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, or parts per billion, 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. Standard carbon pitchers do not.
Arsenic: what is it, and where does it come 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, already in the groundwater before it reaches your home's plumbing. 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.
Arsenic: why does it matter?
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
Arsenic: what do we grade it against?
Arsenic is held to two different numbers, and they do not agree. The health-based level for Arsenic (ppb) is 0 (federal MCLG; no safe level for a carcinogen), from US EPA Arsenic Rule / NPDWR. The enforceable legal limit is 10. The table below lists every level on this page next to the source that set it.
| Contaminant | Health-based level | Legal limit | Source (health-based) |
|---|---|---|---|
| Arsenic (ppb) | 0 federal MCLG; no safe level for a carcinogen | 10 EPA MCL, 2001 rule effective 2006; a feasibility compromise | US EPA Arsenic Rule / NPDWR |
| Arsenic, California health goal (ppb) | 0.004 OEHHA PHG, one extra cancer per million lifetime; 2,500x below the federal limit | 10 matched by California's enforceable MCL | CA OEHHA PHG, 2004 |
| Arsenic, WHO guideline (ppb) | 10 provisional 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, while legal limits are enforceable compromises. Your report grades to the health column.
Arsenic across US water systems: what we measured
15,115 US community water systems have an arsenic result on record in our dataset. 3,532 of them, 23.4%, found enough to report, which means 2 parts per billion (ppb) or more. 188 are at or above the 10 ppb federal limit, and those serve 758,000 people between them.
The count is detections, not exceedances of a health level. The health-based goal for arsenic sits about three orders of magnitude below the 2 ppb any certified laboratory is required to report, so grading against it would flag every detection by arithmetic and mean nothing. Between the detection count and the violation count, 3,344 systems have detectable arsenic while staying fully legal. The rules require no notification for those.
Half the systems reporting a detection sit at or below 2 ppb; one in ten is above 6.8 ppb, and the highest system average in the set is 89 ppb.
5,478 of the results rest on a single sampling point. Their median detected value is 2 ppb against 1.9 ppb for systems with several points, and their detection share is 53.7% against 49.4%. What separates the halves is scale: a median 420 people served against 1,650. Small groundwater systems typically draw from a single well, and single-well groundwater is where arsenic concentrates.
The reporting gap between systems that treat their water and systems that buy it
43.5% of the 30,916 systems that treat their own water have an arsenic result. Among the 7,416 systems that buy finished water from a wholesaler, only 22.6% do. That 20.9-point gap is a paperwork boundary, not a testing gap. Arsenic is sampled at the treatment plant, and when the plant belongs to the wholesaler, the result is filed under the wholesaler's permit number rather than the retail system's.
No figure here is a share of all US utilities. Because buyers report at half the rate of treaters, a figure like “61% of US water utilities do not test for arsenic” would follow from the arithmetic and still be false. Where we can follow the supply chain, we do: 615 results in this set were measured at a supplying wholesaler's plant and attached to the buying system. That tracing is not complete. The district we tested first shows the rest of the problem: Moulton Niguel Water District's published report prints arsenic at 1.9 ppb from its treatment plant, while a bulk reconstruction of state records alone would file that district as having no arsenic data.
Which states report the most arsenic?
Nebraska leads the arsenic ranking: 74.7% of its 525 reporting systems detect arsenic at or above 2 ppb, at a state median of 3.75 ppb. Nevada (59.7%), Idaho (43.1%), Washington (42.6%) and New Mexico (41.1%) follow.
