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The number on the box, decoded.

A filter is certified for specific claims, not for a whole standard. Two filters can both say “NSF/ANSI 53” and reduce different things. Check the claim on the certifier’s listing.

42

NSF/ANSI 42: Taste, odor and appearance

A product certified to NSF/ANSI 42 was tested to reduce things that affect taste, smell or look, not health. It is certified only for the specific claims on its listing.

Covers

  • Chlorine, for taste and odor (listing claim: "Chlorine Reduction")
  • Chloramine (listing claim: "Chloramine Reduction")
  • General taste and odor (listing claim: "Taste and Odor Reduction")
  • Particles, graded by size class (listing claim: "Nominal Particulate Reduction")
  • Zinc (listing claim: "Zinc Reduction")

Often misread

Many sites say a filter certified to NSF/ANSI 42 removes lead. It does not. Every 42 claim is aesthetic, and lead reduction is a separate health claim under NSF/ANSI 53 or 58.

53

NSF/ANSI 53: Health-related contaminants

Covers filters tested to reduce contaminants with health effects, such as lead, cysts, VOCs and PFOA/PFOS. A certified product holds only the claims on its listing, not every claim in the standard.

Covers

  • Lead (listing claim: "Lead Reduction")
  • Copper (listing claim: "Copper Reduction")
  • Cysts such as Cryptosporidium (listing claim: "Cyst Reduction")
  • PFOA and PFOS (listing claims: "PFOA Reduction" and "PFOS Reduction")
  • A set of seven PFAS (listing claim: "Total PFAS Reduction")

Often misread

Reading "certified to NSF/ANSI 53" as "removes everything in 53." Certification is per claim. A filter certified to 53 only for cyst reduction has not been shown to reduce lead.

58

NSF/ANSI 58: Reverse osmosis systems

Covers point-of-use reverse osmosis (RO) systems. Every certified system must reduce total dissolved solids (TDS). Other claims, like lead or nitrate, are optional and listed per product.

Covers

  • Total dissolved solids, required for every system (listing claim: "TDS Reduction")
  • Lead and copper (listing claims: "Lead Reduction", "Copper Reduction")
  • Nitrate and nitrite (listing claim: "Nitrate/Nitrite Reduction")
  • Fluoride (listing claim: "Fluoride Reduction")
  • Pentavalent arsenic at 50 or 300 ppb or less

Often misread

Assuming every RO system removes everything RO can remove. Only TDS reduction is required. Lead, nitrate, arsenic and PFAS are separate claims that a given system may or may not hold.

401

NSF/ANSI 401: Emerging contaminants

Covers filters tested to reduce trace contaminants that EPA does not regulate, such as some drugs, pesticides, BPA and microplastics. Each compound is its own claim.

Covers

  • Atenolol
  • Bisphenol A (BPA)
  • Carbamazepine
  • DEET
  • Estrone

Often misread

Saying a 401 filter "removes pharmaceuticals" in general. Each claim covers one named compound, and a product may hold only some of them.

P473

NSF P473: Retired PFOA/PFOS protocol

A former NSF test protocol for PFOA and PFOS reduction. NSF moved its claims into NSF/ANSI 53 and 58 and removed all products and references to P473.

Covers

  • Nothing today. PFOA and PFOS reduction claims now sit under NSF/ANSI 53, and under 58 on some listings.

Often misread

Calling it "NSF/ANSI 473" and treating it as a live PFAS standard. NSF calls it Protocol P473, and its claims were folded into NSF/ANSI 53 and 58.

244

NSF/ANSI 244: Backup germ filtration

Covers filters for water that is already treated or shown to be safe. They guard against occasional, accidental germ contamination, such as before a boil-water advisory.

Covers

  • Bacteria, with at least a 3-log (99.9%) reduction (WQA listing claim: "Bacteria")
  • Viruses, with at least a 3-log (99.9%) reduction (WQA listing claim: "Virus")
  • Cysts (WQA listing claim: "Cyst")

Often misread

Calling a 244 filter a purifier. It is a backup for water that is already safe. Products meant to make germ-contaminated water safe are certified under NSF/ANSI 55 Class A or NSF P231.

P231

NSF P231: Microbiological water purifiers

An NSF protocol for purifiers that claim to reduce bacteria, viruses and cysts. It is based on EPA's 1987 Guide Standard and Protocol for Testing Microbiological Water Purifiers.

Covers

  • Bacteria (listing claim: "Bacteria Reduction")
  • Viruses (listing claim: "Virus Reduction")
  • Cysts (listing claim: "Live Cyst Reduction")

Often misread

Mixing it up with NSF/ANSI 244. P231 covers purifiers for bacteria, viruses and cysts. 244 is only a backup for water that is already treated or shown to be safe.

55

NSF/ANSI 55: UV disinfection systems

Covers ultraviolet (UV) light systems. Class A systems inactivate or kill bacteria, viruses and cysts in contaminated water. Class B systems only reduce non-disease-causing bacteria in water that is already disinfected.

Covers

  • Bacteria, viruses and cysts in contaminated water (listing claim: "Disinfection Performance, Class A")
  • Non-disease-causing bacteria in disinfected water (listing claim: "Disinfection Performance, Class B")

Often misread

Treating any UV certification as disinfection. Only Class A is a disinfection claim. Class B units are meant for water that is already disinfected.

62

NSF/ANSI 62: Water distillers

Covers distillers, which boil water and collect the steam as it condenses. NSF notes that heavy metals stay behind, but some chemicals that turn to gas easily can carry over.

Covers

  • Heavy metals, which NSF says are left behind when the water boils
  • Other reduction claims as shown on each product listing

Often misread

Assuming distilled water is free of everything. NSF notes that chemicals that turn to gas easily, such as some VOCs, can carry over.

44

NSF/ANSI 44: Water softeners

Covers home water softeners that trade calcium and magnesium for sodium or potassium. A certified softener was tested for hardness reduction and may hold a few extra claims.

Covers

  • Hardness from calcium and magnesium (listing claim: "Hardness Reduction")
  • Barium (listing claim: "Barium Reduction")
  • Radium 226/228 (listing claim: "Radium 226/228 Reduction")
  • Salt and water efficiency (listing term: "Efficiency Rated")

Often misread

Treating a softener as a drinking water filter. A 44 certification says nothing about lead, PFAS or chlorine. Those need their own claims under other standards.

177

NSF/ANSI 177: Shower filters

Covers shower filters that attach in front of the showerhead. NSF says they are certified to reduce only free available chlorine.

Covers

  • Free available chlorine in shower water (listing claim: "Free Available Chlorine Reduction")

Often misread

Treating a certified shower filter as a health filter. NSF/ANSI 177 is an aesthetic standard that covers free chlorine only.

372

NSF/ANSI/CAN 372: Lead content of materials

A materials standard. It checks how much lead is in the parts of a product that touch drinking water. It is not a claim that the product removes lead from water.

Covers

  • Weighted average lead content of wetted surfaces: 0.25% maximum for most components
  • Solder and flux: 0.2% maximum
  • Faucets, valves, pipes, fittings, gaskets, coatings and water meters
  • Water filters, softeners, RO systems and UV units, as products

Often misread

Reading a 372 mark on a filter as proof it removes lead. It only means the product's own parts meet the lead-free content limit.