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Selenium is the chemical element Se, atomic number 34, and atomic weight 78.97. It is a trace nutrient: the body uses it to make selenoproteins involved in antioxidant defense, redox control, thyroid-hormone metabolism, reproduction, DNA production, cell protection, and resistance to infection. The same element is also a photoconductive and photovoltaic material used in electronic and optical devices.
Selenium is essential in appropriate amounts but toxic in excess. This article covers the element, what it does in the body, food sources, recommended intake, deficiency and overdose concerns, workplace exposure, and its industrial uses. It refers to the chemical element—not the separately named browser-automation software that some developers also call Selenium.
Selenium at a glance
| Property | What is established |
|---|---|
| Chemical symbol | Se |
| Atomic number | 34 |
| Atomic weight | 78.97 |
| Periodic-table location | Group 16 (old group VIA) |
| Common classification | Metalloid, because it has both metallic and nonmetallic properties |
| Biological status | Essential trace nutrient, but toxic in large amounts |
| Discovery | Identified by Jöns Jacob Berzelius in 1818 |
National Library of Medicine terminology describes selenium as an element with the symbol Se, atomic number 34, and atomic weight 78.97. Chemistry references commonly call it a metalloid rather than a typical metal or nonmetal.
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Jöns Jacob Berzelius identified selenium in 1818 while examining an elemental residue produced during the oxidation of sulfur dioxide from copper pyrites in sulfuric-acid production. The National Academies’ chemistry chapter places selenium in group VIA, now called group 16, alongside other elements with related outer-electron behavior.
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Its intermediate properties explain the metalloid label. Selenium can conduct electricity in ways that change with conditions such as light exposure, yet it does not behave like a conventional structural metal. Those electrical properties are important in technology, while its ability to form selenoproteins makes it biologically important.
What selenium does in the body
The body does not use selenium as a general-purpose “antioxidant” on its own. Instead, it incorporates selenium into specialized proteins called selenoproteins. The NIH Office of Dietary Supplements identifies 25 selenoproteins, including glutathione peroxidases, thioredoxin reductases, and selenoprotein P.
Antioxidant and redox systems
Glutathione peroxidases and thioredoxin reductases participate in reactions that control oxidation and reduction. These systems help cells handle reactive molecules and maintain a workable redox balance. Selenium therefore supports antioxidant defenses as a structural component of enzymes; it is not evidence that taking large doses will provide extra protection.
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Thyroid-hormone metabolism
The thyroid needs selenium to produce and regulate hormones properly. Several selenoproteins help process thyroid hormones and protect thyroid tissue from oxidative damage associated with hormone production.
Other established roles
NIH consumer guidance lists selenium as necessary for reproduction, DNA production, cellular protection, and resistance to infection. These are biological functions of selenium-containing proteins, not claims that selenium supplements treat infections, infertility, thyroid disease, or cancer.
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How much selenium do you need?
The NIH Office of Dietary Supplements lists these daily recommended amounts in its 2025 consumer fact sheet:
| Life stage | Recommended amount | Qualification |
|---|---|---|
| Adults | 55 micrograms (mcg) per day | NIH Office of Dietary Supplements, 2025 consumer fact sheet |
| Pregnant teenagers and women | 60 mcg per day | NIH Office of Dietary Supplements, 2025 consumer fact sheet |
| Breastfeeding teenagers and women | 70 mcg per day | NIH Office of Dietary Supplements, 2025 consumer fact sheet |
These figures are intake targets, not a reason to take a high-dose product. The amount on a supplement label is normally stated in micrograms, while workplace limits may be stated in milligrams per cubic meter; those units describe very different exposure situations.
Which foods contain selenium?
Useful dietary sources include seafood, beef, pork, poultry, eggs, dairy products, breads, cereals, beans, lentils, and nuts. Animal foods and grains can contribute substantially to daily intake, but the amount is not fixed for every serving or region.
Why plant-food values vary
Plants obtain selenium from soil, so concentration depends strongly on the soil in which they grow and on other soil conditions. Two portions of the same plant food can therefore contain different amounts when produced in different regions. A database value is an estimate for the listed food and serving, not a guarantee for every package.
Brazil nuts require particular caution
USDA FoodData Central values reproduced in the NIH 2025 professional fact sheet give an average of 544 mcg of selenium in 1 ounce (about 6–8 Brazil nuts). NIH also warns that Brazil-nut values vary widely and can be exceptionally concentrated. Do not treat one nut as a guaranteed dose or use a large handful as a routine way to increase intake.
