What percentage of arsenic does reverse osmosis remove?

RO systems certified to NSF/ANSI 58 typically achieve 85-95%+ reduction of arsenic in controlled testing. Real-world performance depends on membrane condition, water pressure, and whether arsenic is in its oxidized form (arsenate, which RO handles well) or its reduced form (arsenite, which is harder to remove). If your water has significant iron or sulfide, pre-treatment may be needed so arsenic is in the right form before the membrane.

Does a whole-house filter remove arsenic?

Whole-house arsenic removal is possible with a whole-house activated alumina or point-of-entry RO system. It's appropriate if your arsenic level is very high (above 50 ppb), you have young children bathing in the water, or you want to protect plumbing and appliances. Most households can address drinking and cooking exposure with an under-sink RO at a much lower cost.

Does boiling water remove arsenic?

No. Boiling concentrates arsenic by evaporating water and leaving dissolved solids behind. Never use boiling as arsenic treatment.

How long does an RO membrane last for arsenic removal?

Most RO membranes last 2-5 years with proper maintenance. Arsenic removal efficiency declines as the membrane ages. Check your filtered water with a TDS meter periodically. A rising TDS reading in your filtered water is a sign the membrane needs replacement. Annual testing of filtered water for arsenic is the definitive check.

Is arsenic removal different for different arsenic forms?

Yes. Arsenic exists in two main forms: arsenate (As V) and arsenite (As III). RO and activated alumina both remove arsenate effectively. Arsenite is more difficult. If your well has high iron or is low in dissolved oxygen, arsenite may be the dominant form. A lab that specifies arsenic speciation can tell you. In those cases, pre-oxidation with a sediment filter or aeration step before the treatment unit improves removal.

How to Remove Arsenic from Well Water: What Works and What Doesn't

Health disclaimer: This page provides general information about arsenic treatment options. It is not medical advice. If your well tests above the EPA limit of 10 ppb for arsenic, consult a licensed water treatment professional before selecting a system. Always verify treatment effectiveness with post-treatment testing.

Your arsenic test result tells you your level. Now you need to pick the right treatment. For context on where arsenic comes from and health effects, see the arsenic in well water overview. The choice depends on three things: how high your level is, whether you want point-of-use or whole-house protection, and your budget.

Two technologies have solid evidence behind them. Most other common filters don’t work for arsenic at all. Knowing the difference before you spend money matters.

The Two Technologies That Work

Reverse osmosis is the most widely used approach for household arsenic removal. An RO system forces water through a semi-permeable membrane under pressure, blocking dissolved ions including arsenic. Systems certified to NSF/ANSI 58 have been independently tested and confirmed to reduce arsenic. Well-maintained RO systems typically achieve 85-95% or better reduction.

Under-sink RO units treat water at the tap where you drink and cook. That covers the primary exposure pathway at a cost of $150-400 for a quality unit. They install under the kitchen sink and deliver filtered water through a dedicated faucet.

Activated alumina is the other proven option. It’s a porous aluminum oxide media that arsenic ions bind to as water passes through. It works well for arsenate (As V), the oxidized form of arsenic more common in oxygenated well water. Activated alumina is often used in whole-house or point-of-entry systems, and it’s what many municipal water systems use at scale.

Both work. RO is more widely available for residential use and easier to install yourself. Activated alumina is better suited to whole-house applications or situations where high flow rate matters.

Why Common Filters Don’t Work for Arsenic

Activated carbon filters, including Brita pitchers and standard under-sink carbon units, are certified for taste, odor, and specific organic compounds. They don’t bind arsenic ions. Carbon filters have the wrong chemistry for this problem.

Water softeners are also ineffective. They use ion exchange to swap calcium and magnesium ions for sodium ions, which reduces hardness. Arsenic ions don’t get exchanged in this process. A softener won’t lower your arsenic level.

Boiling is worth calling out specifically because it makes arsenic worse, not better. Boiling drives off water as steam and leaves dissolved solids behind, concentrating arsenic in whatever’s left in the pot. Don’t boil water as an arsenic fix.

