NSF/ANSI water filter standards explained: what 42, 53, 58, 401, 55, 44, 177, 372 and 61 each certify, and how to check a listing
NSF/ANSI 42 is taste, 53 is health contaminants, 58 is RO at 75% TDS reduction, 55 is UV at 40 mJ/cm². Only 'certified' means a listing exists.
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Short answer
Each NSF/ANSI water filter standard certifies one job, and the number on the box tells you which. NSF/ANSI 42 covers aesthetic effects such as chlorine taste and odor; NSF/ANSI 53 covers health-effect contaminants such as lead, cysts and VOCs; NSF/ANSI 58 covers reverse osmosis and, per NSF’s standard 58 page, requires at least 75% reduction of total dissolved solids; NSF/ANSI 401 covers up to 15 emerging compounds such as pharmaceuticals; NSF/ANSI 55 covers UV disinfection, Class A at 40 mJ/cm² and Class B at 16 mJ/cm²; NSF/ANSI 44 covers cation-exchange softeners; NSF/ANSI 177 covers shower filters against a 2.0 mg/L free chlorine challenge; NSF/ANSI/CAN 372 covers lead content in the fitting itself, at most 0.25% by weighted average; and NSF/ANSI 61 covers wetted materials, not performance. Only the word certified means an accredited third party listed that model. Tested to and meets are the maker’s own claim, so check the model number in the NSF, WQA or IAPMO database before you trust the box.
If the box says “tested to NSF standards” with no certifier’s mark and no database listing, you can stop reading here: that phrase is marketing copy, and the table below will not make it anything else.
Which standard covers which claim?
An NSF/ANSI standard is a published test protocol: challenge water at a set concentration, a set flow and pressure, and a minimum reduction the product must hold across its rated capacity. The index below is NSF’s own grouping, read from its knowledge library and the ANSI blog summaries on 2026-09-14.
| Standard | What it certifies | Typical claim on the box | Product class |
|---|---|---|---|
| NSF/ANSI 42 | Aesthetic effects: chlorine taste and odor, particulate | “Reduces chlorine taste and odor” | Pitcher, faucet, under-sink and whole-house carbon filters |
| NSF/ANSI 53 | Health-effect contaminants: lead, cysts, VOCs, and in later revisions PFOA and PFOS | “Reduces lead”, “Reduces PFOA/PFOS” | Pitcher, faucet, under-sink, whole-house carbon and adsorptive filters |
| NSF/ANSI 58 | Reverse osmosis systems: minimum 75% TDS reduction from a 750 mg/L challenge at 50 psig over a 7-day protocol | “Reduces TDS”, “Reduces arsenic”, “Reduces fluoride” | Under-sink and countertop RO |
| NSF/ANSI 401 | Up to 15 emerging compounds (drugs, some pesticides, flame retardants, detergents) at a minimum 95% reduction | “Reduces pharmaceuticals and pesticides” | Carbon and RO systems |
| NSF/ANSI 55 | UV disinfection: Class A at 40 mJ/cm² with sensor and alarm, Class B at 16 mJ/cm² | “Class A disinfection”, “Class B supplemental” | UV units |
| NSF/ANSI 44 | Residential cation-exchange softeners regenerated with sodium or potassium chloride | “Reduces hardness” | Water softeners |
| NSF/ANSI 177 | Shower filters: a 2.0 mg/L free available chlorine challenge reduced by at least 50% through rated life | “Reduces free available chlorine” | Shower filters |
| NSF/ANSI/CAN 372 | Lead content of wetted parts: 0.25% weighted average maximum, 0.2% for solder and flux | “Lead free” | Faucets, valves, fittings |
| NSF/ANSI 61 | Health effects of materials that touch drinking water, no performance claim | Materials-safety mark only | Housings, valves, gaskets, any wetted component |
Two of these trip readers most often. NSF/ANSI 61 is a materials standard, so a 61 mark on a filter housing says the plastic is acceptable in contact with water and nothing about what the cartridge removes. NSF/ANSI 401 is not a PFAS standard: NSF’s 42, 53 and 401 page puts PFOA and PFOS under 53 for carbon and anion exchange, and under 58 for reverse osmosis.
What is the difference between certified, tested to and meets?
