Arsenic is a naturally occurring heavy metal that enters drinking water primarily through groundwater. The US Geological Survey estimates that 2.1 million Americans on public water systems and at least another 23 million on private wells are exposed to arsenic levels that exceed what modern science considers safe. The contamination is heaviest in the southwestern US, parts of New England, the Upper Midwest, and the Appalachian region — but arsenic detection is documented in every state.
The EPA’s legal limit for arsenic in drinking water is 10 parts per billion. The Environmental Working Group’s health guideline is 0.004 ppb — a 2,500x difference between what’s legal and what’s considered protective of long-term health. That gap is what makes arsenic such a stubborn problem: many US utilities are fully legally compliant while still delivering water with documented cancer risk.
Where arsenic in drinking water comes from
The dominant source is natural geology. Arsenic occurs in many rock formations and gradually leaches into groundwater over millennia. The southwestern US — Arizona, New Mexico, Nevada, and parts of California — has the highest natural arsenic concentrations in the country because of the volcanic and mineral-rich bedrock. New England (particularly Maine and New Hampshire) has arsenic-rich bedrock that contaminates private well water at high rates.
Secondary sources include:
- Mining and smelting — copper and gold mining release arsenic into watersheds; legacy mining sites are still affecting water supplies decades after operations ceased
- Coal-fired power plants — fly ash contains arsenic; runoff can affect surface water sources
- Agricultural runoff — older arsenic-based pesticides (banned in the 1990s but persistent in soils) continue to leach
- Wood preservatives — chromated copper arsenate (CCA) used on pressure-treated wood until 2003 contributes locally
Why arsenic in drinking water is dangerous
Arsenic is a known human carcinogen, classified Group 1 by the International Agency for Research on Cancer (IARC). Long-term exposure to arsenic in drinking water is associated with cancers of the skin, bladder, lung, kidney, and liver. The exposure-response relationship is well-established and roughly linear: there is no clear threshold below which exposure becomes “safe.”
The current EPA limit of 10 ppb was set in 2001, down from the previous limit of 50 ppb. The change was estimated to prevent thousands of bladder and lung cancers per year. Many public health researchers argue the limit should be lower still — the National Research Council’s 2001 assessment estimated that lifetime exposure at 10 ppb still carries a cancer risk roughly 10 times higher than the EPA’s typical “acceptable” risk threshold.
EWG’s guideline of 0.004 ppb reflects modern cancer-risk modeling: it’s the level at which lifetime risk drops to one in a million.
In addition to cancer, chronic arsenic exposure is associated with:
- Cardiovascular disease (increased risk of heart attack and stroke)
- Type 2 diabetes
- Adverse pregnancy and birth outcomes
- Developmental and cognitive effects in children
- Skin lesions and chronic skin conditions
Who is most at risk
Private well users in arsenic-rich geology. Roughly 13% of private wells in the US exceed 10 ppb. Wells in New England, the Southwest, and the Upper Midwest test highest. Private wells are not regulated by the EPA — testing is the well owner’s responsibility, and roughly two-thirds of well owners have never tested for arsenic.
Public water customers in the Southwest. Phoenix, Tucson, Las Vegas, Albuquerque, and many smaller cities in Arizona, New Mexico, and Nevada have arsenic detection levels above EWG’s guideline, sometimes by hundreds of times. These utilities are typically legally compliant but the water is far from health-protective levels.
Children and pregnant residents. Developmental effects from arsenic exposure in utero and in early childhood are well-documented. Pregnant residents in any area with documented arsenic should filter aggressively.
People with cardiovascular risk factors. The arsenic-cardiovascular link is significant enough that high-arsenic exposure should be considered a modifiable risk factor on par with hypertension or high cholesterol.
How to know if arsenic is in your tap water
Three options:
Check your utility’s TapWaterSafety page. We pull EWG-aggregated arsenic detection data per utility and flag it as a Top Concern when detected above 1 ppb. Phoenix, for example, has arsenic at typical levels of 2.4 ppb — below the EPA legal limit but 600x above the EWG guideline.
Read your utility’s CCR. Federal law requires utilities to test for and report arsenic. The CCR will list both the detected level and the EPA legal limit. What it won’t tell you is how that level compares to modern cancer-risk-based health guidelines.
Test your tap water directly. Mail-in lab tests for arsenic run $25-80. SimpleLab’s Tap Score, Watercheck, and similar services include it in basic heavy metals panels. This is essential if you’re on a private well.
How to filter arsenic
Standard carbon filters do not remove arsenic. This is the same critical point we make for chromium-6 and lead: basic pitcher filters and most refrigerator filters are useless against arsenic. Don’t assume your filter handles it unless the certification specifically states it does.
Three technologies work effectively:
Reverse osmosis (most reliable)
RO membranes block arsenic with very high efficacy — typically 95-99% removal for both arsenic-3 (arsenite) and arsenic-5 (arsenate). NSF/ANSI Standard 58 specifically tests for arsenic reduction.
This is the recommended technology for any home where arsenic is a documented concern. RO also handles chromium-6, lead, PFAS, nitrate, and most other concerning contaminants in the same system.
Iron-coated sand or specialty adsorptive media
Some specialty point-of-use filters use iron-oxide-coated sand or specialty resins that selectively adsorb arsenic. These work well but require periodic media replacement and aren’t widely available in consumer products.
Distillation (impractical for daily use)
Distillation removes arsenic effectively but is too slow and energy-intensive for typical home water needs. Used mainly in laboratory settings or emergency situations.
What does NOT remove arsenic
- Basic carbon pitcher filters
- Most refrigerator filters
- Boiling water (concentrates arsenic by removing water as vapor)
- Water softeners
- UV disinfection
- Activated carbon alone (some removal, but unreliable)
Top filter recommendations for arsenic
Best countertop option: AquaTru Countertop Reverse Osmosis. NSF/ANSI 58 certified for arsenic. Removes both common forms of arsenic. No plumbing installation. $449-599.
Best under-sink option: Aquasana OptimH2O Reverse Osmosis + Claryum. Same arsenic reduction with a dedicated drinking faucet and longer membrane life. $429-549.
Whole-house option (only for severe contamination): A whole-house RO system runs $2,000-5,000 installed. Almost always overkill. Arsenic is dangerous through ingestion, not skin contact, so kitchen-tap RO is sufficient for the vast majority of homes.
A note on well water specifically
If you’re on a private well in known arsenic geology (Maine, New Hampshire, parts of Arizona, New Mexico, Nevada), the recommendation is stronger: test for arsenic at the wellhead and after your home plumbing, then install a whole-house solution if levels warrant. Private well water gets no federal oversight, no required testing, and no automatic notification when arsenic spikes. The responsibility is entirely yours.
Pregnant residents and families with young children on private wells in arsenic regions should consider an under-sink RO system as a baseline default, even before testing — the cost is low relative to the documented health risks.
The bottom line
Arsenic in drinking water is one of the best-documented and most-underestimated cancer risks in US homes. Legal compliance with the EPA’s 10 ppb limit is not the same as safety. If your TapWaterSafety utility page shows arsenic detection at any level above the EWG guideline of 0.004 ppb, a reverse osmosis system at the kitchen tap is the practical solution.
Find your water by ZIP code to see your arsenic detection level and get a filter recommendation matched to your full water profile.