What Does a Reverse Osmosis System Remove? Full Contaminant List

Quick answer: A reverse osmosis system removes 95 to 99 percent of total dissolved solids. That includes lead, arsenic, fluoride, nitrates, chromium-6, uranium, radium, sodium, PFAS forever chemicals, and microplastics. It does not remove dissolved gases like radon or hydrogen sulfide. It also needs a carbon pre-filter, not the membrane, to take out chlorine.

This guide gives you the full list. You will see real removal rates. You will also see what RO misses, and what to use instead.

How Reverse Osmosis Removes Contaminants

An RO system pushes water through a semipermeable membrane. A pump or your home water pressure does the pushing. Water molecules pass through. Most other things stay behind.

The pores in that membrane are tiny. They measure about 0.0001 micron across. That is smaller than almost every contaminant in tap water.

There is one simple rule behind the whole list below:

Big or charged things get blocked. Small, neutral, or gas-like things slip through.

Membrane makers put it in technical terms. An RO membrane rejects material larger than about 100 molecular weight units. In general, it removes 95 to 99 percent of inorganic material.

That one rule explains both lists in this guide. It explains why RO stops lead. It also explains why RO lets radon gas pass.

The Four Stages and What Each One Removes

Here is a point people often miss. A "reverse osmosis system" is not just a membrane. It is a set of stages. Each stage does a different job.

The carbon pre-filter matters more than most buyers think. Most RO membranes are badly damaged by chlorine. So the carbon stage has to strip chlorine before water reaches the membrane.

The clean water that comes out is called permeate. The wastewater that carries the rejected junk down the drain is called concentrate, or brine. How much of each you get is the system's recovery ratio. Most home units are rated in GPD, or gallons per day.

Most home membranes today are thin-film composite (TFC) membranes. The DOW FilmTec membrane is often treated as the benchmark that other brands are measured against.

Full Contaminant List: What Reverse Osmosis Removes

Below are the main groups. Each row shows a published rejection rate. Rates come from membrane spec sheets and from system makers.

One honest note first. RO does not promise 100 percent removal of any single thing. It rejects a percentage. That percentage shifts with water pressure, water temperature, membrane type, and how well you keep up with filter changes.

Heavy Metals

RO is very strong in removing heavy metals. This is the main reason people buy one.

These rates come from published membrane rejection tables.

Lead gets in through old pipes and solder. Homes built before 1986 are the main risk. No safe level of lead is known for young children.

One detail most pages skip. Arsenic comes in two forms. RO removes arsenic V well. Arsenic III isn't charged the same way, so it slips through more often. Water with arsenic III usually needs an oxidation step first. Ask about this if your test shows arsenic.

RO also cuts chromium, including chromium-6 (hexavalent chromium), plus copper, selenium, antimony, beryllium, thallium, silver, and manganese.

PFAS and Forever Chemicals

PFAS stands for per- and polyfluoroalkyl substances. People call them forever chemicals because they do not break down. They come from nonstick pans, waterproof gear, food wrappers, and firefighting foam.

RO handles them well. Published removal for PFOS runs about 90 to 99 percent.

The EPA has said that high-pressure membranes, meaning nanofiltration and reverse osmosis, are very effective at removing PFAS.

RO also targets PFOA, PFHxS, PFNA, GenX (also called HFPO-DA), and PFBS.

Fluoride

This is one of the most searched questions.

RO removes up to about 92 percent of fluoride in drinking water.

Here is the key point. Carbon filters usually cannot make a fluoride claim at all. That is why NSF/ANSI 58 is the standard to shop for if fluoride is your reason for buying.

Nitrates and Farm Runoff

Nitrates come from fertilizer, manure, and septic systems. They matter most for well owners in farm areas.

Removal rates vary more for nitrates than for metals. One source lists 60 to 75 percent. Another lists 85 to 90 percent.

Why the gap? Nitrate is a small, single-charged ion. It is harder to reject than a heavy metal. Pressure, temperature, and membrane age all move the number. So treat nitrate removal as a range, not a fixed promise.

High nitrate levels are linked to methemoglobinemia in babies. People call it blue baby syndrome. If you have an infant and a private well, test first and pick a system with a listed nitrate claim.

Salts, Minerals, and Total Dissolved Solids

This is the membrane's core job.

Rates from published rejection data.

RO also cuts chloride and silica. Together, all of these make up TDS, or total dissolved solids. A good system drops TDS by 95 to 99 percent.

Radiological Contaminants

RO removes radium and uranium, plus gross alpha particles. These show up in some well water and some rock formations.

Radon is the exception. Because it's a gas, it passes through. More on that below.

Microplastics and Nanoplastics

RO blocks these easily. The size math is not close. Membrane pores are near 0.0001 micron. The smallest nanoplastics are far larger than that. So they get rejected along with dissolved solids.

Chlorine, Chloramine, and Disinfection Byproducts

Be clear about which stage does this work. The activated carbon pre-filter removes chlorine, not the membrane. Published tables list chlorine reduction near 98 percent.

Chloramine is harder. It needs catalytic carbon, a denser, more reactive carbon media.

Carbon also handles disinfection byproducts like trihalomethanes and haloacetic acids.

Cysts, Bacteria, and Viruses

Here you need caution. Do not trust marketing that says RO kills germs.

RO can physically remove microorganisms. But it is not recommended for that job. Only coliform-free water should be fed into the system. Bacteria growing in the housings can break down the membrane over time.

