A water filter’s certification number tells you which category of claim was tested, and the categories are not interchangeable. NSF/ANSI 42 is aesthetics. NSF/ANSI 53 is health effects. NSF/ANSI 401 is emerging compounds. A filter certified only to 42 has not been tested for lead, and nothing about a 42 certification implies it removes anything harmful.
This is the most consequential misreading in the category, because the certifications look equivalent on packaging and are not.
The three you will see most
NSF/ANSI 42 — Aesthetic Effects
Sets minimum requirements for systems designed to reduce non-health-related contaminants. The claims available under it include chlorine, chloramine, taste and odor, particulate, iron, manganese, zinc and total dissolved solids. It covers both point-of-use and point-of-entry systems.
These are real improvements — chlorine taste is the single most common complaint about tap water, and a 42-certified carbon filter fixes it. But every claim in this standard is about how the water looks, tastes and smells.
NSF/ANSI 53 — Health Effects
Sets minimum requirements for systems designed to reduce specific health-related contaminants, using health limits set by the EPA and Health Canada. It offers over fifty contaminant reduction claims, of which lead, Cryptosporidium, volatile organic compounds and chromium are among the most commonly sought.
The critical point about 53: it is claim-specific, not blanket. A filter is certified under 53 for the particular contaminants it was tested against, not for the standard as a whole. “NSF/ANSI 53 certified” with no contaminant named is an incomplete statement. A filter certified to 53 for chlorine-resistant cysts has not necessarily been tested for lead, and a filter certified to 53 for lead has not necessarily been tested for VOCs.
Always read the claim list, not the standard number.
NSF/ANSI 401 — Emerging Compounds and Incidental Contaminants
Addresses the reduction of up to 15 individual contaminants that published studies have found in drinking water at trace levels — compounds not yet regulated by the EPA or Health Canada. The most common claim categories are prescription drugs, over-the-counter medications, herbicides, pesticides and chemical compounds.
401 sits between the other two. The compounds it covers are present at trace levels and are not currently regulated, so it is not a health-effects standard in the same sense as 53 — but it is not an aesthetic standard either.
What all three have in common
Every one of the three also requires material safety testing, under NSF/ANSI/CAN 372, and structural integrity testing. So a certification under any of them means the housing will not leach something unpleasant into your water and will not fail under pressure — which is worth knowing when comparing against an uncertified product.
The rest of the family
| Standard | Covers |
|---|---|
| NSF/ANSI 42 | Aesthetic effects — chlorine, chloramine, taste, odor, particulate, iron, manganese, zinc, TDS |
| NSF/ANSI 44 | Water softeners using ion-exchange resin, replacing calcium and magnesium with sodium or potassium |
| NSF/ANSI 53 | Health effects — 50+ contaminant claims against EPA and Health Canada health limits |
| NSF/ANSI 55 | Ultraviolet systems. Class A inactivates bacteria, viruses and cysts in contaminated water; Class B reduces non-disease-causing bacteria in already disinfected water |
| NSF/ANSI 58 | Reverse osmosis systems, reducing contaminants regulated by the EPA and Health Canada |
| NSF/ANSI 62 | Distillation systems. Removes heavy metals; note that some volatile organic chemicals can pass through with the vapor |
| NSF/ANSI 177 | Shower filters — reduces free available chlorine only |
| NSF/ANSI 244 | Filters for supplies already treated or considered microbiologically safe, guarding against intermittent microbiological contamination; claims may cover bacteria, viruses and cysts |
| NSF/ANSI 401 | Emerging contaminants — up to 15 compounds not yet regulated |
| NSF P231 | Microbiological water purifiers, certified against EPA 1987 task force recommendations |
Two entries deserve a second look. NSF/ANSI 177 is narrow by design: a shower filter certified under it is certified for free available chlorine and nothing else, which is worth knowing before paying for one on broader grounds. And NSF/ANSI 62 carries an explicit caveat — distillation is excellent on heavy metals and can let some volatile organics through with the steam, which is the opposite of the usual assumption that distillation removes everything.
PFAS: which standards carry the claim
PFOA and PFOS reduction claims live in NSF/ANSI 53, for devices using activated carbon or anion exchange resins, and NSF/ANSI 58, for reverse osmosis systems. Both absorbed the claims that originally sat in NSF Protocol P473.
The numbers have moved as the science and the regulations have:
- In 2016 the EPA’s health advisory limit for combined PFOA and PFOS was 70 parts per trillion.
- Updates in 2022 lowered the combined PFAS limit used in the NSF standards to 20 ppt, and added PFHpA, PFHxS, PFNA and PFBS to a “Total PFAS” reduction claim.
- The EPA’s April 2024 rule set maximum contaminant levels of 4.0 ppt for PFOA and PFOS, with 10 ppt for PFHxS, PFNA and HFPO-DA (GenX), plus a hazard index for mixtures.
- NSF committees have been working to align Standards 53 and 58 with the EPA rule; as of our last check that alignment was not complete.
The regulatory picture is also still moving. On 18 May 2026 the EPA proposed rescinding the MCLs for PFHxS, PFNA, HFPO-DA and the Hazard Index, and formally extending the PFOA and PFOS compliance deadline to 2031. Neither proposal had been finalized when we last checked this in October 2026, and the 4.0 ppt MCLs for PFOA and PFOS remain in force.
For a buyer, the practical consequence is unchanged: if PFAS is your reason for filtering, look for a named PFOA/PFOS or Total PFAS reduction claim under 53 or 58 and check the reduction level stated on the performance data sheet. The regulatory threshold may move; the filter’s tested claim is what you are buying.
How to verify a claim rather than trust the box
- Find the performance data sheet. Certified systems come with one, listing each contaminant, the challenge concentration used, and the reduction achieved. If a manufacturer will not produce it, that is the answer.
- Check the certifier’s own listing rather than the retailer’s description. Certification bodies publish searchable databases of certified products and the specific claims each holds.
- Read the words around the number. “Tested to NSF/ANSI 53 standards” is not “certified to NSF/ANSI 53.” “Meets the requirements of” is a self-assertion. Certification is performed by a third party and is listed.
- Match the claim to your water. A lead claim is irrelevant if your service line is plastic and your utility reports no lead; a cyst claim matters on a private well and much less on a treated municipal supply.
The short version
Certified to 42 only: your water will taste better. Certified to 53 for a named contaminant: that contaminant was tested, at a stated reduction. Certified to 401: some trace unregulated compounds were tested. Nothing else is implied by any of them, and the contaminant list is the specification — not the standard number printed on the front.
Related guides
- How to choose a water filter, starting from your own water
- Pitcher, faucet, under-sink or RO: choosing by contaminant
- Reverse osmosis: what NSF/ANSI 58 covers
Sources: NSF, NSF/ANSI 42, 53 and 401: Filtration Systems Standards, for the scope of each standard, the claim categories and the material safety and structural integrity requirements; NSF, Standards for Water Treatment Systems, for the wider standards table; NSF, Forever Chemicals and the Advancement of Filtration Standards, for the PFAS claim history and limits; Harvard EELP PFAS in Drinking Water regulatory tracker for the status of the May 2026 proposals. Regulatory status last checked October 2026.