Arsenic is one of the most common health-relevant contaminants in US drinking water, but the risk is heavily concentrated in a single category: private well water. The USGS estimates that roughly 2.1 million people are served by domestic wells with arsenic concentrations above the federal 10 ppb standard, and almost none of those wells are subject to monitoring requirements. Public utilities have been held to the 10 ppb limit since 2006, and most stay well below it.
The right way to think about arsenic is geographic and source-specific. If you are on a private well in the western US — Colorado, Nevada, Arizona, New Mexico, Montana — or in parts of New England or the Upper Midwest, your starting probability is meaningfully higher than the national average and a one-time test is cheap insurance. If you are on city water, your annual Consumer Confidence Report tells you where you stand.
This article covers what arsenic is, where it comes from, what we know about exposure, and which filter mechanisms actually remove it. Standard activated-carbon pitchers do not remove arsenic — that is the misconception that wastes the most money in this category.
Where arsenic in drinking water comes from
The vast majority of arsenic in US tap water is naturally occurring. Arsenic is a metalloid found in volcanic and metamorphic bedrock, and groundwater that flows through arsenic-bearing rock dissolves it over time. Concentrations vary dramatically by geology — wells a quarter-mile apart can read very different numbers depending on the aquifer they tap.
A smaller share of arsenic exposure traces to industrial activity. Historical mining and ore-processing left contaminated groundwater in parts of the West. Until 2003, chromated copper arsenate (CCA) was the standard pressure-treatment chemical for outdoor lumber — playgrounds, decks, fence posts. CCA was phased out for residential use, but legacy structures and the soils around them remain a localized exposure source separate from drinking water.
For the dose that matters, drinking water is the largest route of arsenic exposure for most people, ahead of food and incidental soil contact. The geography of risk follows the geology. The USGS has published maps and modeled estimates of where private-well arsenic is most likely to exceed 10 ppb — the high-risk zones cluster in the Southwest, parts of New England (Maine, New Hampshire), the Upper Midwest, and pockets of the Rocky Mountain and Pacific Northwest regions.
Inorganic versus organic arsenic
Drinking-water exposure is almost entirely inorganic arsenic, and the inorganic forms — arsenite (As III) and arsenate (As V) — are the more toxic forms relevant to public health. Organic arsenic compounds, found mainly in seafood and some plants, are generally considered substantially less toxic and are not the form regulated under the EPA Maximum Contaminant Level.
The distinction matters for filtration because the two inorganic forms behave differently in water. Arsenate (As V) carries a negative charge at typical drinking-water pH, which makes it adsorb readily to certain media. Arsenite (As III) is uncharged at the same pH, which makes it harder to capture for some technologies. Reverse osmosis and iron-oxide media handle both forms; activated alumina works best on As V and may need a pre-oxidation step (often added as part of a complete treatment system) when As III dominates.
What the federal standard says
The EPA enforceable Maximum Contaminant Level for arsenic is 10 micrograms per liter — 10 ppb, or 0.010 mg/L. This is the limit public water systems must meet. EPA lowered the standard from 50 ppb to 10 ppb in January 2001, with full compliance required by January 2006.
A few things to note about that 10 ppb threshold:
- The same 10 ppb appears as the WHO provisional guideline value. WHO classifies it as provisional in part because the achievable detection and treatment limit is around this concentration — there is scientific evidence of effects at lower exposures, but practical reduction below 10 ppb is harder.
- A few US states have adopted lower state-level standards. New Jersey set its own MCL at 5 ppb in 2006. New Hampshire followed at 5 ppb in 2021. These apply only within those states.
- Private wells are not subject to the federal MCL. The EPA standard regulates public water systems serving 25 or more people. Private well owners are responsible for their own testing and treatment.
What we know about exposure
The International Agency for Research on Cancer classifies arsenic as a Group 1 carcinogen — carcinogenic to humans, with sufficient evidence for cancers of the urinary bladder, lung, and skin. That is the highest evidentiary tier in the IARC framework, alongside designations for substances such as tobacco smoke and asbestos.
The ATSDR Toxicological Profile for Arsenic — the joint CDC/ATSDR reference for the toxicology — documents that long-term exposure to inorganic arsenic in drinking water is associated with skin lesions, the cancers IARC enumerates, and cardiovascular disease, with developmental effects in children at sufficient chronic exposure. The CDC does not stake out a "safe" exposure level the way it does with lead, but the regulatory and toxicological framework treats chronic exposure above 10 ppb as a meaningful concern.
This is context, not medical advice. If you have specific questions about your exposure history, primary care and occupational-medicine clinics can order arsenic testing through urine or hair samples.
Where you fit on the risk map
Practically, the question is: which side of the public-versus-private-well line are you on, and where are you geographically?