| State | Typical system (ppb) | Found 2 ppb or more | Above 10 ppb |
|---|---|---|---|
| Nebraska 525 systems reporting, 96% of the state | 3.75 | 75% 392 systems | 11 |
| Nevada 154 systems reporting, 100% of the state | 2.55 | 60% 92 systems | 8 |
| Idaho 587 systems reporting, 100% of the state | 1.07 | 43% 253 systems | 11 |
| Washington 1,028 systems reporting, 59% of the state | 1.4 | 43% 438 systems | 15 |
| New Mexico 314 systems reporting, 65% of the state | 1 | 41% 129 systems | 8 |
| California 2,381 systems reporting, 88% of the state | 0.23 | 35% 828 systems | 68 |
| Montana 353 systems reporting, 65% of the state | 0 | 24% 85 systems | 4 |
| Oregon 419 systems reporting, 56% of the state | 0 | 22% 93 systems | 5 |
| Oklahoma 555 systems reporting, 68% of the state | 0 | 19% 103 systems | 9 |
| Illinois 920 systems reporting, 58% of the state | 0 | 18% 166 systems | 13 |
| Wisconsin 690 systems reporting, 80% of the state | 0 | 18% 124 systems | 3 |
| Alaska 316 systems reporting, 99% of the state | 0 | 17% 54 systems | 3 |
| Iowa 553 systems reporting, 57% of the state | 0 | 15% 84 systems | 6 |
| Maryland 424 systems reporting, 95% of the state | 0 | 15% 62 systems | 0 |
| Massachusetts 303 systems reporting, 76% of the state | 0 | 13% 40 systems | 4 |
22 states qualify. A state is ranked only with at least 30 reporting systems and at least 25% of its systems covered, because ranking a state on a small slice of it would publish our own collection coverage as that state's water quality. The 14 states held back average 1.44 times higher above the line than the ranked ones. The full accounting is in the method block below.
This ranks states, not individual utilities. Arsenic values in this dataset often rest on a single sampling point that we cannot check against a utility's own published number, so naming an individual system here would not be reliable.
The map itself is a geology map. Arsenic dissolves out of arsenic-bearing rock and sediment into groundwater, so the states at the top are groundwater states over the right rock, and nothing a utility does causes it. What a utility does determines whether it comes out of the tap.
How this was built, and what it cannot tell you
Every arsenic figure above is drawn from one dataset of 38,332 community water systems, and here is what it cannot tell you.
- Records
- 15,115 community water systems with a arsenic result on record, drawn from 38,332 systems that passed every filter listed here. The full panel set holds 42,081 systems.
- Filters applied
- Community water systems only (316 non-community systems removed). Wholesalers removed (2,164), because their population is already counted under the retail systems that buy from them. Systems whose most recent report predates 2023 removed (929). Systems EPA’s inventory does not classify removed (340).
- Sample dates
- July 2016 to July 2026. Nine in ten records are dated September 2024 or later. The median is March 2026. Some older samples are valid: several analytes are on a three-year monitoring cycle, so a sample taken years ago can still appear in a current report.
- Sources
- Contaminant levels: state drinking-water monitoring records and utility reports, assembled into one panel set dated 2026-06-16. System type, population served and purchased-water status: EPA SDWIS, vintage 2026Q1 (file dated 2026-04-08). Health and legal lines: the Stasis threshold registry, version 2026-08-08, which is the same registry that grades a customer report, so a page and a report can never disagree.
- Provenance
- 6,055 of 15,115 results are reconstructions we computed from raw state monitoring records, rather than figures taken straight from a utility’s published water quality report. Full breakdown: 8,441 the utility tested and reported no detection; 5,115 reconstructed from the state's raw monitoring records, finished (delivered) water only; 940 reconstructed from source-water sampling only; this system has no finished-water record; 615 measured at the supplying wholesaler's treatment plant, inherited with attribution; 4 read from the utility's published water quality report. We compile and interpret these figures; we do not take the samples ourselves.
- Scoring rule
- A system is counted against the health-based level. Both lines are printed on every figure so the gap between them is visible. Non-detects are counted as zero and included in the denominator, so a share above a line is never inflated by dropping clean systems.
