Selenium is the chemical element with the symbol Se, atomic number 34, and atomic weight 78.97. Your body needs small amounts of selenium to make selenoproteins involved in antioxidant defense, thyroid-hormone metabolism, reproduction, DNA production, cell protection, and resistance to infection. The same element can be useful at nutritional levels and harmful when exposure becomes excessive.
This article is about selenium the element and nutrient. If you meant a browser-automation product with the same name, that is a different subject.
Selenium at a glance
| Property | What is established |
|---|---|
| Chemical symbol | Se |
| Atomic number | 34 |
| Atomic weight | 78.97 |
| Periodic-table position | Group 16, formerly called group VIA |
| Common classification | Metalloid, because it shows both metallic and nonmetallic properties |
| Biological status | Essential micronutrient, but toxic in large amounts |
These identity details come from the U.S. National Library of Medicine (NLM) MeSH record and the National Academies chemistry chapter hosted by NCBI.
How selenium was discovered
Jöns Jacob Berzelius identified selenium in 1818. The National Academies’ chemistry account says he found it as an elemental residue while oxidizing sulfur dioxide from copper pyrites during sulfuric-acid production. Selenium’s position in group 16 places it alongside oxygen, sulfur, tellurium, and polonium. Its mixed electrical and chemical behavior is why references commonly describe it as a metalloid rather than simply a metal or a nonmetal.
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What selenium does in the body
Selenium is not a general-purpose “antioxidant” treatment. It is incorporated into specific proteins, and those proteins perform the work.
Selenoproteins and redox control
The NIH Office of Dietary Supplements (ODS) professional fact sheet identifies 25 selenoproteins. They include glutathione peroxidases, thioredoxin reductases, and selenoprotein P. These proteins help control oxidation-reduction reactions, protect cells from damaging oxidative reactions, and transport or manage selenium in the body.
Thyroid function
The thyroid needs selenium to produce hormones and function properly. Selenium-dependent enzymes participate in thyroid-hormone metabolism, so adequate intake supports this system without making selenium a stand-alone treatment for thyroid disease.
Other biological roles
NIH’s consumer fact sheet also lists roles in reproduction, DNA production, cellular protection, and resistance to infection. Those statements describe physiological functions of adequate selenium status; they do not establish that taking extra selenium prevents or cures unrelated illnesses.
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The following amounts are NIH recommendations published in its 2025 consumer fact sheet:
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| Life stage | Recommended amount |
|---|---|
| Adults | 55 micrograms (mcg) per day |
| Pregnant teens and women | 60 mcg per day |
| Breastfeeding teens and women | 70 mcg per day |
These are daily intake targets, not instructions to take a supplement. Needs for children and other life stages should be checked in the current NIH guidance rather than inferred from the adult figure.
Which foods contain selenium?
NIH lists seafood, beef, pork, poultry, eggs, dairy products, breads, cereals, beans, lentils, and nuts as food sources. The amount in plant foods can change substantially with soil selenium concentration and other soil factors, so an identical food grown in two regions may not provide the same amount.
Brazil nuts are unusually concentrated
USDA FoodData Central values reproduced in the NIH 2025 professional fact sheet give an average of 544 mcg of selenium for 1 ounce, about six to eight Brazil nuts. That is an average, not a guaranteed amount: individual nuts can vary widely. Treat Brazil nuts as a concentrated food rather than an exact-dose supplement, especially if you also use a selenium product.
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A practical food-first approach
- Use a varied diet that includes the protein foods, grains, beans, lentils, dairy, eggs, or nuts that fit your eating pattern.
- Do not assume a plant-based diet is automatically low in selenium; soil conditions determine much of the variation.
- Do not assume one serving of any food supplies a fixed number of micrograms unless a reliable label or database value applies to that specific product.
Can you be deficient in selenium?
Most people in the United States consume enough selenium from food, according to NIH. Inadequate intake is more plausible in particular circumstances, including kidney dialysis, HIV infection, or diets based heavily on plant foods grown in local soils that are low in selenium.
A risk factor is not a diagnosis. If you have one of these conditions, discuss diet and laboratory evaluation with a qualified clinician instead of self-prescribing a high-dose product.
