NSF Water Filter Certifications Explained: 42, 53, 58, 401, 244, 61 and P473

NSF/ANSI standards are the main way to tell whether a home water filter does what it claims. In short: NSF/ANSI 42 covers taste and odor, such as chlorine. NSF/ANSI 53 covers health-related contaminants such as lead. NSF/ANSI 58 covers reverse osmosis systems. NSF/ANSI 401 covers “emerging” contaminants such as some pharmaceuticals. NSF/ANSI 244 covers protection against occasional germs in already-treated water. NSF/ANSI/CAN 61 is about the materials in water system parts, not filtering. The old P473 PFAS protocol no longer stands alone. NSF folded it into standards 53 and 58.

Certification is always claim-by-claim. The standard number tells you the category. The listed claims tell you what the filter actually reduces.

Why these standards matter

There is no federal requirement for home water filters to be tested or certified. EPA says certification is voluntary under the Safe Drinking Water Act. NSF/ANSI standards fill that gap. They are written through a consensus process that includes industry, regulators and public health officials, and accredited bodies certify products against them.

NSF notes one thing that confuses many shoppers: the numbers are not a ranking. They reflect the order the standards were created. A “58” is not better than a “53.” It simply covers a different type of system.

The performance standards, one by one

NSF/ANSI 42: Aesthetic effects

This standard covers contaminants that affect taste, smell or appearance but are not health risks. Possible claims include chlorine, taste and odor, chloramine, particulates, iron, manganese, zinc and total dissolved solids. Testing also covers material safety and structural integrity.

What it does not mean: a filter certified only to NSF/ANSI 42 has no certified claim for lead, PFAS or any other health contaminant.

NSF/ANSI 53: Health effects

This is the most important standard for most carbon-based filters. NSF says it offers more than 50 contaminant reduction claims. Common ones include lead, Cryptosporidium, volatile organic chemicals (VOCs) and chromium. The health limits used in testing come from EPA and Health Canada.

A filter can be certified to NSF/ANSI 53 for just one claim, such as cysts. That is why you must check the specific claim for the contaminant you care about. EPA’s consumer guide for lead filters says to look for NSF/ANSI 53 with a lead reduction claim. It also suggests NSF/ANSI 42 particulate reduction (Class I), because some particles can contain lead.

NSF/ANSI 58: Reverse osmosis

This covers reverse osmosis (RO) systems, which push water through a semi-permeable membrane. Claims cover contaminants regulated by EPA and Health Canada, and RO systems can also be certified to reduce total dissolved solids. EPA’s WaterSense label for RO systems requires NSF/ANSI 58 certification. See our reverse osmosis vs. carbon filters guide.

NSF/ANSI 401: Emerging compounds and incidental contaminants

This standard covers up to 15 specific contaminants that have been found at trace levels in drinking water. Most are not regulated by EPA, such as certain prescription and over-the-counter drugs, herbicides and pesticides. NSF also lists NSF/ANSI 401 as the standard for filters that reduce microplastics. See our post on microplastics in drinking water.

NSF/ANSI 244: Supplemental microbiological protection

This standard is for filters used on public water that is already treated or known to be microbiologically safe. It covers protection against occasional contamination, such as a short-term problem before a boil water notice is issued. It is not for treating untreated water such as surface water or an unsafe well. Products can claim bacteria, virus and cyst reduction. NSF also says the standard covers material safety and structural integrity.

Other NSF standards cover systems meant for water that may be contaminated, such as Class A ultraviolet systems under NSF/ANSI 55 and microbiological water purifiers under NSF P231. During an active boil water advisory, follow your utility’s instructions.

Other standards you may see

NSF/ANSI 44: water softeners. It covers hardness reduction, not health contaminants. See hard water explained.
NSF/ANSI 55: ultraviolet (UV) systems.
NSF/ANSI 62: distillation systems.
NSF/ANSI 177: shower filters, certified only for free chlorine reduction.