- Automation
- Software assembled the dataset. It reads state monitoring records, normalises them and combines them, and every figure on this page comes out of one program rather than being typed by hand. This is how 42,081 systems are held to one standard. A person chooses which comparisons to draw and which limits to name, and reviews what the numbers mean. The health thresholds come from the cited primary literature, not a model.
- Raw data
- The row-level extract is not published. It was assembled out of several hundred separate state and federal sources. The findings, the method, the denominators and the limits are all on this page so the work can be checked, and every threshold cited comes from a public primary source linked in place, but the compilation itself is not a free download. Any figure can still be checked at retail: against a utility’s own published report, or by putting one address into the lookup and comparing what it returns. Researchers and reporters who need specific rows can write to data@stasishome.com and say what they are working on.
- Known limitation: how states qualify to be ranked
- A state is ranked only if at least 30 of its systems have a arsenic result AND at least 25% of its community systems do. 22 states clear both. 14 more clear the first and fail the second, so we hold them back from the ranking. The gate exists because coverage in this dataset runs from under 3% of a state’s systems to over 99%, and ranking a state on a 3% subsample would publish our own collection coverage as that state’s water. The excluded states average 32.1% above the line, against 22.3% for the ranked ones, 1.44x higher. The direction differs by analyte, and we recompute this figure from the data on every build.
- Known limitation: purchased water
- 7,416 of the 38,332 systems here buy finished water from a wholesaler instead of treating their own. For those systems, plant-level analytes are often sampled under the seller’s permit number, so our record shows “not reported” where the water was in fact tested upstream. A claim that some share of utilities does not test for arsenic would therefore be false, and nothing on this page makes one. Reporting rates are stated against systems that treat their own water: 43.5% of those 30,916 systems have a arsenic result, against 22.6% of the 7,416 that purchase. There is a 20.9-point gap between the two rates, and it is the hole this limitation describes.
- Scoring rule: why the count is detections
- Arsenic's health-based reference is 0.004 ppb (California public health goal 0.004 ppb (OEHHA, 2004)), which sits about three orders of magnitude below the 2 ppb any certified laboratory is required to report. Counting systems above the health goal would return every detection by arithmetic and carry no information. So the health-side count here is detection at or above the 2 ppb reporting floor, and the health goal is shown as context rather than as a grading line.
- Known limitation: source water versus delivered water
- 940 results come from source-water sampling, taken before treatment, because the system has no finished-water record in the state's files. Those are an upper bound on the tap, not a measurement of it, so they are counted separately and never blended into delivered-water figures. A raw-water number treated as a tap number overstates what reaches you.
- Known limitation: private wells are not here
- This dataset covers community water systems only. The highest US arsenic exposures are on private domestic wells, which no utility tests and no federal rule covers, and an estimated 2.1 million people in the lower 48 states draw well water above the 10 ppb limit. None of them appear in any figure here.
- Corrections
- If a figure here disagrees with your utility’s published report, the published report is more likely to be right and we want to know. Write to data@stasishome.com with the system name and we will check it against the source and correct the record.
These figures describe water systems as a whole. They cannot tell you what is at your own tap, and a system with no result on record is one nobody has tested rather than one known to be clean.
What helps against Arsenic?
5 approaches below reduce Arsenic in a home, ordered from the most direct fix to the smallest change in habit. The most direct is Under-sink reverse osmosis (RO) at the kitchen tap (NSF/ANSI 58). Each entry also names what it cannot do, because no single fix covers every exposure route.
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. This is the standard 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. 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 about Arsenic
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. The 10 ppb legal limit is a starting point, and lower is better.
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, so the safe move is to test rather than wait for symptoms.
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 must treat to the 10 ppb limit. That limit is a feasibility compromise set below the health goal, so compliant municipal water can still sit above California's health goal.
Sources cited for Arsenic
Every number on this Arsenic page traces to one of the 7 sources below: 2 peer-reviewed, 2 government or regulatory.
Peer-reviewed
Government & regulatory
Related water science
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