Arsenic is a naturally occurring element found in rocks and soil. In some areas, groundwater can pick up arsenic as water moves through arsenic-bearing geological formations.
Human activities can also contribute arsenic to the environment, including certain mining, metal processing, historical pesticide use, and industrial activities.
For households, the most important concern is long-term exposure to inorganic arsenic through drinking water, particularly in regions where groundwater naturally contains elevated concentrations.
Quick answer: The U.S. EPA Maximum Contaminant Level for arsenic in public drinking water is 10 parts per billion (ppb), or 0.010 mg/L. Arsenic cannot reliably be detected by taste, smell, or appearance, so laboratory testing and public-water monitoring are the appropriate ways to determine whether it is present.
In This Guide
- What Is Arsenic?
- How Arsenic Gets Into Water
- Arsenic III vs. Arsenic V
- Health Effects
- EPA Arsenic Limit
- Public Water Systems
- Private Wells
- Testing for Arsenic
- Arsenic Treatment Options
- Reverse Osmosis
- Adsorptive Media
- Ion Exchange
- Why Oxidation Matters
- Does Boiling Remove Arsenic?
- Choosing a Home Filter
- Berkey® & PF-2 Filters
- Frequently Asked Questions
What Is Arsenic?
Arsenic is a naturally occurring element distributed throughout Earth's crust.
It can occur in:
- Rock
- Soil
- Groundwater
- Surface water
- Air
- Food
Arsenic exists in both organic and inorganic chemical forms.
From a drinking-water perspective, inorganic arsenic is the primary concern because it is generally more toxic than the organic forms commonly found in seafood.
“Organic arsenic” does not mean organically grown. In chemistry, organic and inorganic refer to molecular structure. Many arsenic compounds found in seafood are chemically different from the inorganic arsenic associated with drinking-water concerns.
How Does Arsenic Get Into Drinking Water?
Natural Geology
The most important source of drinking-water arsenic in many affected regions is natural geology.
Groundwater moving through arsenic-containing rock and sediments can dissolve arsenic and carry it into aquifers.
This means arsenic contamination can occur even where there is no nearby industrial pollution.
Mining and Metal Processing
Mining, smelting, and processing ores containing arsenic can release arsenic into soil, air, and water.
Historical Agricultural Uses
Arsenic compounds were historically used in some pesticides and agricultural products.
Wood Preservation
Arsenic compounds were formerly used in certain pressure-treated wood preservatives, although residential use of chromated copper arsenate-treated lumber has been substantially restricted in the United States.
Other Industrial Sources
Arsenic compounds have also been associated historically with:
- Glass manufacturing
- Pigments
- Metal alloys
- Semiconductor production
- Other industrial processes
Arsenic III vs. Arsenic V: Why the Form Matters
One of the most important—and often overlooked—parts of arsenic treatment is speciation.
In groundwater, inorganic arsenic commonly occurs in two oxidation states:
- Arsenite — As(III)
- Arsenate — As(V)
| Characteristic | As(III) — Arsenite | As(V) — Arsenate |
|---|---|---|
| Common environment | More common under reducing, low-oxygen groundwater conditions | More common under oxidizing conditions |
| Treatment | Can be more difficult for several treatment technologies | Often more readily removed by adsorption and ion-exchange processes |
| Pre-treatment | May be oxidized to As(V) before treatment | Often treated directly by an appropriate system |
This distinction matters when choosing a filter. A system tested for pentavalent arsenic, As(V), should not automatically be assumed to provide the same performance for untreated As(III).
What Are the Health Effects of Arsenic Exposure?
Health effects depend on:
- The chemical form of arsenic
- Exposure level
- Length of exposure
- Route of exposure
- Individual health factors
Acute High-Level Exposure
Very high inorganic arsenic exposure can cause severe poisoning.
Possible symptoms can include:
- Nausea
- Vomiting
- Abdominal pain
- Diarrhea
- Numbness or tingling
- Muscle cramps
- Cardiovascular or neurological complications in severe cases
Acute arsenic poisoning is different from the much lower-level chronic exposure typically considered in drinking-water regulation.
