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When you’re shopping for a water filter, NSF certification numbers come up constantly. NSF 42. NSF 53. NSF 58. They’re not interchangeable, and a filter certified under one standard tells you nothing about whether it works for another contaminant.

Here’s what each standard actually covers.

NSF/ANSI 42, Aesthetic Contaminants

Covers: chlorine (taste and odor), chloramines (taste and odor), particulates, and some aesthetic issues.

Does NOT cover: lead, PFAS, arsenic, nitrates, bacteria, or any health-effect contaminant.

A filter with only NSF 42 certification will make your water taste and smell better. That’s it. Most basic pitcher filters carry only NSF 42. Brita’s standard pitcher filter, for example, is certified under NSF 42 but not NSF 53.

If you’re filtering for health reasons, lead, PFAS, anything with a health-based EPA limit, NSF 42 alone is not sufficient.

NSF/ANSI 53, Health-Effect Contaminants

Covers: lead, cysts (Cryptosporidium, Giardia), certain volatile organic compounds (VOCs), and other health-effect contaminants.

Does NOT cover: PFAS, arsenic, nitrates, fluoride, or other dissolved inorganic contaminants.

NSF 53 is the standard to look for if you’re filtering for lead. A solid carbon block filter certified to NSF 53 for lead reduction will reduce lead by at least the certified rate (typically 97%+). But NSF 53 certification is specific, a filter can be NSF 53 certified for cysts but not for lead. Check which specific contaminants the certification covers.

For pitcher filters: Clearly Filtered and ZeroWater both carry NSF 53 certification. Standard Brita filters don’t, though Brita does offer some products that do.

NSF/ANSI 58, Reverse Osmosis Systems

Covers: PFAS, arsenic, nitrates, fluoride, lead, dissolved solids, and a broad range of contaminants removed through the RO membrane process.

This is the certification that matters if you’re dealing with PFAS contamination, high arsenic (common in some well water), fluoride, or nitrates. Reverse osmosis is the most broadly effective treatment technology for dissolved contaminants, and NSF 58 is its certification standard.

An RO system certified to NSF 58 has been tested against the specific contaminants listed in its certification. Not all NSF 58 systems are certified for all contaminants, the certification document shows exactly what’s covered.

NSF/ANSI 401, Emerging Contaminants

Covers: pharmaceuticals, pesticides, herbicides, and other “emerging” contaminants that don’t yet have EPA regulatory limits.

This is the newest of the major filter standards and covers things like ibuprofen, atenolol, and certain agricultural chemicals. If you’re concerned about pharmaceutical contamination (a real issue in some water supplies), NSF 401 is the standard to look for.

NSF/ANSI 55, UV Water Treatment

Covers: ultraviolet disinfection systems.

Two classes:

Class A, Inactivates and/or removes microorganisms at 40 mJ/cm² or higher. This is the purification level used for disinfecting water with unknown or unsafe microbial quality. If you have a private well or need protection against bacteria and viruses, you need Class A.

Class B, Treats water with already-safe microbial quality for additional reduction. Not a purification system.

This distinction is critical. Many UV systems sold for home use are Class B. A Class B system won’t make unsafe water safe, it’s a supplemental step for already-treated water. Well owners and anyone dealing with bacterial contamination need Class A.

NSF/ANSI 177, Shower Filtration

Covers: free chlorine reduction in shower filters.

This is a narrower standard. It certifies that the shower filter reduces free chlorine. It does not cover chloramine removal, chloramines require different filter media (KDF-55 or vitamin C/ascorbic acid) and those are not covered under NSF 177.

If you’re on a municipal water supply that uses chloramines (about half of US utilities), verify specifically whether a shower filter is certified or tested for chloramine removal, not just chlorine. Many are not.

“Tested to NSF Standards” vs. NSF Certified

This is the most common source of confusion when buying water filters.

NSF certified means NSF International independently tested the product, verified its performance, audits the manufacturing facility, and re-tests periodically. The product is listed in NSF’s public database.

“Tested to NSF standards” means the manufacturer conducted internal testing using NSF protocols. NSF didn’t test it. NSF didn’t certify it. It’s not in the database.

The distinction matters. Some manufacturers use NSF-like language on packaging and marketing without pursuing actual certification. The only way to confirm certification is to look up the product at info.nsf.org/Certified/DWTU/.

How to Verify a Certification

  1. Go to info.nsf.org/Certified/DWTU/
  2. Search by company name or product name
  3. Find the specific product and click through to see the full certification
  4. Confirm the standard (42, 53, 58, etc.) and the specific contaminants covered

Do this before buying, not after. A filter marketed for lead removal that isn’t in the NSF 53 database for lead is relying on your trust, not on independent verification.

Related: What Water Filter Do I Need?, match your specific contaminant to the right NSF standard and filter type.

Frequently Asked Questions

What does NSF certified mean for a water filter?
NSF International is an independent testing organization that certifies water treatment products against specific performance standards. When a filter is NSF certified, it means NSF tested that specific product and verified that it reduces the claimed contaminants to the claimed levels. It's not a general quality seal, each certification covers specific contaminants under specific test conditions.
What is the difference between NSF 42 and NSF 53?
NSF 42 covers aesthetic contaminants, chlorine, taste, odor, and particulates. It does not cover health-effect contaminants. NSF 53 covers health-effect contaminants like lead, cysts (Cryptosporidium, Giardia), and certain VOCs. A filter with only NSF 42 certification does not remove lead, even if it improves taste and odor.
Does NSF 58 cover PFAS removal?
Yes. NSF/ANSI 58 is the certification standard for reverse osmosis systems. PFAS reduction (specifically PFOA and PFOS) is covered under NSF 58 testing. An RO system certified under NSF 58 has been tested and verified to reduce the contaminants listed in its certification, which typically includes PFAS, arsenic, nitrates, fluoride, and other dissolved solids.
What does "tested to NSF standards" mean?
"Tested to NSF standards" means the manufacturer ran internal tests using NSF methodology. It does NOT mean NSF independently tested or certified the product. The certification itself requires NSF to purchase and test the product, conduct unannounced factory audits, and verify ongoing production consistency. If you need certainty, look up the product in NSF's online certified product database.
How do I verify a water filter is NSF certified?
Go to info.nsf.org/Certified/DWTU/ and search by brand, model name, or company. The database shows every certified product, the standard it's certified under, and the specific contaminants covered. This is the only reliable way to verify a claim, not the product packaging, not the manufacturer's website.