oxidant stress from vitamin E deficiency.<ref>{{cite book| title =Essentials of Human Nutrition |edition= 2nd |publisher= Oxford University Press |date= 2002 |isbn= 978-0-19-262756-8 |first1= Jim |last1= Mann |first2= A. Stewart |last2= Truswell}}</ref>

Selenium interacts with other nutrients, such as [[iodine]] and [[vitamin E]]. The effect of selenium deficiency on health remains uncertain, particularly concerning [[Kashin–Beck disease]].<ref>{{cite journal |title= Selenium and iodine supplementation of rural Tibetan children affected by Kashin-Beck osteoarthropathy |first1=R. |last1= Moreno-Reyes |first2=F. |last2= Mathieu |first3=M. |last3= Boelaert |first4=F. |last4= Begaux |first5=C. |last5= Suetens |first6=M. T. |last6= Rivera |first7=J. |last7= Nève |first8=N. |last8= Perlmutter |first9=J. |last9= Vanderpas |journal= American Journal of Clinical Nutrition |volume= 78 |issue= 1 |pages= 137–144 |date= 2003 |pmid= 12816783 |display-authors= 3|doi= 10.1093/ajcn/78.1.137 |doi-access= free }}</ref> Also, selenium interacts with other minerals, such as [[zinc]] and [[copper]]. High doses of selenium supplements in pregnant animals might disturb the zinc:copper ratio and lead to zinc reduction; in such treatment cases, zinc levels should be monitored. Further studies are needed to confirm these interactions.<ref>{{cite journal|title= The effect of dietary organic and inorganic selenium supplementation on serum Se, Cu, Fe and Zn status during the late pregnancy in Merghoz goats and their kids|first1=R.|last1 =Kachuee |first2=M. |last2= Moeini |first3=M. |last3= Suori |volume= 110 |issue= 1 |pages= 20–27 |journal= Small Ruminant Research |date=2013 |doi=10.1016/j.smallrumres.2012.08.010}}</ref>

In the regions (e.g., regions within North America) where low selenium soil levels lead to low concentrations in the plants, some animal species may be deficient unless selenium is supplemented with diet or injection.<ref>National Research Council, Subcommittee on Sheep Nutrition (1985). ''Nutrient requirements of sheep''. 6th ed., National Academy Press, Washington, {{ISBN|0309035961}}.</ref> [[Ruminant]]s are particularly susceptible. In general, absorption of dietary selenium is lower in ruminants than in other animals and is lower in forages than in grain.<ref name="NRCsr2007">National Research Council, Committee on Nutrient Requirements of Small Ruminants (2007). ''Nutrient requirements of small ruminants''. National Academies Press, Washington, {{ISBN|0-309-10213-8}}.</ref> Ruminants grazing certain forages, e.g., some [[white clover]] varieties containing [[cyanogenic glycoside]]s, may have higher selenium requirements,<ref name="NRCsr2007" /> presumably because cyanide is released from the [[aglycone]] by [[glucosidase]] activity in the rumen<ref>{{cite journal|last1=Coop |first1=I. E.|last2=Blakely |first2=R. L. |date=1949|title= The metabolism and toxicity of cyanides and cyanogenic glycosides in sheep|journal= New Zealand Journal of Science and Technology|volume= 30|pages= 277–291}}</ref> and glutathione peroxidases are deactivated by the cyanide acting on the glutathione [[Moiety (chemistry)|moiety]].<ref>{{cite journal|last1=Kraus |first1=R. J.|last2=Prohaska |first2=J. R.|last3=Ganther |first3=H. E. |date=1980|title= Oxidized forms of ovine erythrocyte glutathione peroxidase. Cyanide inhibition of 4-glutathione:4-selenoenzyme|journal= Biochimica et Biophysica Acta (BBA) - Enzymology |pmid=7426660|volume=615|pages= 19–26|doi=10.1016/0005-2744(80)90004-2|issue=1}}</ref> Neonate ruminants at risk of [[Nutritional muscular dystrophy|white muscle disease]] may be administered both selenium and vitamin E by injection; some of the WMD [[Myopathy|myopathies]] respond only to selenium, some only to vitamin E, and some to either.<ref>Kahn, C. M. (ed.) (2005). ''Merck Veterinary Manual''. 9th ed. Merck & Co., Inc., Whitehouse Station, {{ISBN|0911910506}}.</ref>

==== Nutritional sources of selenium ====
Dietary selenium comes from meat, nuts, cereals, and mushrooms. [[Brazil nut]]s are the richest dietary source (though this is soil-dependent since the Brazil nut does not require high levels of the element for its own needs).<ref>{{cite journal |last1=Barclay |first1=Margaret N. I. |last2=MacPherson |first2=Allan |last3=Dixon |first3=James |date=1995 |title=Selenium content of a range of UK food |journal=Journal of Food Composition and Analysis |volume=8 |issue=4 |pages=307–318 |doi=10.1006/jfca.1995.1025}}</ref><ref>{{cite web |title=Selenium Fact Sheet |url=http://ods.od.nih.gov/factsheets/selenium.asp#h2 |publisher=The Office of Dietary Supplements, [[National Institutes of Health]] |place=United States}} Includes a list of selenium-rich foods.</ref>