Most makers say the same thing. RO is not designed to remove bacteria and viruses. If bacteria get in, they keep growing in the pre-filters.

RO does hold back larger cysts and oocysts, plus asbestos fibers.

If you are on untreated well water, add UV disinfection after the RO. That combination is the right answer.

Pesticides, Herbicides, and Emerging Compounds

Published removal for pesticides can reach 99 percent.

RO handles select pesticides such as endrin, heptachlor, lindane, and pentachlorophenol. Common ones like atrazine show reductions near 95 percent. Simazine and glyphosate are also commonly listed.

Then there is the newer group. NSF/ANSI 401 covers 15 "emerging" compounds that are not federally regulated yet. The list includes drug traces like ibuprofen, naproxen, and estrone. It also includes atrazine, DEET, BPA, and TCEP.

What Reverse Osmosis Does Not Remove

This is the section most guides skip. It is also the most useful one.

RO units do not effectively remove most dissolved gases like carbon dioxide, methane, and radon. NDMA is a known weak spot because of its low molecular weight, and it is usually treated by advanced oxidation instead.

The iron and hardness point is worth repeating. This is not really a failure. It's the wrong tool for the job. RO is a point-of-use unit that sits under one sink. Iron and hardness are whole-house problems. Treat them upstream with an ion exchange softener or an iron filter. Your membrane will last much longer.

If you want near-total gas removal too, distillation is the other option. It boils water and catches the steam. It is slower and uses more energy, but it works differently.

How to Check the Claims: NSF/ANSI Certifications

Marketing claims are cheap. Third-party testing is not. NSF International and ANSI set the standards that separate the two.

Here is what each one covers.

NSF/ANSI 58 is the one built for RO. It certifies the whole system, not one cartridge. It requires a TDS reduction test. It then allows optional claims for arsenic V, fluoride, nitrate and nitrite, lead, chromium-6, and PFAS.

The standard sets a minimum of 75 percent TDS reduction. Most certified systems achieve 90-96 percent. NSF 58 also tests structural strength, recovery ratio, and material safety.

Now, the trap almost everyone falls into.

Certification is claim-specific. A filter certified for lead is not automatically certified for mercury or VOCs. One NSF 53 filter may cover lead and cysts but not VOCs. Another may do the reverse. Certification to one claim does not include the others.

So do this before you buy:

1. Write down the exact model number.
2. Look it up in the official NSF certification database.
3. Confirm your specific contaminant is named on that listing.

Also watch the wording. Phrases like "tested to NSF standards" or "NSF-grade" are not certification. They mean nothing on their own.

The Water Quality Association (WQA) also runs a Gold Seal program that tests to the same NSF/ANSI standards. Either mark is fine.

Frequently Asked Questions

Does reverse osmosis remove chlorine?

Yes, but the carbon pre-filter does that work, not the membrane. Published reduction is about 98 percent. The carbon stage is required, since chlorine damages RO membranes.

Does reverse osmosis remove fluoride?

Yes. RO removes up to about 92 percent of fluoride. Carbon-only filters usually cannot make this claim, so look for an NSF/ANSI 58 listing that names fluoride.

Does reverse osmosis remove PFAS?

Yes. Removal of PFOS runs about 90 to 99 percent. The EPA has described high-pressure membranes like RO as very effective against PFAS.

Does reverse osmosis remove lead?

Yes. Typical removal is around 98 percent. Check that lead is listed on the system's NSF 53 or NSF 58 certification, since claims are contaminant-specific.

Does reverse osmosis remove bacteria and viruses?

Not as a rated claim. RO can hold back microorganisms, but it is not recommended for that use. Feed only coliform-free water to the system. Add UV disinfection for untreated well water.

Does reverse osmosis remove microplastics?

Yes. Membrane pores near 0.0001 micron are far smaller than any plastic particle found in drinking water.

Does reverse osmosis remove hard water minerals?

It removes calcium and magnesium at 94-98 percent. But a whole-house softener should handle hardness first. Hard water fouls the membrane and shortens its life.

Does reverse osmosis remove arsenic?

It removes arsenic V at 92 to 96 percent. Arsenic III is removed less reliably and usually needs an oxidation step before the RO.

Does reverse osmosis remove nitrates from well water?

It reduces them. Published ranges run from 60 to 75 percent, up to 85 to 90 percent, depending on the source and conditions. Pick a system with a listed nitrate claim if this is your main concern.

Is reverse osmosis water safe to drink every day?

For most people, yes. The main debate is mineral content, since RO strips calcium and magnesium. A remineralization stage or a normal balanced diet handles that.

Does reverse osmosis remove 100 percent of contaminants?

No. RO does not guarantee full removal of any single contaminant. It rejects a percentage, and that percentage changes with the contaminant, the membrane, and operating conditions.

How often should I replace RO filters?

Sediment and carbon pre-filters usually need changing every 6 to 12 months. The membrane usually lasts 2 to 5 years. Skipping changes drops your removal rates, so follow the maker's schedule.

The Short Version

Reverse osmosis is the broadest treatment you can put under a kitchen sink. It removes 95 to 99 percent of dissolved solids, nearly all heavy metals, fluoride, nitrates, radiological contaminants, microplastics, and PFAS.

It is not magic. It misses dissolved gases. It leans on a carbon stage for chlorine. It is not a germ killer. And it works best when hardness and iron are handled upstream.

Test your water. Match the result to the right tool. Then check the exact model against the NSF database before you spend a dollar.