Material standards: 61 and 372

These two standards are about what a product is made of, not what it removes.

NSF/ANSI/CAN 61 sets health-effects requirements for products and materials that touch drinking water. That includes pipes, fittings, coatings, valves and other system parts, from source to tap. It checks whether chemicals leach from the material into water at levels above health limits. Most U.S. states require drinking water system parts to meet it. If you see “61” on a faucet or pipe, it means the material is safe to contact drinking water. It does not mean the product filters anything.

NSF/ANSI/CAN 372 covers lead content. It sets a maximum weighted average of 0.25% lead for parts that contact drinking water. Many certified filters also carry a 372 listing for their materials. A “372” mark on a pitcher means the pitcher is “lead free.” It is not a lead-removal claim.

What happened to NSF P473 (PFAS)?

NSF created Protocol P473 in 2016 to test whether filters reduce PFOA and PFOS. At the time, EPA’s health advisory for the two combined was 70 parts per trillion.

Starting in 2017, an NSF joint committee worked to move the P473 test methods into the formal standards. P473 claims were added to NSF/ANSI 53 (for carbon and anion exchange resin filters) and NSF/ANSI 58 (for RO). NSF says this resulted in all products and references to P473 being removed and replaced by NSF/ANSI 53 or 58.

The 2022 updates to both standards went further. They added a “Total PFAS” reduction claim that also covers PFHpA, PFHxS, PFNA and PFBS. They also lowered the combined PFAS level a filter must reach from 70 ppt to 20 ppt. NSF says committees are working on further updates to match EPA’s 2024 PFAS drinking water rule. Our EPA PFAS rule explainer covers where that rule stands as of October 2026.

What this means for you: to find a PFAS filter, look for NSF/ANSI 53 or 58 with a PFOA/PFOS or Total PFAS claim. A product page that still lists “P473” as its current certification is out of date and should be checked. See our PFAS filter guide.

“Tested to” vs. “certified”

These phrases sound alike but mean different things.

Certified means an independent, accredited body tested the product against the standard and keeps checking it. NSF says certification includes product testing, material analysis, technical review and on-site inspection of factories. It is ongoing, with regular audits.
“Meets,” “complies with” or “tested to” NSF standards usually means the company is reporting its own results or results from a lab it hired. NSF says compliance claims are often self-declared, with no guarantee of validity, and they do not show how or by whom the result was reached.

Lab test reports can still be useful information. But only certification includes independent verification and ongoing audits.

NSF is not the only certifier. EPA’s consumer guide lists other ANSI-accredited certification bodies for drinking water filters, including IAPMO, WQA, UL and CSA. Each has its own mark and public listing directory. “Certified by IAPMO to NSF/ANSI 53” is a legitimate certification.

How to check a filter’s certification yourself

Find the mark and standard on the box. EPA says the certification mark and standard numbers are on the filter or its smallest package.
Read the performance data sheet. It lists each certified claim, the test challenge level and the percent reduction. It also usually lists the rated capacity in gallons.
Look up the model in the certifier’s listing. For NSF, use the NSF listings for drinking water treatment units. You can search by manufacturer, by brand or model, or by standard. The results show each model, its replacement cartridge, its service cycle in gallons, and its exact claims. For other certifiers, use their directories: IAPMO (pld.iapmo.org), WQA (wqa.org) and the UL and CSA product directories.
Match the model and cartridge numbers exactly. Certifications apply to specific models and replacement cartridges.
If you cannot find it, ask the company for the certifier and file number. If it cannot give one, treat the claim as unverified.

When companies are listed under a parent or legal name, a search by brand may come up empty. Try searching by model number.

Bottom line

Start by learning what is in your water. Look up your water system or read your water quality report. Then choose a filter with a certified claim for that contaminant: NSF/ANSI 42 for taste, 53 for health contaminants, 58 for RO, 401 for emerging chemicals. Verify the model in the certifier’s public listing before you buy. A standard number alone, or a “tested to” claim, is not enough.