Arsenic
Arsenic causes cancer, so the federal health goal is zero. The enforceable limit of 10 ppb is a feasibility compromise, and California's health-based goal sits 2,500 times lower at 0.004 ppb.
Arsenic in water is a confirmed human carcinogen with no safe level, so the federal health goal is zero. The enforceable EPA limit of 10 ppb is a feasibility compromise, and California's health-based goal sits 2,500 times lower at 0.004 ppb. It comes from natural geology in groundwater, mostly private wells. Reverse osmosis removes it well; carbon pitchers do not.
What it is, and where it comes from
Arsenic is a naturally occurring metalloid, and in drinking water the health-relevant form is inorganic arsenic, which the WHO calls highly toxic. It reaches taps almost entirely through natural geology: arsenic-bearing rock and sediment dissolve into groundwater, so it is a source-water contaminant, not something your premise plumbing adds. The highest US exposures are on private wells, which no utility tests for you. Researchers estimated about 2.1 million people in the lower 48 states draw domestic-well water above the 10 ppb limit, a modeled range of 1.5 to 2.9 million, concentrated in areas with arsenic-bearing geology. Two chemical forms drive how hard it is to remove: arsenate, As(V), is charged and filters out readily, while arsenite, As(III), is uncharged at typical water pH and slips through most filters unless it is oxidized to As(V) first.
Why it matters
Arsenic is a Group 1 carcinogen, the category the International Agency for Research on Cancer reserves for agents confirmed to cause cancer in humans, with sufficient evidence for cancers of the bladder, lung, and skin. That is why the federal health goal is zero: for a carcinogen, no dose is known to be risk-free. The National Research Council's 2001 assessment estimated that men drinking water at the 10 ppb legal limit for a lifetime face about 23 extra bladder cancers per 10,000, a risk high enough that it kept the EPA from loosening the standard. Non-cancer harm is well documented too: long-term exposure causes skin lesions, pigmentation changes, and hard patches on the palms and soles, and it is linked to cardiovascular disease and diabetes. In children, higher exposures track with lower measured intellectual function; the clearest effects in the landmark Bangladesh study appeared above 50 ppb, well above typical US levels. You take arsenic in by drinking and cooking, so the fix belongs at the kitchen tap.
- Arsenic is a confirmed human carcinogen (IARC Group 1), with sufficient evidence for cancers of the bladder, lung, and skin from long-term exposure to inorganic arsenic.IARC Monographs Vol. 100C, Arsenic and inorganic arsenic compounds, Group 1, 2012
- The National Research Council's 2001 assessment estimated about 23 extra bladder cancers per 10,000 US men who drink water at the 10 ppb legal limit over a lifetime, a finding that led the EPA to hold the standard rather than loosen it.National Research Council, Arsenic in Drinking Water: 2001 Update (National Academies Press)
- In a study of 201 ten-year-olds in Araihazar, Bangladesh, children whose water arsenic exceeded 50 ug/L scored significantly lower on intellectual function tests than those below 5.5 ug/L, in a dose-response pattern.Wasserman et al., 2004 (Environ Health Perspect; childhood intellectual function, Araihazar, Bangladesh)
- About 2.1 million people in the conterminous US are estimated to draw domestic-well water with arsenic above the 10 ppb limit (95% confidence interval 1.5 to 2.9 million), a modeled estimate concentrated in areas with arsenic-bearing geology.Ayotte et al., 2017 (Environ Sci Technol; US high-arsenic domestic-well population)
- The WHO reports that the first signs of long-term high arsenic exposure are skin pigmentation changes, skin lesions, and hard patches on the palms and soles, with cardiovascular disease and diabetes also associated.WHO, Arsenic fact sheet
What we grade it against
| Contaminant | Health-based level | Legal limit | Source (health-based) |
|---|---|---|---|
| Arsenic (ppb) | 0federal MCLG; no safe level for a carcinogen | 10EPA MCL, 2001 rule effective 2006; a feasibility compromise | US EPA Arsenic Rule / NPDWR |
| Arsenic, California health goal (ppb) | 0.004OEHHA PHG, one extra cancer per million lifetime; 2,500x below the federal limit | 10matched by California's enforceable MCL | CA OEHHA PHG, 2004 |
| Arsenic, WHO guideline (ppb) | 10provisional guideline set by treatment feasibility, not a no-risk level | none set | WHO Arsenic Fact Sheet |
Health-based levels come from peer-reviewed research and government risk scientists working without cost constraints. Legal limits are enforceable compromises. Your report grades to the health column.
What helps
Direct fixes
- Under-sink reverse osmosis (RO) at the kitchen tap (NSF/ANSI 58)
Removes arsenate, As(V), at roughly 95 to 99 percent, and the same unit covers PFAS, lead, and most heavy metals at the tap you drink from.
