What Is NSF/ANSI 401? Filters for Emerging Contaminants

Some filter boxes promise to reduce “pharmaceuticals” or “emerging contaminants.” NSF/ANSI 401 is the standard that lets you check that kind of claim, compound by compound.

What does NSF/ANSI 401 cover?

When NSF announced the standard in August 2014, it covered up to 15 compounds that published studies had found in drinking water:

• Prescription drugs: atenolol, carbamazepine, estrone, meprobamate, phenytoin and trimethoprim.
• Over-the-counter painkillers: ibuprofen and naproxen.
• Herbicides and pesticides: DEET (the insect repellent), linuron and metolachlor.
• Industrial chemicals: BPA, nonylphenol and the flame retardants TCEP and TCPP.

Those same 15 appear as separate reduction claims on NSF’s current listings for the standard, on pitchers, faucet-mount, countertop, under-sink and refrigerator filters. A model can be certified for some compounds and not others, so read its claims.

Does a 401 filter remove microplastics?

NSF’s 401 listings also show a “Microplastics Reduction” claim on many models. Where NSF adds a footnote, it says the claim “is bracketed by Testing of Std. 42 Particulate Class I Reduction”: in other words, it’s tied to the fine-particle test under NSF/ANSI 42 rather than to the chemical tests above.

Do you need an NSF/ANSI 401 filter?

Probably not as a first step. In its 2014 announcement, NSF itself described these contaminants as “not a public health issue,” while noting that they have been found in drinking water at trace levels and can affect how some people feel about their water. If they concern you, a 401 claim tells you a filter was actually tested for them.

What 401 doesn’t do is replace NSF/ANSI 53. If your water has a known health contaminant such as lead or PFAS, that’s the claim to look for first, ideally on the same filter. Some systems carry both: in our comparison of reverse osmosis systems, one model was listed under 401 alongside its 53 and 58 claims.