Long-Term Exposure
Long-term exposure to elevated inorganic arsenic concentrations has been associated with:
- Changes in skin pigmentation
- Hyperkeratosis, or thickened skin
- Peripheral neurological effects
- Cardiovascular effects
- Developmental effects
- Other chronic health outcomes
Arsenic and Cancer
Inorganic arsenic is a recognized human carcinogen.
Long-term exposure has been associated particularly with increased risks of:
- Skin cancer
- Bladder cancer
- Lung cancer
Finding arsenic in water does not mean an individual will develop one of these conditions. Health risk depends on concentration and cumulative exposure over time.
What Is the EPA Limit for Arsenic in Drinking Water?
EPA's enforceable Maximum Contaminant Level for arsenic in regulated public drinking-water systems is:
10 ppb = 0.010 mg/L = 10 micrograms per liter
The current 10-ppb federal standard replaced the previous 50-ppb limit.
The World Health Organization also uses 10 μg/L as its provisional guideline value.
The MCL should not be interpreted as a sharp boundary between “zero risk” and “danger.” Drinking-water standards incorporate health-risk assessment, treatment, and regulatory considerations.
Arsenic and Public Water Systems
Community water systems subject to federal drinking-water requirements monitor arsenic and must comply with the federal standard.
If you receive municipal water, start by reviewing your utility's annual Consumer Confidence Report.
It can tell you:
- Your water source
- Whether arsenic was detected
- The reported concentration
- The applicable federal standard
If your report is unclear, contact the utility directly.
Arsenic and Private Wells
Private wells require more homeowner involvement because they are generally not monitored under the federal regulations that apply to public water systems.
Arsenic testing deserves particular consideration when:
- You rely on groundwater.
- Arsenic is known to occur naturally in your region.
- Nearby wells have tested high.
- Your state or local health agency recommends testing.
- The property is near historical mining or industrial activity.
You cannot determine whether well water contains arsenic by looking at it. Arsenic may be present in completely clear, odorless water.
See our Well Water Testing & Treatment Guide for broader guidance on private wells.
How Do You Test Water for Arsenic?
Laboratory testing is the most reliable way to determine arsenic concentration.
1. Use an Appropriate Laboratory
Look for a state-certified or otherwise appropriately accredited drinking-water laboratory.
2. Ask What the Test Measures
A standard arsenic test may report total arsenic. If treatment decisions depend on As(III) versus As(V), ask whether arsenic speciation is available.
3. Follow Sampling Instructions Carefully
The laboratory may provide specific bottles, preservation instructions, flushing requirements, or holding-time requirements.
4. Compare the Result With Applicable Standards
In the U.S. public water context, EPA's federal MCL is 10 ppb.
5. Retest After Installing Treatment
Post-treatment laboratory testing can help confirm whether the installed system is performing as expected under your actual water conditions.
How Can Arsenic Be Reduced in Drinking Water?
There is no single arsenic-treatment technology that is best for every household.
The appropriate method depends on:
- Arsenic concentration
- Arsenic form
- pH
- Iron and manganese
- Silica
- Sulfate and other competing ions
- Water use
- Whether treatment is point-of-use or whole-house
Common arsenic-treatment approaches include:
- Reverse osmosis
- Adsorptive media
- Ion exchange
- Oxidation followed by filtration or adsorption
- Coagulation and filtration in larger treatment systems
Reverse Osmosis and Arsenic
Reverse osmosis uses pressure to move water across a semipermeable membrane.
Many dissolved substances are rejected by the membrane and leave through a separate concentrate stream.
RO can be an effective arsenic-reduction technology, particularly for As(V), when the finished system is appropriately designed and maintained.
Why As(III) Matters
As(III) may be more difficult for some RO systems to reduce than As(V).
Where As(III) is present, pretreatment that converts it to As(V) may improve overall arsenic reduction.
I would not call reverse osmosis universally “the best arsenic filter.” RO is one established option, but adsorptive media, ion exchange, oxidation, and other technologies can be more appropriate depending on the water chemistry and treatment objective.
For more information, see our Drinking Water Filtration Systems Guide.
Iron-Based and Other Adsorptive Media
Adsorptive media are commonly used for arsenic treatment.
Examples can include:
- Iron oxide
- Iron hydroxide
- Activated alumina
- Other specialized adsorbents
Arsenic binds to the media surface as water passes through.