Standard pitcher filters fail here too, unless a specific model has been tested and certified for arsenic under NSF/ANSI 53 or 58. Very few pitcher filters carry that certification for arsenic. Check the NSF product database before assuming any pitcher filter addresses arsenic.

Point-of-Use vs. Whole-House

For most households with arsenic in their well, an under-sink RO at the kitchen tap addresses the real risk. Drinking and cooking water is where ingestion exposure happens. At $150-400 for the unit plus occasional filter replacements, it’s the cost-effective starting point.

Whole-house treatment makes sense in specific situations. If your arsenic level is very high (above 50 ppb), a point-of-use system may not reduce it enough on its own. Whole-house treatment also matters if you have young children who absorb contaminants through skin during baths. Some homeowners also want to protect their plumbing and appliances from arsenic-laden water.

A whole-house point-of-entry RO or an activated alumina tank system runs $1,500-3,000+ for equipment and professional installation. That’s a meaningful cost difference from under-sink treatment. Get your level tested before committing to whole-house treatment. For arsenic at 15-30 ppb, a well-maintained under-sink RO is the right call for most families.

Arsenic Form Matters

Arsenic in groundwater shows up in two main oxidation states. Arsenate (As V) is the oxidized form, more common in wells with dissolved oxygen. Arsenite (As III) is the reduced form, more common in deep wells, wells with high iron, or wells where dissolved oxygen is low.

RO and activated alumina both remove arsenate well. Arsenite is harder for these technologies to capture at full efficiency. If your RO system is certified to NSF 58 for arsenate but your well primarily has arsenite, you may see lower real-world reduction than the certificate suggests.

A standard lab test reports total arsenic. A speciation test separates the two forms. If your arsenic is above 30 ppb and your well has high iron, low pH, or an obvious sulfide odor, asking for speciation testing is reasonable. Treatment for arsenite-dominant water typically includes a pre-oxidation step, either aeration or a small dose of oxidizing media, before the main treatment unit. This converts arsenite to arsenate, then the RO or activated alumina handles it efficiently.

A licensed water treatment professional can assess your water chemistry and design the right sequence if you’re dealing with high levels or unusual water characteristics.

After Treatment: How to Verify It’s Working

Don’t assume the system is working. Test your filtered water.

Run the system normally for 2-3 weeks after installation to flush the new membrane and establish stable operating conditions. Then collect a sample from the RO output faucet, using the same certified lab you used for your well test. Send it in for arsenic analysis. For certified lab options, see best mail-in water tests.

A passing result means your filtered water has arsenic below your target level, ideally below 5 ppb and ideally as low as the lab can detect. If your result is higher than expected, check membrane age, water pressure, and whether any pre-filters need replacement.

A TDS meter gives you an ongoing rough check between annual arsenic tests. As a membrane ages, its rejection rate declines for all dissolved solids. If your filtered TDS reading starts climbing significantly compared to your original baseline, the membrane may be approaching end of life. Annual arsenic testing on your filtered water is the definitive confirmation.

Replace membranes on schedule. Most RO membranes are rated for 2-5 years. Deferring membrane replacement to save money is a false economy when you’re relying on the system for arsenic reduction.

For system options, see the best under-sink RO systems page.

Sources

EPA, “Arsenic in Drinking Water”: epa.gov/ground-water-and-drinking-water/basic-information-about-arsenic-drinking-water
NSF International, “NSF/ANSI 58, Reverse Osmosis Drinking Water Treatment Systems”: nsf.org
USGS, “Arsenic and Drinking Water from Private Wells”: usgs.gov
EPA, “Technologies for Arsenic Removal from Drinking Water”: epa.gov

Health disclaimer: This page provides general information about arsenic treatment options. It is not medical advice. If your well tests above 10 ppb for arsenic, consult a licensed water treatment professional. Always verify treatment effectiveness by testing your filtered water output through a certified laboratory.