NSF 42 vs 53 vs 58 vs 401, decoded

The one rule: claims, not standards

A standard is a list of possible claims. A product is certified for the specific claims on its listing, and nothing else. Two filters can both say "certified to NSF/ANSI 53" while one reduces lead and the other only reduces cysts.

So the number on the box starts the question. The listing answers it. Every certifier publishes its listings, and each one names the claims, word for word, such as "Lead Reduction" or "PFOA Reduction."

NSF/ANSI 42: taste, odor and appearance

NSF/ANSI 42 covers what you can taste, smell or see, not health. Its claims include chlorine, chloramine, general taste and odor, particles graded by size, and zinc.

The mistake to avoid: a 42 certification does not mean lead removal. Many sites say it does. Lead reduction is a separate health claim under 53 or 58.

NSF/ANSI 53: health contaminants

NSF/ANSI 53 is where the health claims live. Depending on the product, its listing may show:

• Lead and copper
• Cysts such as Cryptosporidium
• PFOA and PFOS, and a Total PFAS claim covering seven PFAS
• Trihalomethanes and a wider VOC group
• Pentavalent arsenic, chromium, mercury and other metals

What 53 does not cover is just as useful. NSF lists nitrate and fluoride claims under 58, not 53. No NSF or WQA listing shows a claim for GenX (HFPO-DA). Bacteria and viruses are outside it.

One more thing about PFAS. NSF's PFAS claims require reducing PFAS below a level that is still above EPA's new 4.0 ppt limits for PFOA and PFOS, and NSF says work to align 53 with EPA's 2024 rule is still underway. A certified claim is the best evidence there is today, but it is not yet proof a filter gets you under EPA's line.

NSF/ANSI 58: reverse osmosis

NSF/ANSI 58 covers reverse osmosis systems. Every certified system must reduce total dissolved solids. Everything else is optional, claim by claim: lead, copper, nitrate and nitrite, fluoride, pentavalent arsenic, chromium, radium, cysts, VOCs and more.

The mistake to avoid: assuming every RO system removes everything RO can remove. If your water has nitrate, the listing needs a nitrate claim.

PFOA and PFOS claims for RO systems are a split case. WQA lists them under 58. NSF lists RO systems' PFOA and PFOS claims under 53.

NSF/ANSI 401: emerging contaminants

NSF/ANSI 401 covers trace chemicals EPA does not regulate. Each compound is its own claim: atenolol, bisphenol A, carbamazepine, DEET, estrone, ibuprofen, linuron, meprobamate, metolachlor, naproxen, nonylphenol, phenytoin, TCEP, TCPP and trimethoprim, plus microplastics.

A 401 filter does not "remove pharmaceuticals" in general. It reduces the ones on its listing.

The numbers people confuse with these

• NSF/ANSI/CAN 372 is about the lead content of a product's own parts. It is not a lead-removal claim.
• NSF P473 was NSF's PFOA and PFOS test. It was retired and its claims moved into 53 and 58. There is no live "NSF/ANSI 473."
• NSF/ANSI 244 is a backup for water that is already treated. NSF P231 covers purifiers for bacteria, viruses and cysts.
• NSF/ANSI 55 covers UV systems. Only Class A is a disinfection claim.
• NSF/ANSI 44 covers water softeners. It says nothing about lead, PFAS or chlorine.

Who can certify

NSF is not the only certifier. EPA names five that are accredited to test filters to these standards: CSA Group, IAPMO R&T, NSF, UL Solutions and the Water Quality Association. A WQA Gold Seal listing to NSF/ANSI 58 is a real certification. They word claims differently, so WQA's "Lead" is NSF's "Lead Reduction."

How to check a filter in two minutes

1. Find the certifier's mark on the box or the maker's page.
2. Open that certifier's listing search and find the brand and model.
3. Read the claims. Look for the exact contaminant you care about.
4. Match the cartridge, too. A claim belongs to a specific model and cartridge.

Not sure which contaminant to look for? Start with your water.