The uncharged form, arsenite As(III), is removed far less well without pre-oxidation to As(V). A system that oxidizes the water first, or one certified for total arsenic, closes that gap; confirm the certification covers your arsenic form.
Bigger retrofits
- Anion exchange (point-of-entry or utility scale)
Binds charged arsenate, As(V), to a resin and removes it; it is one of the EPA's Best Available Technologies for arsenic and is common at utility scale.
Like RO, it removes the uncharged As(III) form poorly unless the water is oxidized to As(V) first.
- Oxidation followed by adsorptive media (e.g., granular ferric hydroxide)
Chlorine, permanganate, or a solid oxidizing medium first converts As(III) to As(V), then an iron-based adsorptive medium captures it. The standard dedicated whole-house or well-treatment approach in high-arsenic areas.
Free and behavioral
- Carbon pitcher or standard carbon block
Good for chlorine taste and some other contaminants.
Most carbon pitchers and blocks do not remove arsenic; in independent testing only devices specifically certified for arsenic reduced it. A home with detected arsenic needs RO or a dedicated arsenic medium, not a standard pitcher.
- Test the well first, then treat to the result
Arsenic is invisible and tasteless, and utility reports do not cover private wells, so testing is the only way to know your number. The EPA and CDC advise testing a private well every year for bacteria, nitrate, and pH, and at least once for arsenic and other metals, more often in known high-arsenic areas.
Arsenic is the sharpest example in water of the gap between legal and safe. The federal health goal is zero because arsenic causes cancer, yet the enforceable limit is 10 ppb, a level the government's own scientists tied to about 23 extra bladder cancers per 10,000 men over a lifetime. California's health-based goal sits 2,500 times below that legal limit. The exposure that matters most is invisible and on private wells, where no utility report will ever flag it. The honest position is simple: a compliant number is not a safe number, arsenic is measurable, and once you know your number the fix at the kitchen tap is straightforward, as long as it accounts for which chemical form you have.
Common questions
What is a safe level of arsenic in water?
There is no level known to be risk-free, because arsenic is a confirmed human carcinogen, which is why the federal health goal (MCLG) is zero. The enforceable EPA limit is 10 ppb, a feasibility compromise, and California's health-based goal is 0.004 ppb, 2,500 times lower. Aim as low as your treatment can reach; below the 10 ppb legal limit is the floor, not the target.
What are the symptoms of arsenic in drinking water?
Arsenic has no taste, color, or smell, so you cannot detect it without a test. The first signs of long-term high exposure that the WHO describes are skin changes: pigmentation changes, skin lesions, and hard patches on the palms and soles. Most effects appear only after years of elevated exposure, not from short-term contact, which is why testing rather than waiting for symptoms is the right move.
What are the health effects of arsenic in water?
Arsenic is a Group 1 carcinogen with sufficient evidence for bladder, lung, and skin cancer. The National Research Council estimated about 23 extra bladder cancers per 10,000 US men who drink water at the 10 ppb limit over a lifetime. Non-cancer effects include skin lesions, cardiovascular disease, and diabetes, and higher childhood exposures track with lower intellectual function, with the clearest effects seen above 50 ppb.
How do I test my water for arsenic?
A certified drinking-water lab test is the only reliable way, because arsenic is invisible and tasteless. If you are on a private well this matters most, since utility reports do not cover wells. The EPA and CDC advise testing a well at least once for arsenic and other metals, and every year for bacteria, nitrate, and pH. Ask the lab whether it reports total arsenic or the As(III) and As(V) split, since the form affects which filter you need.
Does reverse osmosis remove arsenic?
Yes, for the charged form. Under-sink RO removes arsenate, As(V), at roughly 95 to 99 percent. The catch is arsenite, As(III), which is uncharged at normal water pH and passes through RO and anion exchange far more easily unless it is oxidized to As(V) first. Choose a system that oxidizes the water or is certified for total arsenic, and confirm the certification covers your form.
What is the best water filter for arsenic?
For a home, under-sink reverse osmosis at the kitchen tap is the most practical fix, and it also handles PFAS, lead, and most heavy metals. Anion exchange and iron-based adsorptive media are the other proven options, common for whole-house or well treatment. Standard carbon pitchers and blocks do not reliably remove arsenic; only devices specifically certified for arsenic reduction do. Whatever you choose, confirm it is certified (NSF/ANSI 58 for RO, or NSF/ANSI 53) and that it covers your arsenic form.
Is arsenic in water only a private-well problem?
The highest US exposures are on private wells, which no utility tests for you, and an estimated 2.1 million people in the lower 48 draw well water above the 10 ppb limit (range 1.5 to 2.9 million). Public utilities on arsenic-bearing groundwater are required to treat to the 10 ppb limit, but that limit is a feasibility compromise, not a health-based level, so compliant municipal water can still sit above California's health goal.
Sources
Peer-reviewed
Government & regulatory
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