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- Antioxidant Support: Promotes the body’s natural antioxidant capabilities to reduce oxidative damage in the body caused by environmental toxins, stress, and the effects of poor diet and lifestyle choices*
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Food-first interpretation
For most people in the United States, ordinary food provides enough selenium. A varied diet that includes some of the listed sources is generally more predictable than adding a concentrated product without a specific reason.
Can you be deficient in selenium?
NIH reports that most people in the United States consume adequate selenium. Inadequate intake is more plausible in particular circumstances rather than as a universal concern.
- People receiving kidney dialysis can be at higher risk.
- People living with HIV can be at higher risk.
- People whose diets rely heavily on plant foods grown in selenium-poor local soils can be at higher risk.
Those categories do not diagnose a deficiency. If one applies to you, a clinician can decide whether dietary assessment or laboratory evaluation is appropriate. Do not use nonspecific symptoms alone to self-prescribe selenium.
Is selenium dangerous?
Yes, excessive selenium can cause harm. NLM characterizes it as essential but toxic in large amounts. Risk rises when concentrated supplements or industrial compounds are treated like ordinary food.
Supplements and label reading
NIH says supplements may contain selenomethionine, selenium-enriched yeast, sodium selenite, or sodium selenate. The Supplement Facts panel states the amount of selenium provided. Check the serving size and add selenium from all products rather than assuming that two “different” supplements have unrelated amounts.
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- IMMUNE & ANTIOXIDANT SUPPORT: Selenium supports immune system health and provides antioxidant support (1)
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Most people do not need a selenium supplement simply because selenium is important. Consider one only when a qualified health professional identifies a dietary or medical reason, and follow the product label and professional advice. More is not better for thyroid, immune, or antioxidant function.
Concentrated compounds and workplace exposure
Elemental selenium and selenium compounds used in laboratories or industry are not equivalent to trace amounts in food. NIOSH’s Pocket Guide to Chemical Hazards lists an 8-hour time-weighted-average exposure limit of 0.2 mg/m3 for selenium and many selenium compounds, accessed in 2026, and an IDLH (immediately dangerous to life or health) value of 1 mg/m3 as Se. These are airborne occupational limits, not dietary targets. Follow the safety data sheet, engineering controls, respiratory protection program, and workplace regulations that apply to the specific compound and task.
Elemental selenium versus selenium in food and supplements
“Selenium” can describe several chemically different exposures:
| Context | Examples | How to interpret it |
|---|---|---|
| Elemental selenium | Material used in scientific or industrial applications | Has physical properties such as photoconductivity; not a food serving |
| Organic dietary forms | Selenomethionine, selenium-enriched yeast | Forms used in some foods and supplements; read the labeled selenium amount |
| Inorganic dietary forms | Sodium selenite, sodium selenate | Forms used in some supplements; dose and context matter |
| Occupational exposure | Dusts, fumes, or concentrated compounds | Evaluate by exposure route and workplace limits, not by food recommendations |
The chemical form, dose, and route of exposure all matter. A trace amount in a meal, a concentrated capsule, and airborne industrial dust should never be treated as interchangeable.
What is selenium used for outside nutrition?
Selenium’s electrical behavior is the basis for several material-science applications. PubChem describes it as photoconductive: its electrical resistance changes when light strikes it. That property supports photocells and photographic exposure meters. Selenium’s photovoltaic behavior also enables conversion of light into electrical signals in suitable devices.
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Elemental selenium has additionally been used in rectifiers and other electronic and solid-state applications. These uses concern the material properties of elemental selenium and do not mean that eating more selenium improves electronic, visual, or “energy” performance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to decide whether a supplement makes sense
- Start with your diet. Identify regular sources such as seafood, meat, poultry, eggs, dairy, grains, beans, lentils, or nuts.
- Check your circumstances. Kidney dialysis, HIV, or dependence on foods from selenium-poor soils can increase the chance of inadequate intake.
- Read the entire label. Record the selenium amount per serving and the chemical form, and include selenium from multivitamins or other products.
- Ask a clinician when risk is specific. A professional can determine whether testing, dietary changes, or supplementation is justified.
- Avoid dose escalation. Do not use Brazil nuts or multiple supplements as a way to chase a higher number.
Common misunderstandings
- “Selenium is an antioxidant, so more must be better.” Selenium supports antioxidant enzymes, but excess intake is toxic.
- “Every Brazil nut contains the same amount.” Selenium concentration varies widely; the 544-mcg-per-ounce figure is an average from USDA data reproduced by NIH.
- “A food database value applies everywhere.” Soil conditions can change selenium levels in plant foods.
- “Dietary and workplace numbers can be compared directly.” Micrograms per day in food and milligrams per cubic meter in air measure different routes and risks.
- “The element and browser-automation software are the same thing.” Chemical selenium is Se, atomic number 34; the software is a separate technology with the same name.
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