Water Filters Compared: What Reverse Osmosis, Carbon, and Ion Exchange Actually Remove

Walk into any store and the water-filter aisle implies they're interchangeable — a wall of pitchers and faucet attachments all promising "cleaner water." They are not interchangeable. A filter is a specific technology that removes specific contaminants, and the gap between "filters water" and "removes the thing that's actually in my water" is exactly where people waste money and, occasionally, keep drinking something they think they've handled.

So this piece skips the brand comparisons and does the thing that actually matters: it maps the technologies to the contaminants. Match those two correctly and the brand barely matters.

The contaminant-by-filter matrix

This is the whole article in one table. Rows are the contaminants covered across the tap-water Atlas; columns are the four main point-of-use technologies. "Certified" means it works if the specific product carries the relevant NSF/ANSI certification — not every carbon filter is created equal.

*Distillation can miss volatile organics like THMs unless the unit is vented for them (they evaporate and carry over). †RO membranes reduce microbes but are not a certified disinfection method — don't rely on RO alone for microbiologically unsafe water. Inferred — compiled from the established removal mechanisms and NSF/ANSI certification scopes below

Activated carbon — the common one, and its blind spot

Activated carbon is the technology in most pitchers, faucet filters, and refrigerator filters. It works by adsorption — organic molecules and chlorine stick to the vast surface area of the carbon as water passes through. That makes it genuinely excellent at what it does: removing chlorine and the taste and odor that come with it, and capturing trihalomethanes, other volatile organic compounds, and many pesticides. A carbon block filter specifically certified to NSF/ANSI 53 also reduces lead. Industry

Here is the blind spot that matters: carbon does not remove dissolved inorganic ions. Nitrate, arsenic, and hexavalent chromium pass straight through a standard carbon filter. If you're on a private well with a nitrate or arsenic problem, a carbon pitcher gives you cleaner-tasting water and a false sense of security. This single distinction — carbon handles organics and chlorine, not dissolved metals and anions — resolves most of the confusion in the whole category. Inferred from the adsorption mechanism and NSF certification scopes

Reverse osmosis — the all-rounder

Reverse osmosis (RO) pushes water through a semi-permeable membrane whose pores are small enough to block almost everything larger than a water molecule. That's why it's the most comprehensive point-of-use option: it removes lead, arsenic, nitrate, chromium-6, PFAS, fluoride, and dissolved solids in one system, and most RO units pair the membrane with carbon stages that also handle chlorine and organics. It's certified under NSF/ANSI 58, and the EPA lists RO among the effective technologies for both PFAS and the inorganic contaminants. Regulatory

The trade-offs are real, which is why RO isn't automatically the answer. It's installed under the sink with a storage tank, it's slower than a faucet filter, and it wastes some water to drain in the process. It also strips the beneficial minerals along with everything else (some units re-mineralize). If your only issue is chlorine taste, RO is overkill; if you have a nitrate or arsenic exceedance, it's the straightforward fix. Inferred from RO membrane rejection characteristics and NSF 58 scope

Ion exchange and water softeners

Ion exchange swaps unwanted ions in the water for harmless ones held on a resin. The version most homes know is the water softener — a cation-exchange resin that trades hardness minerals (calcium and magnesium) for sodium. Crucially, a standard softener addresses hardness, not health contaminants; it won't touch nitrate or arsenic. Industry

The health-relevant version is anion exchange, which targets negatively-charged contaminants: it's a recognized treatment for nitrate, hexavalent chromium, and PFAS. Specific adsorptive media fill the gaps — activated alumina for fluoride and arsenic, for instance. Ion exchange shines when you have one specific ion to remove and don't want the whole-water treatment of RO. Regulatory

Distillation and specialty media

Distillation boils water and condenses the steam, leaving dissolved solids — metals, nitrate, arsenic, salts — behind in the boiling chamber. It's extremely thorough for those, and it kills microbes. Its one gap is volatile organic compounds: THMs and similar can evaporate with the water and carry over into the distillate unless the unit vents them off. It's also slow and energy-intensive, which is why it's more common for specific needs than as a whole-home solution. Inferred from distillation's non-volatile separation mechanism

Beyond these, targeted media exist for specific problems — activated alumina and iron-oxide media for arsenic and fluoride, KDF for some metals — and are usually combined with carbon or RO in multi-stage systems. The theme is the same: each medium has a defined job.

Read the certification, not the marketing

"Filters up to 99% of contaminants" is a marketing sentence, not a specification. What makes a claim real is third-party certification to the relevant NSF/ANSI standard, which tells you exactly what the unit was tested to reduce: Industry

• NSF/ANSI 42 — aesthetic effects (chlorine, taste, odor). Not a health claim.
• NSF/ANSI 53 — health effects (lead, certain VOCs, cysts, and specific named contaminants). Check which contaminants.
• NSF/ANSI 58 — reverse osmosis systems.
• NSF/ANSI 44 — cation-exchange water softeners.
• NSF/ANSI 401 — "emerging" contaminants (some pharmaceuticals, PFOA/PFOS-adjacent).
• NSF/ANSI P473 — specifically PFOA and PFOS reduction.

The practical move: identify the contaminant you need to remove, then look for the filter certified to reduce that specific contaminant — not just a filter with a certification logo. A unit certified to NSF 42 for chlorine tells you nothing about its lead or PFAS performance.

How to choose for your water

The whole decision collapses to three steps:

1. Find out what's in your water. On a public system, read the annual Consumer Confidence Report or look your address up in a database like the EWG Tap Water Database. On a private well, get a certified lab test — nobody tests it for you. Start with the tap-water hub for what to look for.
2. Match the technology to the contaminant using the matrix above. Chlorine taste only? Carbon. Nitrate or arsenic? RO or the right ion-exchange/adsorptive media. A mix, or you want thorough? RO. Just PFAS? RO, certified carbon block, or anion exchange.
3. Verify the certification covers your specific contaminant, and commit to replacing the filter on schedule — a spent filter is worse than none, because it can release captured contaminants back into the water.

Do that and you've skipped the two failure modes that define this category: buying a filter that doesn't remove your actual problem, and trusting a filter long after it stopped working. For the individual contaminants and the health context behind each, the tap-water Atlas covers them one by one.

---

Embr is an independent, evidence-tiered guide to the chemistry of the home. We have no affiliate relationships and no financial stake in what you buy — this piece names technologies and certifications, not brands, on purpose.