Performance depends strongly on:
- Arsenic speciation
- pH
- Competing ions
- Media capacity
- Contact time
As the adsorption sites become occupied, the media eventually requires replacement or regeneration, depending on the system design.
Ion Exchange and Arsenic
Some anion-exchange resins can reduce arsenate (As(V)) by exchanging ions between the resin and the water.
Performance may be affected by competing ions such as sulfate.
Ion exchange also creates a waste stream during regeneration, so treatment design must account for disposal and maintenance.
An ordinary household water softener should not automatically be considered an arsenic-treatment system. Arsenic treatment requires the correct resin, configuration, water chemistry, and operating conditions.
Why Oxidation Can Improve Arsenic Treatment
Some treatment methods are more effective for As(V) than for As(III).
For this reason, arsenic treatment can include an oxidation step to convert:
As(III) → As(V)
Oxidation may be followed by treatment approaches appropriate to the water system, such as adsorption, filtration, ion exchange, or another arsenic-reduction process.
This is why water testing should come before buying treatment. Knowing only the total arsenic concentration may not provide enough information to design the most effective system.
Does Boiling Water Remove Arsenic?
No.
Boiling does not remove dissolved arsenic.
Because some water evaporates while arsenic remains behind, prolonged boiling can increase the arsenic concentration in the remaining water.
See our Does Boiling Water Purify It? Guide.
Does Activated Carbon Remove Arsenic?
Ordinary activated carbon should not be assumed to provide reliable arsenic reduction.
Carbon-based finished filters may contain additional media specifically designed to adsorb arsenic, but the performance comes from the complete filter formulation rather than ordinary carbon alone.
See our How Activated Carbon Filters Work guide.
Can a TDS Meter Detect Arsenic?
No.
A TDS meter measures electrical conductivity and estimates the overall concentration of dissolved ionic material.
It does not identify individual substances such as arsenic.
A low TDS reading does not mean arsenic is absent, and a high TDS reading does not mean arsenic is present. Arsenic requires contaminant-specific testing.
How to Choose a Home Arsenic Filter
If arsenic is actually present in your water, evaluate a treatment system using the following criteria.
1. Confirm the Arsenic Concentration
Start with laboratory data rather than assuming arsenic is present because of taste, color, or location alone.
2. Determine the Arsenic Form When Necessary
If treatment performance depends on the speciation of arsenic (As(III) versus As(V), consider arsenic speciation.
3. Look for Arsenic-Specific Performance
Do not rely on general descriptions such as:
- “Heavy-metal filter”
- “Activated-carbon filter”
- “Multi-stage filter”
Look for testing or certification for arsenic reduction on the exact model.
4. Check Test Conditions
Pay attention to:
- Influent arsenic concentration
- Arsenic oxidation state
- Rated capacity
- Flow rate
- Water chemistry limitations
5. Maintain the System
Adsorption media and other treatment components have finite capacity.
Follow replacement, regeneration, cleaning, and monitoring requirements.
6. Verify Performance
For a private well or known arsenic problem, laboratory testing of treated water provides stronger confirmation than taste or appearance.
What Does NSF Certification for Arsenic Mean?
NSF/ANSI 53 includes health-effects contaminant reduction claims for drinking-water treatment systems.
NSF's certification listings include an arsenic-reduction category specifically for pentavalent arsenic (As(V)) under specified influent conditions.
An arsenic certification claim is condition-specific. Do not assume that a system certified for As(V) provides equivalent performance for untreated As(III), higher arsenic concentrations, or different water chemistry unless the product documentation specifically supports that use.
Where Do Berkey® and PF-2 Filters Fit?
Berkey PF-2™ Fluoride and Arsenic Reduction Elements are supplemental filter elements designed for use with compatible Berkey® systems using Black Berkey® Elements.
The PF-2™ elements use specialized media designed for fluoride and arsenic reduction, rather than relying solely on ordinary activated carbon.
For an arsenic concern, however, the same principle applies as with any other treatment product:
- Know the arsenic concentration.
- Understand whether As(III) or As(V) is present when relevant.
- Review current product-specific test information.
- Follow installation and replacement instructions.