Public water (most US households). Your utility is required to meet the 10 ppb MCL and to report violations to customers. Your annual Consumer Confidence Report lists the detected arsenic concentration. Most utilities are well below 10 ppb. Search for your utility name plus "Consumer Confidence Report" — every public system publishes one annually.
Private well water. No federal monitoring applies. The EPA and USGS recommend testing every private well for arsenic at least once, and retesting every three to five years or after any well work. The probability of an exceedance is heaviest in the geological zones the USGS maps highlight — the Southwest, parts of New England, the Upper Midwest. A one-time arsenic test through a state-certified lab typically runs in the low double digits and tells you definitively whether you are above 10 ppb.
If your test comes back above 10 ppb, the question is no longer whether to treat — it is which mechanism.
Which filter mechanisms actually remove arsenic
Arsenic is small, often charged, and fully dissolved. Like fluoride, it is harder to remove than chlorine, particulates, or chlorinated organics. The mechanisms that work are specific.
Reverse osmosis (NSF/ANSI 58). RO physically excludes arsenic across both inorganic forms. The NSF/ANSI 58 standard requires certified systems to reduce pentavalent arsenic from a 0.30 mg/L challenge to below the 0.010 mg/L MCL. Most quality RO systems achieve well above the 95 percent reduction floor. RO is the most reliable consumer-grade option for arsenic and addresses many other contaminants in the same pass. A countertop unit like AquaTru carries NSF/ANSI 58 certification and requires no plumbing. An under-sink option like Hydroviv uses targeted media tuned to your zip code's known contaminant profile, including arsenic in regions where the local water testing flags it.
Iron-oxide and iron-hydroxide adsorption media. Granular ferric hydroxide and similar iron-based media are highly effective for both As III and As V. They are the workhorse media in many municipal arsenic-removal systems and increasingly available in residential whole-house and point-of-use cartridges. Performance depends on water chemistry (pH, competing ions); reputable systems publish performance specs at relevant test conditions.
Activated alumina. Highly effective for As V — capacity drops on As III unless the system pre-oxidizes the influent water (often via chlorination) to convert As III to As V before the alumina bed. Activated alumina is a common choice in dedicated arsenic-removal cartridges and is the same chemistry used in many fluoride-targeted gravity filter elements.
Combination systems. Some under-sink systems pair RO with activated alumina post-filtration as a belt-and-suspenders configuration for very high starting concentrations. For most well users above 10 ppb but below roughly 50 ppb, a properly maintained NSF/ANSI 58 RO system is sufficient on its own.
Distillation. Distilling tap water removes arsenic completely along with virtually everything else. Distillers are slow, energy-intensive, and produce demineralized water, but they are the most thorough single-batch option.
What does not work for arsenic
Several common assumptions are wrong, and they cost households money or — worse — false confidence in unsuitable equipment.
- Standard activated-carbon pitchers and faucet filters (Brita, PUR Classic, refrigerator filters) do not remove arsenic. Granular activated carbon is the wrong chemistry. A handful of pitchers carry NSF certifications that specifically include arsenic — those use composite media beyond plain carbon, and the certification number is what to verify. The default assumption for any uncertified pitcher should be: zero arsenic reduction.
- Boiling concentrates arsenic rather than removing it. Water evaporates; arsenic remains. The same applies to letting water sit out.
- Sediment filters capture particulates and rust. They do nothing to dissolved arsenic.
- Water softeners swap calcium and magnesium for sodium. They do not remove arsenic in either oxidation state. Households relying on a softener for arsenic protection are unprotected.
- Big Berkey base black filters are not certified for arsenic reduction. The brand sells optional arsenic/fluoride elements (the same PF-series add-ons relevant to fluoride) that bolt onto the base filters and add activated alumina chemistry. The combination is the configuration that addresses arsenic; the base filter alone does not.
What a reader can actually do
If you are on city water and want to know where you stand on arsenic, your annual Consumer Confidence Report lists it. Most US cities are well below 10 ppb.
If you are on a private well, the steps are:
- Test once. A state-certified lab arsenic test through your state environmental or health department's recommended provider list. The test report will give you a specific concentration in ppb or mg/L.
- If you are above 10 ppb, install one of the certified treatment options above. RO is the highest-confidence general-purpose choice; iron-oxide media or a combination system is the right pick if your starting concentration is high enough that an extra layer of redundancy matters.
- Retest after installation. A treated-water sample tells you the system is working as specified.
- Retest every three to five years. Arsenic concentration in well water is not constant. Aquifer chemistry, well age, and seasonal water-table changes all influence the number.
The certification labels to look for are NSF/ANSI 58 for reverse osmosis and any NSF/ANSI 53 listing that explicitly includes arsenic as a tested claim. A filter without one of those certifications has not been independently verified to reduce arsenic, regardless of marketing copy.
For broader context on filter technologies, see RO vs Carbon vs Gravity. For the related public-water-system contaminant story, see Lead in Tap Water.
