Reverse osmosis does remove arsenic from drinking water, and it’s one of the most effective home treatment methods available. A well-maintained RO system typically removes around 95% of arsenic, with some systems achieving over 99% removal. But the actual performance depends heavily on which form of arsenic is in your water, how well you maintain the system, and how contaminated your water is to begin with.
How Well RO Removes Arsenic
In laboratory conditions, RO membranes can remove up to 99% of arsenic. In real-world household use, results vary more widely. A study of household RO systems in a region of the western U.S. with high groundwater arsenic found a median removal rate of 95%, but an average of 80.2% due to some systems performing poorly. Individual systems ranged from nearly zero removal to over 99%, largely depending on maintenance and water chemistry.
That wide range matters. If your well water contains 100 parts per billion (ppb) of arsenic and your system removes 95%, you’d end up with 5 ppb, which is below the EPA’s legal limit of 10 ppb. But if your water starts at 200 ppb and your system only removes 80%, you’d still have 40 ppb in your glass. High starting concentrations can overwhelm even a good system.
Why Arsenic Type Makes a Big Difference
Arsenic exists in two chemical forms in water, and RO handles them very differently. Arsenate (the oxidized form, sometimes written as As(V)) carries an electrical charge, which makes it easy for an RO membrane to reject. Removal rates for arsenate consistently exceed 95% and often reach 99% or higher.
Arsenite (the reduced form, As(III)) is the problem. At the pH levels typical of groundwater, arsenite is electrically neutral, meaning it can slip through an RO membrane much more easily. Removal rates for arsenite range from about 30% to 80%, with many systems landing around 50 to 60%. That’s a substantial gap. If your water contains mostly arsenite, a standard RO system alone may not bring your levels below the safe threshold.
Groundwater from private wells often contains a mix of both forms, and the ratio depends on local geology and water chemistry. Without testing, you won’t know which form dominates in your water.
Converting Arsenite for Better Removal
The most effective way to improve RO’s performance against arsenite is to convert it into arsenate before the water reaches the membrane. This conversion, called oxidation, can be done with common disinfectants like chlorine, potassium permanganate, or ozone. Once arsenite is oxidized to arsenate, it picks up an electrical charge and becomes far easier for the membrane to reject.
For household systems, this adds complexity. Most oxidation methods require chemical dosing equipment or an additional filter containing a solid oxidizing medium. Some newer approaches use small electrolytic cells that generate oxidants directly in the water without added chemicals, but these aren’t widely available in consumer products yet. If your water has high arsenite levels, a water treatment professional can help you set up an oxidation step ahead of your RO unit.
What Makes an RO System Underperform
Several factors can erode your system’s arsenic removal over time:
- Membrane age and fouling. RO membranes degrade with use. Hard water, sediment, and biological growth can all reduce the membrane’s ability to reject contaminants. Replacing filters and membranes on schedule is essential.
- Low water pressure. RO systems need adequate feed pressure to force water through the membrane. If your home’s water pressure is low, the system won’t perform at its rated efficiency.
- Water chemistry. High pH, high temperature, and elevated levels of dissolved solids can all reduce rejection rates. Very hard water is particularly damaging to membranes over time.
- Improper installation. Systems that aren’t set up correctly, or that bypass the membrane for some water flow, can deliver untreated water to your tap without any obvious sign.
The study of real-world household systems found that poor maintenance and improper setup were major reasons some units failed to remove arsenic effectively. A system that worked well when new can deteriorate significantly if neglected.
Choosing a Certified System
If you’re buying an RO system specifically for arsenic, look for one certified under the NSF/ANSI 58 standard. This is the testing protocol for point-of-use reverse osmosis systems. Under this standard, arsenic reduction is an optional claim, meaning manufacturers can choose to have their systems tested for it. Not all certified RO systems have been tested for arsenic specifically, so check the product listing for “arsenic reduction” or “pentavalent arsenic reduction” in the claims.
A system certified for arsenic under NSF/ANSI 58 has been independently verified to reduce arsenic under controlled test conditions. That doesn’t guarantee the same performance in your home with your water, but it’s a meaningful baseline of quality.
Why Arsenic in Water Matters
Arsenic is a naturally occurring element that dissolves into groundwater from certain rock formations. It’s tasteless and odorless, so you can’t detect it without testing. The EPA set the maximum contaminant level at 10 ppb in 2001, lowering it from the previous standard of 50 ppb. Public water systems must meet this limit, but private wells are not regulated, and millions of well owners in the U.S. have never tested for arsenic.
Long-term exposure to arsenic in drinking water is linked to skin changes (including darkened patches and thickened skin on the palms and soles), cancers of the skin, bladder, and lungs, and increased risk of cardiovascular disease, diabetes, and developmental problems. Skin changes are typically the earliest visible sign of chronic exposure. These effects develop over years, not days, which makes ongoing low-level exposure easy to overlook but important to address.
Testing Your Water
If you’re on a private well, the only way to know your arsenic level is to have your water tested by a certified laboratory. Many state health departments offer low-cost testing programs. A basic arsenic test typically costs $15 to $50. For the best information, ask for speciated arsenic testing, which tells you how much arsenite versus arsenate your water contains. This helps you determine whether a standard RO system will be sufficient or whether you’ll need a pre-oxidation step.
If you already have an RO system, test the water coming out of the dedicated faucet, not just the raw well water. This tells you whether your system is actually performing. Retesting every year or two, and after replacing membranes, helps you catch declining performance before it becomes a health issue.