What Does Reverse Osmosis Remove? Full Contaminant Guide

What Does Reverse Osmosis Remove From Your Tap Water?

The EPA regulates about 90 contaminants in U.S. drinking water. Independent testing keeps finding thousands more that no federal rule covers. Pharmaceuticals, PFAS, microplastics, and industrial chemicals can all show up in your tap water without breaking a single regulation.

Reverse osmosis takes a different approach to that problem. It does not work off a regulation list at all. It blocks contaminants by physical size, catching almost anything larger than a water molecule. So what does reverse osmosis remove, exactly, and what slips through?

Those answers matter more than any marketing claim. Here is what the science actually shows.

Key Takeaways

Reverse Osmosis Removal Rates at a Glance

Reverse osmosis removes up to 99% of most dissolved contaminants from water, including lead, arsenic, chlorine, PFAS, bacteria, and hundreds more. The exact rate depends on the contaminant (fluoride and nitrate sit a little lower), but RO is one of the most effective residential water treatment methods available. The EPA notes that point-of-use RO systems can remove contaminants such as lead, VOCs, PFAS, arsenic, bacteria, and viruses.

Here is a high-level view of what RO handles:

*Chlorine and chloramine removal is primarily handled by carbon pre-filters in multi-stage RO systems.
**VOC removal varies by compound and system design. Carbon pre-filtration significantly improves VOC reduction.

The key to RO's reach is the membrane's pore size: about 0.0001 microns. Picture it this way. If a human hair were a highway, an RO membrane pore would be a crack in the sidewalk. Only water molecules are small enough to fit through.

How Reverse Osmosis Filtration Works

A reverse osmosis membrane works like a screen door at the molecular level. It lets water through and keeps contaminants out.

The process itself is simple. Pressurized tap water pushes against a semi-permeable membrane. The membrane's microscopic pores (about 0.0001 microns) allow water molecules to pass while blocking dissolved salts, metals, chemicals, and microorganisms. The rejected contaminants flush down the drain as wastewater. For a deeper look at the science, see our complete guide to reverse osmosis water filtration.

A single membrane on its own does not tell the whole story. The best-performing systems use multi-stage filtration, a series of specialized filters before and after the membrane, each handling a different job. Think of it like an assembly line where each worker handles one task:

• Pre-filters (sediment and carbon) remove large particles and chlorine that would otherwise damage the membrane
• The RO membrane does the heavy lifting: dissolved metals, salts, fluoride, and PFAS
• Post-filters (carbon polishing) catch anything remaining and improve taste

This is why a 12-stage system catches contaminants that a basic 3-stage setup can miss. More stages means more specialized treatment at every step. Crystal Quest's Thunder line, for example, combines an ultrafiltration (UF) membrane at 0.02 microns with an RO membrane at 0.0001 microns, plus Eagle Redox Alloy (ERA) media. ERA uses the same copper-zinc redox reaction as KDF, working like a tiny battery that neutralizes contaminants on contact, alongside ion exchange resin. That is 12 stages working together.

Heavy Metals Reverse Osmosis Removes

Lead

RO removes 95-99% of lead from drinking water. That matters because the EPA sets a lead action level of just 15 parts per billion (ppb), and there is no safe level of lead exposure, especially for children. Long-term exposure has been linked to developmental delays in children and kidney problems in adults.

Aging pipes, solder, and fixtures can leach lead into your water between the treatment plant and your faucet. An RO system installed at the point of use, right where you draw drinking water, catches it before it reaches your glass.

Arsenic

RO removes 94-99% of Arsenic V (the most common form in drinking water) and is one of the most effective residential methods for arsenic reduction. The EPA's maximum contaminant level (MCL) for arsenic is 10 ppb, and arsenic occurs naturally in groundwater across many regions of the U.S.

If you are on well water with elevated arsenic, a multi-stage RO system with a dedicated arsenic removal cartridge adds protection beyond the membrane alone. Our arsenic in drinking water guide covers testing and treatment options in detail.

Mercury, Chromium, Cadmium, and Iron

RO also reduces mercury, chromium, and cadmium, typically in the 90% and higher range for these dissolved heavy metals. The CDC confirms that reverse osmosis removes chemicals including lead, copper, and chromium. These metals can enter water supplies through industrial discharge, mining, and natural geological deposits, and all have been linked to serious health effects with prolonged exposure.

Iron is another common concern, especially for well water users who notice orange or brown staining. RO removes 90% or more of dissolved (ferrous) iron. Particulate iron, the visible kind that discolors water, gets caught by the sediment pre-filter before water ever reaches the membrane. If your well water runs high in iron, the multi-stage approach earns its keep: the pre-filter handles visible iron particles, and the membrane handles the dissolved iron that clear water still carries.

Does Reverse Osmosis Remove Fluoride?

Yes. RO removes 85-95% of fluoride from drinking water. That makes reverse osmosis one of the few effective residential methods for fluoride reduction. Most standard carbon-only filters do not remove fluoride at all.