The three phrases are not grades of the same thing. NSF’s page on being certified by NSF versus certified to NSF draws the line at who did the work and whether anyone can look it up.
| Phrase on the listing | Who verified it | Is the model in a database? | What it is worth |
|---|---|---|---|
| “Certified to NSF/ANSI 53” with an NSF, WQA Gold Seal or IAPMO R&T mark | An accredited certifier bought the retail product, ran the full protocol across rated capacity, and audits the factory on a schedule | Yes, by brand and model | The only phrase that carries a third-party claim |
| “Tested to NSF/ANSI 53” or “Tested by an independent lab to NSF standards” | Whoever the maker hired, on whatever sample the maker sent, for however many gallons the maker paid for | No | A maker’s claim about a test nobody outside the company can see |
| “Meets NSF/ANSI 53” or “Meets NSF standards” | The maker, by its own reading of the standard | No | A maker’s claim with no test named at all |
Certification also covers a specific model line, not a brand. A listing for one pitcher does not certify the brand’s other pitcher, and a listing for a system does not always cover every replacement cartridge sold for it.
How do you check a listing?
Search the certifier’s database, not the maker’s site, and match the model number stamped on your unit to the model number in the listing. The brand name alone proves nothing.
| Certifier | Where to search | How the search works |
|---|---|---|
| NSF | info.nsf.org/Certified/DWTU/Listings.asp | Search by company, brand or model, or by reduction claim. The claim search takes a URL of the form Listings.asp?ProductFunction=058%7CFluoride+Reduction&submit2=SEARCH, where the 3-digit code is the standard and the text after the bar is the claim |
| WQA Gold Seal | find.wqa.org/find-products | Filters for manufacturer, brand or model, product category, common contaminant and product standard; WQA does not publish a query-string pattern for the result page |
| IAPMO R&T | pld.iapmo.org | IAPMO’s help text says the directory searches by listee name, product description, standard, file number, brand or listed model; no query-string pattern is published, so search from the front page |
The NSF claim search is the fastest route when you know the contaminant: the fluoride guide on this site links straight into the 058, fluoride reduction listing, and the same pattern works for any 3-digit standard code and claim name.
Before you buy anything
Do not pay for a certification mark until you have opened the listing yourself. The listing costs nothing and takes about two minutes, and it decides whether “certified” on the box means a third-party protocol or a design choice by the marketing team. If the model number on the listing does not match the model number on the unit, the mark does not apply to what you are holding.
Which standard do you need?
Start from the problem, not the standard, then confirm the mark against the database.
| Your problem | Look for | Where this site goes deeper |
|---|---|---|
| Chlorine taste or smell | NSF/ANSI 42 | Chlorine and chloramine in city water |
| Lead from a service line or old fittings | NSF/ANSI 53 for the filter, 372 for the fitting | Lead in tap water |
| PFAS on a test report | NSF/ANSI 53 for carbon, 58 for RO, never 401 | PFAS in tap water |
| Fluoride or arsenic | NSF/ANSI 58 | Fluoride in tap water, arsenic in well water |
| Coliform or a well needing disinfection | NSF/ANSI 55 Class A, not Class B | UV purifier size calculator |
| Hard water | NSF/ANSI 44 | Softener size calculator |
| Chlorine at the shower head only | NSF/ANSI 177 | Shower head filters |
| Pharmaceuticals or pesticides | NSF/ANSI 401 | The NSF 401 listing, by claim name |
Two questions this page hands off rather than answers: which contaminants a reverse osmosis membrane rejects, and what the EPA limit for each one is. Both are their own tables on this site, and the standard number is only the certification of the reduction, not the limit the water has to reach.
What should you do first?
- Read the mark on the box or the spec sheet and write down the standard number and the exact claim next to it.
- Reject any phrase built on “tested to” or “meets” unless a certifier’s mark and a database listing back it.
- Search the model number, not the brand, in the NSF, WQA or IAPMO database.
- Confirm the listed claim is the contaminant on your test report, since a 42 listing says nothing about lead and a 401 listing says nothing about PFAS.
- For a system with replaceable cartridges, check that the cartridge part number, not only the system, appears in the listing.
Frequently asked questions
What is the difference between NSF/ANSI 42 and NSF/ANSI 53?
NSF's knowledge library groups them by effect. NSF/ANSI 42 covers aesthetic effects only: chlorine taste and odor, and particulate. NSF/ANSI 53 covers health-effect contaminants such as lead, cysts and VOCs, and in its later revisions PFOA and PFOS. A filter can carry both marks, but a 42 mark alone says nothing about lead or any other health contaminant.
Does NSF/ANSI 401 certify PFAS reduction?
No. NSF's 401 page covers up to 15 named emerging compounds, such as prescription drugs, some pesticides, flame retardants and detergents, at a minimum 95% reduction from challenge water. PFOA and PFOS claims fall under NSF/ANSI 53 for carbon and anion-exchange filters and under NSF/ANSI 58 for reverse osmosis systems, so a 401 mark is not a PFAS mark.
