At a glance
| Chemical family | Non-metal / metalloid trace element, essential to human nutrition but toxic in excess. In water it occurs as the oxyanions selenate (Se⁶⁺) and selenite (Se⁴⁺), which are soluble and mobile. |
| CAS number | 7782-49-2 (elemental selenium) |
| Classification | Essential micronutrient with a narrow safe range; chronic excess causes selenosis. Not classified as a human carcinogen (some selenium compounds have even been studied for cancer prevention). The drinking-water limit targets chronic toxicity. EPA MCL 50 µg/L; WHO 40 µg/L. |
| Where you encounter it | Naturally in groundwater and streams draining selenium-rich marine shales and soils — the Great Plains and arid West (parts of Kansas, Nebraska, the Dakotas, Wyoming, Colorado, Utah, California). Irrigation of seleniferous soils, mining, and coal ash amplify it locally. |
| Sleep micro environment relevance | Indirect — an ingestion exposure via drinking water, on the Atlas as part of the private-well and Great Plains tap-water picture the water hub anchors. |
| Activated carbon capture | Not removed by standard carbon. Removal uses reverse osmosis, and specific anion-exchange/adsorptive media suited to selenate/selenite. Inferred from standard selenium water-treatment engineering; the selenium oxyanion is not adsorbed by ordinary carbon |
Regulatory & certification status
Selenium's limits are set to cap chronic toxicity while acknowledging that a little is required. The rows below give the standards and the natural-occurrence context.
| United States | EPA enforceable Maximum Contaminant Level of 50 µg/L (0.05 mg/L), set to protect against selenosis (the chronic-toxicity syndrome). Regulatory — US EPA |
| World Health Organization | WHO guideline value of 40 µg/L, slightly tighter than the US, likewise based on chronic toxicity with allowance for essential dietary intake. Regulatory — WHO |
| Nutrition context | Selenium is an essential nutrient with a Recommended Dietary Allowance (~55 µg/day for adults) and a Tolerable Upper Intake Level (~400 µg/day). Most intake is from food (Brazil nuts, seafood, grains); water is usually a minor contributor unless a supply is unusually high. Regulatory — dietary reference intakes |
| The 72-hour test window | Not applicable. Selenium is a waterborne ingestion exposure measured by a specialized water-lab metals test, unrelated to the VOC-emission chamber tests used for mattresses and foam. Inferred — from the ingestion-via-water route versus the material/VOC focus of product emissions testing |
What it is
Selenium is a trace element that sits chemically between metals and non-metals. Biologically it is indispensable: it is built into selenoproteins such as glutathione peroxidase (a key antioxidant enzyme) and the enzymes that activate thyroid hormone. Too little causes real disease — Keshan disease, a heart condition, is the classic selenium-deficiency illness. The catch is that the beneficial range is narrow: not far above the amount you need, selenium becomes toxic.
Chronic overexposure produces selenosis: the hallmark signs are brittle hair and nails that break off, hair loss, garlic-smelling breath, skin rashes, and, at higher doses, gastrointestinal upset and neurological effects. This is the endpoint the drinking-water limit is written to prevent. In water, selenium travels as the negatively-charged selenate and selenite ions, which is why it is mobile in groundwater and why the treatment that removes it is anion-based rather than carbon.
It is worth saying what selenium in water usually is not: an industrial poison unique to contaminated sites. For most affected households it is a natural feature of the rock their water moves through — the same seleniferous geology that can make local forage toxic to livestock in parts of the Great Plains.
Where you encounter it
Seleniferous geology
The dominant source. Certain Cretaceous marine shales and the soils weathered from them are naturally selenium-rich, and they underlie large parts of the Great Plains and the arid West — including areas of Kansas, Nebraska, the Dakotas, Wyoming, Colorado, and Utah. Groundwater and streams draining these formations carry selenium, and private wells in these regions can exceed 50 µg/L from geology alone.
Irrigation drainage — the Kesterson lesson
Irrigating seleniferous soils leaches selenium into agricultural drainage water, which can then concentrate downstream. The Kesterson Reservoir disaster in California in the 1980s — where selenium-laden irrigation drainage caused severe deformities and death in waterfowl — is the landmark case, and it reshaped how the arid West manages irrigation return flows. Regulatory
Mining and coal ash (secondary)
Mining, ore processing, and coal combustion residuals (coal ash) can release selenium to local water. These are secondary to natural geology for most affected drinking-water wells but matter near those activities. Inferred from documented selenium sources in mining and coal-ash-affected watersheds
What the research says
The narrow-margin toxicology
The ATSDR Toxicological Profile for Selenium and dietary-reference reviews together define selenium's unusual dose-response: a required intake, a recommended allowance, and a tolerable upper limit that is only about sevenfold higher — a much tighter band than most nutrients. Regulatory Selenosis in humans is documented from high-selenium water and food in seleniferous regions, which anchors the toxicity end of the curve.