People search for this more than almost any other RO question. The reasons vary. Some are concerned about dental fluorosis, especially in children. Others want control over their exposure levels, and some simply prefer unfluoridated water. Whatever your reason, RO gives you that choice.

The EPA sets a maximum contaminant level of 4 mg/L for fluoride and a secondary standard of 2 mg/L. If your water exceeds these levels, or if you want to reduce fluoride below them, an RO membrane is the most practical home solution.

For even greater fluoride reduction, Crystal Quest offers an optional fluoride removal cartridge that uses activated alumina to target fluoride specifically. Learn more in our guide on fluoride in tap water and how to filter it.

Does Reverse Osmosis Remove Bacteria and Viruses?

Yes. RO is highly effective at reducing bacteria, viruses, cysts, and parasites. RO membrane pores are about 0.0001 microns wide. Bacteria range from 0.2 to 5 microns. Most viruses range from 0.02 to 0.3 microns. The math is straightforward: these organisms are far too large to pass through.

The EPA lists bacteria and viruses among the contaminants point-of-use RO systems can remove. That covers common waterborne pathogens such as:

• Bacteria: E. coli, Salmonella, Campylobacter, Shigella
• Parasites: Giardia, Cryptosporidium
• Viruses: Hepatitis A, Rotavirus, Norovirus

Systems that pair an ultrafiltration (UF) membrane (0.02 microns) with the RO membrane add a second barrier for biological contaminants. Crystal Quest's Thunder 1000C under-sink system uses this dual-membrane design. UF catches bacteria and viruses early, and RO catches everything else downstream.

Thunder 1000C Under-Sink RO System

12 stages of filtration. UF and RO membranes remove bacteria, viruses, PFAS, lead, and hundreds more contaminants at your kitchen tap.

View Product →

Chemical Contaminants Reverse Osmosis Removes

Chlorine and Chloramine

Most municipal water systems use chlorine or chloramine as disinfectants. These chemicals serve an important purpose at the treatment plant, but you do not need them in your drinking glass.

Multi-stage RO systems typically include activated carbon pre-filters that remove 95-99% of free chlorine before water reaches the membrane. That pre-filter step also protects the membrane, since chlorine degrades RO membranes over time. Chloramine (a combination of chlorine and ammonia) is harder to remove, but systems with catalytic carbon or multiple carbon stages handle it well.

PFAS (Forever Chemicals)

PFAS, short for per- and polyfluoroalkyl substances, are synthetic chemicals found in thousands of everyday products. They earned the name "forever chemicals" because they do not break down naturally in the environment. Imagine glitter: once it is out there, it is everywhere and nearly impossible to clean up.

RO removes 90-99% of common PFAS compounds, including PFOA and PFOS. As the EPA continues to strengthen PFAS regulations, reverse osmosis remains one of the most effective point-of-use treatments available. For a deeper look, see our complete guide to PFAS filtration methods or read about how RO systems specifically remove PFAS and microplastics.

Pesticides, Herbicides, and Pharmaceuticals

Agricultural chemicals and prescription drug residues are increasingly detected in drinking water supplies across the country. RO membranes block most of these compounds, and carbon pre-filters add another layer of removal for organic chemicals.

Pharmaceutical residues, traces of antibiotics, hormones, and pain medications, are a growing concern. Concentrations are typically very low, but RO is one of the few residential technologies effective at reducing them.

VOCs and Disinfection Byproducts

Volatile organic compounds (VOCs) are chemicals that evaporate easily and can enter water from industrial solvents, fuel spills, and manufacturing waste. Disinfection byproducts (DBPs) like trihalomethanes (THMs) form when chlorine reacts with organic matter in water.

The most effective approach combines carbon pre-filtration with the RO membrane. Carbon is excellent at catching VOCs and THMs, and the membrane handles whatever the carbon does not. Multi-stage systems address both categories thoroughly. For a broader look at common contaminants, see our guide to 10 common tap water contaminants and how to remove them.

Dissolved Solids, Nitrates, and Minerals

RO typically reduces total dissolved solids (TDS) by 90-99%. TDS is a broad measurement of everything dissolved in your water. Dissolve a teaspoon of salt in a glass of water, and that salt is now a dissolved solid. High TDS can affect taste, leave residue on fixtures, and signal the presence of specific contaminants.

Nitrates are a particular concern for well water users and homes near agricultural areas. The EPA's MCL for nitrates is 10 mg/L because high nitrate levels are especially dangerous for infants. RO is one of the few effective home treatment options, reducing nitrates by roughly 85-95% depending on the membrane and water conditions. Crystal Quest also offers optional nitrate-specific cartridges for wells with elevated nitrate levels.

RO removes hardness minerals (calcium and magnesium) as well, which eliminates scale buildup on fixtures and appliances. Sediment particles like sand, rust, and silt are handled by the pre-filters before water ever reaches the membrane.

What Reverse Osmosis Does NOT Remove

RO does not effectively remove dissolved gases like carbon dioxide and radon. Radon in water is a dissolved gas, so it slips past the membrane. Reducing waterborne radon calls for aeration or a granular activated carbon system instead.