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Is selenium dangerous?
Yes, excessive exposure can cause harm. NLM characterizes selenium as essential but toxic in large amounts. The risk depends on the chemical form, dose, and route of exposure.
Food and supplement exposure
Normal food intake is not the same as exposure to a concentrated compound. Selenium supplements may contain selenomethionine, selenium-enriched yeast, sodium selenite, or sodium selenate. NIH advises reading the Supplement Facts panel for the amount supplied. Combining a concentrated product with large amounts of selenium-rich foods can make total intake difficult to judge.
- Do not treat “more antioxidant support” as a reason to exceed the labeled serving.
- Check every multivitamin, mineral product, and single-nutrient supplement for selenium before combining them.
- Ask a clinician or pharmacist whether supplementation is appropriate for your medical condition, diet, pregnancy, or medications.
The supplied NIH material establishes that excess intake and supplement-related toxicity are concerns, but it does not provide a single universal dose that is safe for every person. Follow current professional guidance and product labeling rather than improvising a target.
Workplace and laboratory exposure
Dusts, fumes, and concentrated selenium compounds are occupational hazards distinct from dietary selenium. The NIOSH 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 immediately dangerous to life or health (IDLH) value of 1 mg/m3 as Se. Workplace controls, ventilation, protective equipment, and the applicable safety data sheet are required for those settings; food advice cannot be substituted for industrial hygiene.
Should you take a selenium supplement?
There is no basis to assume that every reader needs one. NIH reports that most people in the United States obtain adequate selenium from food. A supplement may be considered when a clinician identifies inadequate intake or a condition associated with low status, but the decision should be based on the person, the product’s labeled amount, and the total diet.
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What to check on a label
- Find selenium in the Supplement Facts panel and note the amount per serving in micrograms.
- Check whether other products you take also contain selenium.
- Identify the chemical form, such as selenomethionine, selenium-enriched yeast, sodium selenite, or sodium selenate.
- Ask a pharmacist or clinician whether the combined intake fits your situation.
Do not use a supplement label as proof that a high dose is beneficial. Selenium’s essential and toxic properties are both real.
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Elemental selenium versus selenium in food
“Selenium” can refer to different chemical contexts:
| Context | Examples | How to interpret it |
|---|---|---|
| Elemental selenium | Se used as a material | A chemical element with photoconductive and photovoltaic behavior |
| Organic dietary forms | Selenomethionine; selenium-enriched yeast | Forms used in some supplements and foods; read the product label |
| Inorganic dietary forms | Sodium selenite; sodium selenate | Forms used in some supplements; dose and safety still matter |
| Occupational compounds or dust | Concentrated laboratory or industrial exposure | Evaluate under workplace chemical-safety rules, not nutrition guidance |
The table separates chemical form and exposure route. It does not mean that one supplement form is universally best for everyone.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is selenium used for outside nutrition?
Elemental selenium has useful electrical behavior. PubChem describes it as photoconductive: its electrical resistance changes when light strikes it. That property supports photocells and photographic exposure meters. Selenium is also used in rectifiers and other electronic and solid-state applications. These material-science uses involve engineered components and controlled industrial handling, not dietary supplementation.
A concise decision checklist
- For general nutrition, start with food variety and the NIH daily target for your life stage.
- For a possible deficiency, consider your medical conditions, dialysis or HIV status, and the selenium content of locally grown plant foods with a clinician.
- For supplements, total the selenium from every product and follow the label unless a professional gives different instructions.
- For Brazil nuts, remember that the NIH-cited 544 mcg per ounce is an average for six to eight nuts and can vary widely.
- For laboratory or workplace exposure, use NIOSH limits and chemical-safety procedures.
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Frequently Asked Questions
What does atomic number 34 mean?
It means every selenium atom has 34 protons in its nucleus. The atomic number identifies the element, while 78.97 is selenium’s listed atomic weight.
Why can two Brazil nuts provide very different selenium amounts?
Selenium uptake by plants depends strongly on soil concentration and related soil factors. The NIH-cited 544 mcg per ounce is an average, not a guaranteed value for each nut.
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