What Does a Water Filter Remove?

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Quick Verdict: What a water filter removes depends entirely on its technology and NSF/ANSI certification — not on marketing claims or price. This guide covers the specific contaminants each filter type addresses, the NSF standards that verify those claims, and the contaminants commonly missed. For curated filter recommendations, see our Best Water Filters guide.

The Key Rule: NSF Certification Tells You What a Filter Actually Removes

Every reputable water filter manufacturer claims their product “removes contaminants.” The only way to verify those claims is NSF/ANSI certification. NSF International independently tests filter products against rigorous standards — a filter certified to NSF/ANSI 53 for lead, for example, has been tested to demonstrate it actually reduces lead to below the standard’s defined threshold, not just that the manufacturer believes it might.

When evaluating any filter, look at:

1. Which NSF/ANSI standard(s) it is certified to
2. For what specific contaminants under that standard — certifications list individual contaminants, not just the standard number

NSF/ANSI Standards and What They Cover

What Activated Carbon Filters Remove

Carbon is the most common filter media. Its massive surface area (1,000+ square meters per gram) adsorbs organic molecules and certain inorganic contaminants. NSF 42-certified carbon filters reliably address:

• Chlorine — the most common treatment chemical in municipal water, responsible for the characteristic “swimming pool” taste
• Chloramine — an alternative disinfectant used by many utilities, harder to remove than free chlorine; requires specific carbon media (catalytic carbon)
• Chlorine disinfection byproducts — trihalomethanes (THMs) and haloacetic acids (HAAs) that form when chlorine reacts with organic matter; covered under NSF 53
• Taste and odor compounds — geosmin (earthy/musty smell), 2-methylisoborneol (MIB), hydrogen sulfide (rotten egg smell)
• Particulate matter — sediment, rust, and fine particles (effectiveness depends on block vs. granular carbon and micron rating)

NSF 53-certified carbon block filters additionally address:

• Lead — via specific blended carbon-ion exchange or activated alumina media; the NSF 53 certification is verified for lead at 200 µg/L challenge concentration
• Volatile organic compounds (VOCs) — benzene, toluene, ethylbenzene, xylene, chloroform, carbon tetrachloride, and others depending on the specific certification
• MTBE — a fuel additive found in water near gasoline spills and underground storage tanks
• Cryptosporidium and Giardia cysts — these parasitic cysts are large enough to be mechanically filtered by sub-micron carbon block filters (typically rated 0.5 micron or finer)
• Selected PFAS compounds — some NSF 53-certified carbon systems now include PFOA and PFOS reduction claims, but verify the specific PFAS compounds in the certification scope
• Asbestos fibers — physically filtered by fine-pore carbon block media
• Turbidity — suspended solids that make water appear cloudy

What Carbon Filters Do NOT Remove

• Nitrates and nitrites (health risk for infants — “blue baby syndrome”)
• Fluoride
• Heavy metals not in the specific certification (arsenic, chromium unless explicitly certified)
• Total dissolved solids (TDS) or hardness minerals
• Bacteria, viruses, or most pathogens (carbon is not a disinfectant)
• Dissolved salts or minerals
• Radium or uranium

What Reverse Osmosis Systems Remove

RO provides the broadest contaminant removal of any consumer filter technology. The semi-permeable membrane physically blocks molecules too large to pass through its sub-nanometer pores. NSF/ANSI 58-certified RO systems reduce:

• Total dissolved solids (TDS) — 90–99% reduction typically, including all dissolved minerals and salts
• Nitrates and nitrites — critical for households with infants or pregnant women
• Fluoride — typically 85–96% reduction
• Lead, arsenic, barium, chromium, selenium, radium — heavy metals and metalloids
• PFAS (PFOA, PFOS, and related compounds) — among the most effective consumer treatments for PFAS
• Bacteria, viruses, and cysts — the 0.0001-micron membrane pore size physically excludes biological organisms
• Pharmaceuticals and hormones — the molecular size of most pharmaceutical compounds exceeds the membrane pore size
• Chlorine and VOCs — via the carbon pre-filter stage included in every multi-stage RO system

What RO Does NOT Remove (or Reduces Rather Than Eliminates)

• Some dissolved gases: Carbon dioxide and hydrogen sulfide can partially pass through RO membranes
• Certain VOCs: Small-molecule organic compounds can have higher membrane permeability — the carbon pre-filter is the primary barrier for VOCs in RO systems
• All minerals: RO removes beneficial calcium and magnesium along with harmful contaminants. Some RO systems include remineralization stages; most buyers find mineral intake from food adequate

What UV Systems Remove

UV systems are a disinfection technology, not a filtration technology — they destroy the DNA of living organisms (bacteria, viruses, protozoa) but remove nothing from the water chemically or physically. UV alone does not reduce lead, nitrates, PFAS, chlorine, or any chemical contaminant. It is best used as a final-stage addition to a carbon or RO system for comprehensive protection, particularly for well water where biological contamination is a concern.

Contaminant Quick-Reference Table

PFAS: A Closer Look

PFAS (per- and polyfluoroalkyl substances) are an emerging concern with new EPA standards finalized in 2024 setting limits for PFOA and PFOS at 4 parts per trillion. The two most effective consumer filter options for PFAS:

• NSF/ANSI 58 certified RO: Consistently high reduction rates (typically 90%+) across a broad range of PFAS compounds due to the membrane’s molecular exclusion.
• NSF/ANSI 53 certified granular activated carbon (GAC) or carbon block: Effective for larger PFAS molecules like PFOA and PFOS; effectiveness varies for shorter-chain PFAS (PFBS, GenX). Some dedicated PFAS-targeting filters using ion exchange resin achieve very high removal rates.

Standard NSF 42-certified pitcher filters do not make certified PFAS reduction claims. If PFAS is your primary concern, verify the specific certification language on the filter model you’re considering.

Frequently Asked Questions

Does a Brita filter remove lead?

The standard Brita pitcher filter (Brita Standard) is certified only to NSF/ANSI 42 and does not have a certified lead reduction claim. The Brita Elite (formerly Brita Longlast) filter is certified to NSF/ANSI 53 for lead reduction. PUR PLUS filters are also NSF 53 certified for lead. Always check the specific filter model’s certification, not just the brand.

Does filtering water remove beneficial minerals?

Activated carbon filters do not significantly remove calcium, magnesium, or other dissolved minerals — they are mineral-neutral. Reverse osmosis systems do remove minerals along with contaminants. The health significance is minimal for most people since dietary mineral intake from food vastly exceeds mineral intake from water. If mineral content is a concern, RO systems with a remineralization post-filter stage are available.

Can any filter remove all contaminants?

No single filter type removes every possible contaminant. The most comprehensive consumer solution is a multi-stage RO system (sediment + carbon pre-filter + RO membrane + carbon post-filter), which addresses chemical, heavy metal, biological, and particulate contaminants. Even RO has limitations with certain dissolved gases. Know what’s in your water first — see How to Test Your Tap Water — and match the filter to your specific contaminants.

Do water filters remove viruses?

Carbon block filters (including RO pre-filters) and most ceramic filters do not remove viruses — their pore sizes are too large. NSF/ANSI 58-certified RO membranes physically block viruses. UV purification (NSF/ANSI 55) kills viruses but doesn’t remove them. For virus protection without RO, UV after a micron pre-filter is the standard approach.

For our recommended filters across every category and budget, see the Best Water Filters guide. To understand the decision process, see How to Choose a Water Filter.