Filter certifications
Water Filter Certifications: NSF/ANSI Standards Explained
For lead or PFAS, look for NSF/ANSI 53 or 58, then check the listing names what you want to reduce. A standard number alone doesn’t say what was tested.
Which certification to look for
Start from what you want to reduce, then check that the product’s listing names it.
| You want to reduce | Look for | Check in the listing |
|---|---|---|
| Chlorine taste and odor | NSF/ANSI 42 | Covers taste and odor. It does not certify lead or PFAS reduction. |
| Lead | NSF/ANSI 53, or 58 for reverse osmosis | Lead is named as a reduction claim. |
| PFAS | NSF/ANSI 53 or 58 | The compound in your result is named, such as PFOA or PFOS. Reverse osmosis systems often carry this claim under NSF/ANSI 53. EPA's home-filter guidance, as of April 2024, says certifications focus on PFOA and PFOS and do not yet show a filter reaches EPA's 2024 limits, though reducing PFAS still limits your exposure. Some listings show a combined Total PFAS claim, which doesn't establish any one compound, so check that the listing names the compound in your result. A listing may not cover PFHxS, PFNA or GenX. |
| Arsenic or nitrate | NSF/ANSI 58 or 53 | The contaminant is named. Most certifications cover arsenic V; arsenic III, found in some wells, is reduced less well unless it’s oxidized first. |
| Pharmaceuticals and other emerging compounds | NSF/ANSI 401 | The compounds you care about are among those verified for that model. |
| Hardness | NSF/ANSI 44 | Softeners only. Hardness is not a health contaminant. |
| Germs in untreated well water | NSF/ANSI 55 Class A | Class A, not Class B. Cloudy water, sediment and iron can shield germs, so most systems need a pre-filter. |
How to check a filter’s certification
- Find the exact modelCopy the model number and the certifier’s mark from the box or the performance data sheet.
- Look it up in the directorySearch that certifier’s product directory. “Tested to NSF standards” on a label is not the same as a listing.
- Confirm your contaminant is namedA standard number alone doesn’t tell you what was tested. The listing has to name what you want to reduce.
EPA lists five ANSI-accredited certifiers, each with a public product directory:
Every standard on this site
- NSF/ANSI 42Drinking Water Treatment Units — Aesthetic EffectsCovers point-of-use and point-of-entry systems designed to reduce non-health-related contaminants such as chlorine, taste, odor, and particulate matter.Standard 42 verifies taste and smell improvements; it does not certify removal of toxic chemicals, heavy metals, or PFAS.Activated Carbon FiltrationMechanical Sediment Filtration
- NSF/ANSI 44Residential Cation Exchange Water SoftenersCation Exchange Water Softening
- NSF/ANSI 53Drinking Water Treatment Units — Health EffectsEstablishes minimum requirements for materials, structural integrity, and verified reduction claims for specific contaminants with regulated health advisory limits (e.g., lead, PFOA/PFOS, VOCs, arsenic).Always check the official manufacturer performance data sheet for the exact chemical compounds certified. Holding standard 53 does NOT mean a filter removes every health contaminant.Activated Carbon FiltrationIon Exchange
- NSF/ANSI 55Ultraviolet Microbiological Water Treatment SystemsSpecifies ultraviolet light treatment systems. Class A systems deliver a minimum UV dose of 40 mJ/cm² to inactivate pathogenic bacteria, viruses, and Cryptosporidium. Class B systems deliver 16 mJ/cm² for supplemental bactericidal treatment of safe water.Ensure Class A certification if treating untreated well water with potential microbial pathogens.Ultraviolet (UV) Disinfection
- NSF/ANSI 58Reverse Osmosis Drinking Water Treatment SystemsThe benchmark standard for residential point-of-use reverse osmosis units. Evaluates membrane TDS reduction, recovery rating, structural burst pressure, and individual chemical reduction claims.Verify that the specific claim (e.g., PFOA/PFOS or arsenic) is explicitly tested, as standard 58 allows optional claim certifications.Reverse Osmosis
- NSF/ANSI 61Drinking Water System ComponentsIon Exchange
- NSF/ANSI 401Emerging Compounds and Incidental ContaminantsEvaluates the reduction of up to 15 emerging compounds, including prescription pharmaceuticals (e.g., atenolol, ibuprofen), over-the-counter drugs, flame retardants (TCEP), and industrial detergents (nonylphenol).Check the specific listing to confirm which of the 15 candidate emerging substances were verified for the product model.Activated Carbon Filtration
A standard names what was tested. Check that the product’s own listing names the contaminant you need.
Order of operations
Finding first, method second, product last.
A method only matters once your water system’s record shows what needs reducing. Then check that a product’s certification names it.