Filters
NSF/ANSI 42, 53, 58, 401 and P473 explained
NSF/ANSI 42 is about taste and particles, 53 is about health-related contaminants such as lead, 58 is for reverse osmosis, and 401 covers trace compounds like some pesticides and medications. A filter is certified only for the specific claims on its listing, so always check the exact model.
Why certification matters
Third-party certification means an accredited body tested the product against a standard and verified its contaminant claims. EPA says that is why it recommends certified filters. Certifiers include NSF, IAPMO R&T, WQA, CSA Group and UL. A manufacturer’s phrase like “tested to NSF standards” is not the same thing as a listing.
The standards you will see
| Standard | What it covers | Typical claims |
|---|---|---|
| NSF/ANSI 42 | Aesthetic effects (non-health) | Chlorine, taste and odor, chloramine, particulates, iron, manganese, zinc, total dissolved solids |
| NSF/ANSI 53 | Health effects | Over 50 claims per NSF, including lead, Cryptosporidium, VOCs and chromium; PFAS reduction claims are also made under this standard |
| NSF/ANSI 58 | Reverse osmosis systems | Dissolved solids and other claims, with contaminant-specific claims depending on the system |
| NSF/ANSI 401 | Emerging compounds and incidental contaminants | Up to 15 specific contaminants such as prescription and over-the-counter drugs, herbicides and pesticides |
| NSF/ANSI 44 | Cation exchange water softeners | Hardness reduction |
| NSF/ANSI 55 | Ultraviolet treatment | Inactivating bacteria, viruses and protozoa (not chemicals) |
The first three descriptions come from NSF’s knowledge library; the softener, UV and RO descriptions come from EPA WaterSense’s treatment guide.
What about “P473”?
NSF/ANSI P473 is a protocol (the “P” stands for protocol, an earlier step than a full standard) that NSF has used to certify PFOA and PFOS reduction claims. EPA’s consumer page now tells shoppers to look for NSF/ANSI 53 or 58 certification for PFAS reduction. You may still see P473 on older listings and packaging. Either way, the listing, not the label, tells you what was tested.
How to read a listing
- Find the exact model number on the certifier’s database (NSF, IAPMO R&T, WQA, CSA Group or UL).
- Read which contaminants are claimed. A filter certified for chlorine under 42 may have no lead claim under 53.
- Check the rated capacity (gallons) and the replacement cartridge the claim applies to.
- Read the performance data sheet for the test concentrations and the filtered-water results. PFAS claims in particular are changing: according to ANSI, NSF/ANSI 53-2025 aligns PFOA and PFOS effluent limits with EPA’s limits, and the certifiers’ joint timeline requires certified products to meet the update by January 1, 2030.
Common traps
- “NSF-compliant” or “tested to NSF standards” is not “NSF-certified”.
- Certification applies to a product configuration. A different cartridge or pitcher body may not be covered.
- One standard does not imply another. Brita’s U.S. Elite filter, for example, has lead and chlorine claims on its data sheet, but no PFAS claim (details).
Sources
Not medical or legal advice. This guide summarizes public documents so you can ask better questions of your utility, your health department or a certified lab. For health concerns, talk to a clinician; for compliance questions, to your state drinking water program. Pages on this site that name products may earn a commission, as disclosed on the affiliate disclosure page.