The two-sided cancer story
Selenium is not a carcinogen; if anything, selenium compounds were studied for possible cancer prevention, though large trials (such as SELECT for prostate cancer) did not show benefit and raised some concerns at supplemental doses. Inferred from the selenium chemoprevention trial literature — benefit not demonstrated, reinforcing that more is not better The takeaway reinforces the theme: with selenium, both too little and too much are problems, and the water limit is about staying in the safe band.
What helps
On a well in seleniferous country, test for it. Selenium can be added to a well-water metals panel. Households in known Great Plains and arid-West selenium regions are the ones who most need the number.
For water over 50 µg/L, use reverse osmosis. Point-of-use RO at the kitchen sink removes selenium for drinking and cooking; specific anion-exchange/adsorptive media handle it at higher flow. See water filters compared.
Keep perspective on diet. Because most selenium comes from food, don't over-correct with high-dose supplements if your water is already elevated — the goal is total intake in the safe range, not zero.
What does NOT help
- Boiling. Selenium is a dissolved element; boiling concentrates it as water evaporates.
- Standard carbon pitcher filters. They don't remove selenium's oxyanion form. Use RO or a system rated for selenium.
- Treating it like a pure toxin to eliminate. Selenium is essential; the aim is the safe band, not the lowest possible level — a distinction that matters for how you think about diet and supplements.
- Assuming city water has it. Public systems monitor and treat for selenium; the concern is overwhelmingly private wells in seleniferous regions.
Open questions
- The precise low-end margin of safety — how close typical intakes sit to both deficiency and excess in different regions — is still refined by nutrition science. Speculation re: exact optimal band; the essential-and-toxic duality is established
- Long-term ecological and human effects of chronic low-level selenium from irrigation drainage in the arid West continue to be studied post-Kesterson. Inferred from ongoing irrigation-drainage monitoring
Where you meet Selenium across your home
The same compound turns up in more than one place you live. Here's where it shows up in Embr — each links to the full breakdown for that part of your home.
Citations
- US Environmental Protection Agency. National Primary Drinking Water Regulations — selenium MCL 50 µg/L. epa.gov Regulatory
- Agency for Toxic Substances and Disease Registry (2003). Toxicological Profile for Selenium. atsdr.cdc.gov Regulatory
- World Health Organization. Selenium in Drinking-water — guideline 40 µg/L. who.int Regulatory
- USGS / US Fish and Wildlife Service. Kesterson Reservoir selenium and waterfowl effects — irrigation drainage in the arid West. usgs.gov Regulatory
Frequently asked questions
Is selenium in drinking water dangerous?
Selenium is unusual: your body needs it, but the margin between essential and toxic is narrow, so too much in water is a genuine concern. It is a required trace nutrient (for antioxidant enzymes and thyroid function), and deficiency causes disease — but chronic excess causes selenosis: brittle, breaking hair and nails, hair loss, skin rashes, and, at high levels, gastrointestinal and neurological effects. The EPA limit of 50 µg/L is set to protect against that chronic toxicity. In water, elevated selenium is almost always natural, from selenium-rich soils and shales in the Great Plains and arid West.
What is the safe level of selenium in water?
The US EPA enforceable Maximum Contaminant Level for selenium is 50 µg/L (0.05 mg/L), and the WHO guideline is 40 µg/L. Both target chronic toxicity (selenosis) while leaving room for the small amount the body actually needs. Because selenium is both essential and toxic, the drinking-water limit isn't a "lower is always better" number the way a pure toxin's would be — it is set to keep total intake below the harmful range, given that most selenium comes from food, not water.
Where does selenium in water come from?
Almost always geology. Selenium is concentrated in certain marine shales and the soils weathered from them, which are widespread across the Great Plains and the arid West — parts of Kansas, Nebraska, the Dakotas, Wyoming, Colorado, Utah, and California. Groundwater and streams draining these formations pick it up. Human activities amplify it locally: irrigation of seleniferous soils mobilizes selenium into drainage water, which caused the notorious Kesterson Reservoir wildlife disaster, and mining and coal ash can add to it.
How do I remove selenium from water?
Selenium is removed by reverse osmosis, and by specific anion-exchange and adsorptive media (its form in water — selenate or selenite — is a negatively-charged ion). A standard activated-carbon filter does not remove it. For a private well over 50 µg/L, point-of-use reverse osmosis at the kitchen sink is the common, effective choice for drinking and cooking water. Test first: selenium is not always on basic well panels and must sometimes be requested specifically, especially in known seleniferous regions.
Related compounds
Embr researches the chemistry of where you live — including the natural elements in well water. See the methodology page for how this Atlas tags claims by evidence strength, the tap-water hub for the other things in your water, and water filters compared for how to remove selenium.
Last reviewed 2026-07-13. If you find a factual error, contact us.