No single filtration technology removes everything. Proper system design, with the right combination of pre-filters, membrane, and post-filters, closes these gaps. This is also why third-party performance standards exist: NSF/ANSI Standard 58 is the industry benchmark for RO system performance, and it requires tested systems to demonstrate specific contaminant reduction claims under controlled conditions. Look for systems tested to this standard when you compare options.

Is Reverse Osmosis Water Safe to Drink?

Yes. RO water is safe to drink. The most common worry is mineral loss, since RO removes calcium and magnesium along with the contaminants. The reassuring part is that your body gets the vast majority of its essential minerals from food, not water.

Consider the scale. A glass of milk provides more calcium than dozens of glasses of tap water. A single banana carries more potassium than you would get from drinking water all day. The minerals in your tap water make up a small fraction of your daily intake.

That said, some people notice that RO water tastes flat compared to mineral-rich tap water. If that describes you, a remineralization post-filter adds back beneficial minerals like calcium and magnesium, giving you the purity of RO with improved taste. Many of Crystal Quest's RO systems offer optional remineralization cartridges for exactly this reason.

How to Choose the Right RO System for Your Home

Now that you know what reverse osmosis removes, the next question is which type of system fits your situation. Most homes choose by where they want filtered water and how much of it they use. If you want to pinpoint exactly which contaminants you are dealing with, a quick home water test helps you size and match the system. The CDC also recommends checking a system's label to confirm it removes the specific chemicals or germs you are concerned about.

Under-sink RO systems are the most popular choice for drinking water. They install beneath your kitchen sink, connect to a dedicated faucet, and filter water on demand. Crystal Quest's Thunder 1000C packs 12 stages of filtration, including both UF and RO membranes.

Countertop RO systems work for renters, apartments, or anyone who cannot modify their plumbing. Crystal Quest's countertop RO offers 10 stages and hooks directly to your faucet, no installation required.

Whole house RO systems filter every tap, shower, and appliance in your home. Crystal Quest's whole house RO ranges from 300 to 7,000 gallons per day, which suits homes with serious well water contamination.

When comparing systems, look for:

• Number of filtration stages, since more stages means broader contaminant coverage
• Systems tested to NSF/ANSI 58 standards for verified RO performance
• Flow rate and tank size matched to your household's daily water needs
• Filter replacement cost and schedule, a key part of long-term ownership
• Waste-to-pure water ratio, since better-engineered systems produce less wastewater

Crystal Quest has manufactured reverse osmosis systems in the USA for over 30 years, hand-assembling every system at its Georgia facility under ISO 9001 certified quality management, with the same engineering behind its commercial systems for hospitals, schools, and restaurants. Browse the full RO system collection to find the right fit, or check the reverse osmosis systems buyer's guide for a detailed comparison.

Need help choosing? Crystal Quest water specialists can match a system to your water test results, your household size, and the contaminants you care most about.

Frequently Asked Questions About What Reverse Osmosis Removes

Does reverse osmosis remove all contaminants?

No. RO removes up to 99% of most dissolved contaminants, but it does not effectively remove dissolved gases like carbon dioxide, and it strips beneficial minerals. A well-designed multi-stage system with carbon pre-filters covers the gaps the membrane alone leaves behind.

What doesn't reverse osmosis remove?

RO does not effectively remove dissolved gases like carbon dioxide and radon, which slip past the membrane and need aeration or activated carbon instead. Certain VOCs at very low concentrations can also pass through without carbon pre-filtration. Beneficial minerals like calcium and magnesium are removed as well.

Does reverse osmosis remove minerals from water?

Yes. RO membranes remove calcium, magnesium, and other dissolved minerals along with harmful contaminants. This is a normal part of how the technology works. If you prefer mineral-rich water, a remineralization post-filter restores beneficial minerals and improves taste.

Does reverse osmosis remove chlorine and chloramine?

Yes. In multi-stage RO systems, activated carbon pre-filters remove the majority of chlorine and chloramine before water reaches the membrane. That dual benefit protects the membrane from chlorine damage and gives you better-tasting water.

Does reverse osmosis remove lead from drinking water?

Yes, RO removes 95-99% of lead. Since there is no safe level of lead exposure, an RO system at the point of use is one of the most effective ways to protect your family, especially if you have older pipes or fixtures that may contain lead solder.

Does reverse osmosis remove PFAS and forever chemicals?

Yes, RO removes 90-99% of common PFAS compounds including PFOA and PFOS. As the EPA continues tightening PFAS regulations, reverse osmosis remains one of the most reliable residential treatment methods.

Is reverse osmosis water safe to drink?

Yes. While RO removes minerals, your body gets the essential minerals it needs primarily from food, not water. If you prefer mineral-rich water, adding a remineralization filter restores calcium and magnesium.

Does reverse osmosis remove viruses and parasites?

Yes. RO membrane pores (about 0.0001 microns) are far smaller than viruses (0.02-0.3 microns) and parasites like Giardia (8-12 microns) and Cryptosporidium (4-6 microns). Systems that combine a UF membrane with the RO membrane add a second barrier. RO is not certified as a microbiological purifier, though, so water known to be unsafe should be disinfected first.