What reverse osmosis removes, and what it doesn't
Reverse osmosis pushes water through a very tight membrane. Most dissolved salts and metals stay behind and go down the drain. The CDC lists lead, copper and chromium among the chemicals RO removes, along with parasites, bacteria and viruses.
A certified system can also reduce arsenic, nitrate, fluoride, PFOA and PFOS, and total dissolved solids. Check the label for each specific claim. Certification under NSF/ANSI 58 means an independent lab tested the system for the contaminants listed on its performance data sheet.
RO has limits you should know:
- Gases and solvents: The membrane lets dissolved gases and many volatile organic chemicals through. The carbon stages built into most RO units handle these.
- Germs: RO alone is not recommended as your only treatment for bacteria. Treat a contaminated well with a proper disinfection step.
- Arsenic III: This form, common in some wells, passes RO unless it's oxidized first.
The mineral question
RO removes calcium and magnesium along with everything else. That's the root of most claims that RO water is unhealthy.
Germany's Federal Institute for Risk Assessment (BfR) reviewed very low-mineral water in 2020. It concluded such water has no long-term adverse health effects, assuming a balanced diet and normal drinking amounts. The agency noted that water is only one of several mineral sources. Your total diet matters most.
The BfR did flag two exceptions:
- People with low mineral intake from food should watch their calcium and magnesium if low-mineral water is their main drink.
- People doing strenuous exercise for more than about 1.5 hours shouldn't rely on it alone to replace fluids.
If you fall into either group, a remineralization stage or a mineral-rich drink can help. For everyone else, it's mostly a taste choice.
Keep the scale in mind. You drink a few liters a day at most. Even mineral-rich tap water supplies only a modest share of daily calcium and magnesium. Dairy, leafy greens, nuts, beans and whole grains do most of the work.
Taste, pH and "acidic water" claims
RO water often tastes flat because the minerals are gone. Many people like that. Others add a remineralization cartridge for taste.
RO water can also test slightly lower in pH. Carbon dioxide passes through the membrane, while the minerals that buffer it don't. EPA treats pH as a secondary standard, with a range of 6.5 to 8.5. Secondary standards cover taste, staining and corrosion. EPA does not consider them a health risk at those levels.
Lower pH matters more for metal plumbing than for you. That's one reason RO systems use plastic tubing and a dedicated faucet.
| Substance | Does RO reduce it? | Notes |
|---|---|---|
| Lead and copper | Yes | Look for the NSF/ANSI 58 claim for each metal. |
| Arsenic V | Yes | The most common form in treated water. |
| Arsenic III | Not reliably | Needs oxidation first; a lab test tells you the form. |
| Nitrate | Yes | Don't boil nitrate water; boiling concentrates it. |
| PFOA and PFOS | Yes | Includes many short-chain PFAS that carbon misses. |
| Fluoride | Yes, most | Fluoridation is a local public-health choice. |
| Calcium and magnesium | Yes | Food supplies most of these minerals. |
| Chlorine and VOCs | Yes, via carbon stages | The membrane alone does not handle them. |
| Bacteria and viruses | Partly | Not a certified disinfection step on its own. |
RO and fluoride
RO removes most fluoride. If your utility fluoridates, your RO tap will deliver far less than the 0.7 mg/L the CDC recommends for fluoridated water.
Community fluoridation is a public-health choice made locally. Some families want to keep it, and some prefer to remove it. If you rely on tap water for cavity prevention, ask your dentist whether you need another fluoride source. See which filters remove fluoride for the full picture.
When RO is worth it, and when it isn't
RO earns its place when a test shows a dissolved contaminant that simpler filters miss. Good reasons include:
- Nitrate or arsenic in a private well
- PFAS, especially shorter-chain types that carbon handles poorly
- High total dissolved solids or a salty taste
- More than one concern at once, such as lead plus nitrate
If your only complaint is chlorine taste, a certified carbon filter is enough. Carbon wastes no water and costs less to run. The same is true if your utility's report shows no contaminant near its limit. Start with a water test, then pick the treatment that matches the result.
RO does send water down the drain. Typical systems waste several gallons per gallon of treated water. EPA WaterSense-labeled units waste 2.3 gallons or less. See how much water RO wastes before you buy.
Keep the system healthy
A neglected RO unit is a bigger risk than the water it makes. Clogged pre-filters let chlorine and sediment damage the membrane. A worn membrane quietly lets more through.
- Replace pre- and post-filters on the maker's schedule.
- Replace the membrane when output drops or a TDS meter shows rising readings.
- Sanitize the system when you change filters, following the manual.
- Keep a simple log of dates.
Retest your water after installation, then once a year. That's the only way to confirm the system still does its job.