NSF/ANSI 42 vs 53 vs 58 vs 401 vs P473 explained

NSF/ANSI 42 covers taste, odor and particles; 53 covers health-related contaminants such as lead and PFAS; 58 covers reverse osmosis (RO) systems; and 401 covers emerging compounds such as some medicines and microplastics. The old PFOA/PFOS test, NSF P473, was replaced by 53, with RO systems also held to 58, while 61 and 372 are materials standards that say nothing about what a filter removes.

A standard is the test. A certification is a listing that names the model and the exact claims it passed, so the claims list matters more than the number on the box.

The standards at a glance

The 42, 53 and 401 examples are three listings for the same PUR faucet filter; the 58 example is an APEC RO system.

42: taste, odor and particles

NSF/ANSI 42 is about how water looks, tastes and smells. For pitcher and point-of-use filters, EPA's lead guide recommends a 42 listing for particulate reduction (Class I), but the lead claim itself must come with testing against NSF/ANSI 53.

53: health effects, including lead and PFAS

For lead, EPA's consumer tool describes certified filters as evaluated for lead reduction to 5 parts per billion or less. For PFAS, EPA points buyers to NSF/ANSI 53 or 58.

PFAS claims come in two forms. A PFOA/PFOS claim covers those two; Pennsylvania lists the current acceptance level as 20 ppt for PFOA and PFOS. A Total PFAS claim covers seven PFAS combined (PFOA, PFOS, PFHxS, PFNA, PFHpA, PFBS and PFDA), reduced below 20 ppt, according to Michigan's PFAS Action Response Team. EPA's limits for PFOA and PFOS are 4.0 ppt each, and EPA says certification standards as of April 2024 do not yet indicate removal down to the levels EPA has now set.

58: reverse osmosis systems

Listings under 58 can be short or long. APEC's ROTL-A1000 lists TDS, PFOA and PFOS; AquaTru's Classic lists 12 claims, among them lead, arsenic and nitrate. Check that your contaminant is on the list, not just that the system says 58.

401: emerging compounds

The PUR faucet filter's 401 listing names 15 compounds, including ibuprofen, bisphenol A and microplastics. None of them is lead or PFAS.

P473: moved into 53 and 58

NSF P473 was a protocol, not an NSF/ANSI standard. In May 2019, NGWA reported that the PFOA and PFOS requirements, previously outlined in P473, had been added to NSF/ANSI 53 and 58 (NGWA).

Michigan's PFAS team says NSF P473 was the accepted PFOA/PFOS standard before March 2019, and that it was then retired and replaced by NSF/ANSI 53, with RO systems also required to meet NSF/ANSI 58. Current listings show both routes: PFOA and PFOS appear on AquaTru's NSF/ANSI 53 listing and on APEC's NSF/ANSI 58 listing.

The name lingers. IAPMO R&T's program page still lists P473, Pennsylvania's list says "NSF/ANSI 53 (or P473)", and Aquasana's product page says "53 (includes P473)". What to look for now is a PFOA/PFOS or Total PFAS claim under 53 or 58.

61 and 372: materials, not removal

Neither shows that a filter removes anything, even when the number is printed next to a reduction claim.

• NSF/ANSI 61 (Drinking Water System Components – Health Effects) limits the chemicals that products and materials put into drinking water. It does not establish performance requirements.
• NSF/ANSI 372 checks lead in the parts that touch water. An IAPMO 372 certificate says its products were verified with weighted average lead content of 0.25% or less, the federal definition of "lead free" for plumbing.

IAPMO R&T files 61 and 372 under "Drinking Water Components and Additives", apart from its treatment-unit standards. A filter whose only listing is 372 has low-lead parts; a lead reduction claim needs 53, or 58 for RO.

Reading a label

1. Find the certifier's mark and the standard. EPA's example wording is "Tested and Certified by (certification body) against NSF/ANSI Standards 42 and 53".
2. Find your contaminant on the performance data sheet, in the certifier's directory or on the maker's website, the three places EPA says claims appear.
3. Look up the exact model or cartridge in IAPMO R&T's directory, NSF's listings or WQA's directory.

How we use listings: How we decide "certified". Who the certifiers are: IAPMO vs NSF vs WQA.