The US [[Recommended Dietary Allowance]] (RDA) of selenium for teenagers and adults is 55&nbsp;[[Microgram|μg]]/day. Selenium as a dietary supplement is available in many forms, including multi-vitamins/mineral supplements, which typically contain 55 or 70&nbsp;μg/serving. Selenium-specific supplements typically contain either 100 or 200&nbsp;μg/serving.{{Citation needed|date=June 2024}} 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]].<ref>{{Cite web |title=FDA Issues Final Rule to Add Selenium to List of Required Nutrients for Infant Formula |url=https://www.fda.gov/Food/NewsEvents/ConstituentUpdates/ucm451982.htm |url-status=dead |archive-url=https://wayback.archive-it.org/7993/20171114120142/https://www.fda.gov/Food/NewsEvents/ConstituentUpdates/ucm451982.htm |archive-date=November 14, 2017 |access-date=2015-09-10 |publisher=Food and Drug Administration |df=mdy-all}}</ref>

===General health effects===
{{Main article|Selenium in biology}}
The effects of selenium intake on cancer have been studied in several [[clinical trial]]s and [[Epidemiology|epidemiologic]] studies in humans. Selenium may have a [[Chemopreventive|chemo-preventive]] role in [[cancer]] risk as an [[Antioxidant|anti-oxidant]], and it might trigger the immune response. At low levels, it is used in the body to create anti-oxidant [[selenoprotein]]s, at higher doses than normal it causes cell death.<ref name=":0" />

Selenium (in close interrelation with [[iodine]]) plays a role in thyroid health. Selenium is a cofactor for the three thyroid hormone [[deiodinase]]s, helping activate and then deactivate various thyroid hormones and their metabolites. Isolated selenium deficiency is now being investigated for its role in the induction of autoimmune reactions in the thyroid gland in [[Hashimoto's thyroiditis|Hashimoto's disease]].<ref>{{Cite journal |last1=Rostami |first1=Rahim |last2=Nourooz-Zadeh |first2=Sarmad |last3=Mohammadi |first3=Afshin |last4=Khalkhali |first4=Hamid Reza |last5=Ferns |first5=Gordon |last6=Nourooz-Zadeh |first6=Jaffar |date=2020-10-31 |title=Serum Selenium Status and Its Interrelationship with Serum Biomarkers of Thyroid Function and Antioxidant Defense in Hashimoto's Thyroiditis |journal=Antioxidants |volume=9 |issue=11 |pages=E1070 |doi=10.3390/antiox9111070 |issn=2076-3921 |pmc=7692168 |pmid=33142736|doi-access=free }}</ref> In a case of combined iodine and selenium deficiency was shown to play a thyroid-protecting role.<ref>{{Cite journal |last1=Vanderpas |first1=J. B. |last2=Contempré |first2=B. |last3=Duale |first3=N. L. |last4=Deckx |first4=H. |last5=Bebe |first5=N. |last6=Longombé |first6=A. O. |last7=Thilly |first7=C. H. |last8=Diplock |first8=A. T. |last9=Dumont |first9=J. E. |date=February 1993 |title=Selenium deficiency mitigates hypothyroxinemia in iodine-deficient subjects |journal=The American Journal of Clinical Nutrition |volume=57 |issue=2 Suppl |pages=271S–275S |doi=10.1093/ajcn/57.2.271S |issn=0002-9165 |pmid=8427203|doi-access=free }}</ref>

==See also==
{{Portal|Chemistry|Nutrition}}
* [[Abundance of elements in Earth's crust]]
* [[ACES (nutritional supplement)]]
* [[Selenium yeast]]

==Notes==
{{Notelist}}

==References==
{{Reflist}}

==External links==
{{Sister project links|d=Q876|n=no|voy=no|s=no|wikt=selenium|q=no|m=no|mw=no|species=no|v=The_periodic_table/Selenium|b=Fundamentals of Human Nutrition/Selenium }}
* [http://www.periodicvideos.com/videos/034.htm Selenium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [https://ods.od.nih.gov/factsheets/selenium-HealthProfessional/ National Institutes of Health page on Selenium]
* [http://www.sas-centre.org/assays/trace_metals/selenium.html Assay] {{Webarchive|url=https://web.archive.org/web/20120226154950/http://www.sas-centre.org/assays/trace_metals/selenium.html |date=2012-02-26 }}
* [https://wwwn.cdc.gov/TSP/ToxProfiles/ToxProfiles.aspx?id=153&tid=28 ATSDR&nbsp;– Toxicological Profile: Selenium]
* [https://www.cdc.gov/niosh/npg/npgd0550.html CDC – NIOSH Pocket Guide to Chemical Hazards]
* [http://elements.vanderkrogt.net/element.php?sym=Se Peter van der Krogt elements site]

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{{Selenium compounds|state=expanded}}
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{{Thyroid hormone receptor modulators}}
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[[Category:Selenium| ]]
[[Category:Chemical elements]]
[[Category:Chalcogens]]
[[Category:Reactive nonmetals]]
[[Category:Polyatomic nonmetals]]
[[Category:Antioxidants]]
[[Category:Dietary minerals]]
[[Category:Native element minerals]]
[[Category:Chemical elements with trigonal structure]]
[[Category:Crystals in space group 152 or 154]]
[[Category:Crystals in space group 14]]
[[Category:Selene]]