What is the difference between NSF/ANSI 55 Class A and Class B?
The ANSI blog summary of NSF/ANSI 55-2024 puts Class A at a minimum UV dose of 40 mJ/cm², rated to inactivate bacteria, viruses, Cryptosporidium oocysts and Giardia cysts, with a UV sensor and a fail-safe alarm required. Class B delivers 16 mJ/cm² and is a supplemental treatment for water that a health authority has already judged acceptable.
What does lead free under NSF/ANSI/CAN 372 actually mean?
The ANSI blog on NSF/ANSI 372 defines lead free as a weighted average of no more than 0.25% lead across the wetted surfaces of a pipe, fitting or fixture, with 0.2% for solder and flux, per the 2011 Reduction of Lead in Drinking Water Act and EPA's 2020 rule. It is a materials-content standard for the part itself, not a claim that the part removes lead from water.
How do I check whether a specific model is actually certified?
Search the brand and model in the certifier's own database, not the maker's marketing page: NSF at info.nsf.org/Certified/DWTU/Listings.asp, WQA at find.wqa.org/find-products, or IAPMO R&T at pld.iapmo.org. NSF's page on certified by versus certified to says only a listing there confirms a third party verified the product; the phrases tested to and meets are the maker's own claim.
Fact sheet and how to cite this page
- Answer
- Each NSF/ANSI number certifies one job: 42 aesthetic effects, 53 health-effect contaminants, 58 reverse osmosis at a minimum 75% TDS reduction, 401 up to 15 emerging compounds, 55 UV at 40 mJ/cm² for Class A, 44 softeners, 177 shower chlorine at a 2.0 mg/L challenge, 372 lead content at most 0.25%. Only a model listed in the NSF, WQA or IAPMO database is certified.
- Basis
- NSF/ANSI standards 42, 44, 53, 55, 58, 61, 177, 372 and 401, published by NSF and ANSI, with the listing databases run by NSF, the Water Quality Association and IAPMO R&T.
- Assumptions
- The reader is checking a residential point-of-use or point-of-entry product sold in North America
- A listing confirms the model line named in the database, not every cartridge or configuration sold under that brand
- Test parameters quoted are the standard’s published challenge conditions, not field performance
- Method
- Standard scopes and test figures from NSF’s knowledge library and the ANSI blog summaries of each standard, database URL patterns from the NSF, WQA and IAPMO listing sites, all read on 2026-09-14; nothing was tested.
- Primary source
- NSF, Are You Certified by NSF or to NSF? (standards or certification body)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "NSF/ANSI water filter standards explained: what 42, 53, 58, 401, 55, 44, 177, 372 and 61 each certify, and how to check a listing", https://ownerspec.com/water/guides/nsf-ansi-water-filter-standards-explained/, facts verified 14 September 2026.
Every figure above also appears, with its source, in the article. All sources · How OwnerSpec verifies numbers
Sources
- NSF, Are You Certified by NSF or to NSF? (standards or certification body)
- NSF, NSF/ANSI 42, 53 and 401: Filtration Systems Standards (standards or certification body)
- NSF, NSF/ANSI 58: Reverse Osmosis Drinking Water Treatment Systems (standards or certification body)
- NSF, NSF/ANSI 44 Technical Requirements (standards or certification body)
- NSF, NSF/ANSI 177 Shower Filter Certification (standards or certification body)
- NSF, NSF/ANSI/CAN 372 Technical Requirements (standards or certification body)
- NSF, NSF/ANSI 401 Testing for Emerging Contaminants (standards or certification body)
- NSF, NSF/ANSI Standard 61: Drinking Water System Components, Health Effects (standards or certification body)
- ANSI Blog, NSF/ANSI 55-2024: Ultraviolet (UV) Water Treatment (standards or certification body)
- ANSI Blog, Lead Free Drinking Water: Regulations and NSF/ANSI 372 (standards or certification body)
- ANSI Blog, NSF/ANSI 53-2023: Drinking Water Units, Health Effect (standards or certification body)
- ANSI Blog, NSF/ANSI 58-2022: Reverse Osmosis for Drinking Water (standards or certification body)
- NSF Official Listings, Drinking Water Treatment Units (fluoride reduction example) (standards or certification body)
- WQA, Find Gold Seal Certified Products (standards or certification body)
- IAPMO R&T Product Listing Directory (standards or certification body)