I would not retain the old blanket claim that PF-2™ “removes up to 97% of fluoride and arsenic ions.” Arsenic performance should be tied to current filter-specific documentation and the arsenic form and test conditions used.
Review our Water Filter Test Results and Berkey® Replacement Filters Guide for current filter-specific information.
If you are choosing among Berkey® system sizes rather than treatment media, see our Berkey® Water Filter Systems Comparison Guide.
Frequently Asked Questions About Arsenic in Drinking Water
What is arsenic?
Arsenic is a naturally occurring element found in Earth's crust. It can occur naturally in groundwater and can also enter the environment through certain industrial and historical agricultural activities.
What is the EPA arsenic limit in drinking water?
The federal Maximum Contaminant Level for arsenic in regulated public drinking-water systems is 10 parts per billion, equivalent to 0.010 mg/L.
Can arsenic occur naturally in well water?
Yes. Natural groundwater contamination from arsenic-bearing rocks and sediments is one of the most important sources of arsenic exposure worldwide.
Can you taste arsenic in water?
No reliable taste, odor, or visual sign identifies arsenic at typical drinking-water concentrations.
What is the difference between As(III) and As(V)?
As(III), or arsenite, and As(V), or arsenate, are different oxidation states of inorganic arsenic. Their chemistry differs, and As(III) can be more difficult for several treatment technologies to reduce.
Why does arsenic speciation matter?
Some treatment methods work substantially better for As(V) than As(III). In those situations, As(III) may first be oxidized to As(V).
Does boiling remove arsenic?
No. Boiling does not remove dissolved arsenic and can increase its concentration as water evaporates.
Does reverse osmosis remove arsenic?
Appropriately designed reverse osmosis systems can reduce arsenic levels, particularly As(V). Performance depends on system design, arsenic form, water chemistry, maintenance, and test conditions.
Does activated carbon remove arsenic?
Ordinary activated carbon should not automatically be relied upon for arsenic reduction. Some finished filters combine carbon with additional arsenic-specific media.
Can iron-based media reduce arsenic?
Iron oxide, iron hydroxide, and related adsorptive media are commonly used for arsenic treatment. Performance depends on the form of arsenic and water chemistry.
Can ion exchange remove arsenic?
Some anion-exchange systems can reduce As(V). Competing ions, particularly sulfate, can affect performance.
Does a water softener remove arsenic?
A conventional household softener should not be assumed to provide arsenic treatment. Specialized treatment media and system design are required.
Can a TDS meter detect arsenic?
No. A TDS meter cannot identify arsenic or tell you whether its concentration exceeds the drinking-water standard.
Should private-well owners test for arsenic?
Testing is particularly important in regions where arsenic occurs naturally in groundwater or where state or local agencies recommend monitoring for arsenic.
How often should I test a private well for arsenic?
There is no single national schedule that fits every well. Follow recommendations from your state or local health agency and consider retesting when water sources, well conditions, or treatment systems change.
Does arsenic cause cancer?
Inorganic arsenic is a recognized human carcinogen. Long-term exposure has been associated particularly with skin, bladder, and lung cancer.
Is arsenic in seafood the same as arsenic in drinking water?
Not generally. Much of the arsenic in many seafoods occurs in organic chemical forms that are less toxic than the inorganic arsenic typically associated with contaminated drinking water.
What should I look for when buying an arsenic filter?
Look for arsenic-specific testing or certification for the exact product, and review the arsenic form, influent concentration, rated capacity, water chemistry limitations, and maintenance requirements.
Final Thoughts
Arsenic differs from many household water-quality concerns in that it usually provides no sensory warning.
You cannot reliably identify it from:
- Taste
- Smell
- Color
- TDS
That makes testing especially important for private wells in regions where arsenic is known to occur.
If arsenic is detected, the next question is not simply, “Which filter removes arsenic?”
The more useful questions are:
- How much arsenic is present?
- Is it primarily As(III) or As(V)?
- What is the water's pH and chemistry?
- Which treatment has demonstrated performance under relevant conditions?
- How will treatment performance be verified over time?
The best arsenic-treatment decision begins with laboratory data. Determine what is actually in the water first, then choose a treatment system with arsenic-specific performance appropriate to those conditions.