PFAS in tap water: what the 4.0 ppt limit means, and which filter certification counts

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Short answer

PFAS in tap water is regulated by EPA’s April 2024 PFAS National Primary Drinking Water Regulation, which sets a Maximum Contaminant Level of 4.0 parts per trillion (ppt) each for PFOA and PFOS, 10 ppt each for PFHxS, PFNA and HFPO-DA, and a Hazard Index of 1 for mixtures. EPA’s May 2026 proposal keeps the 4.0 ppt limits, would move their compliance date from April 26, 2029 to April 26, 2031 for systems that request it, and would rescind the other four limits; it was not final on September 12, 2026. At home, the filters tested for PFOA and PFOS are those certified to NSF/ANSI 53 (carbon block, ion exchange, pitchers) or NSF/ANSI 58 (reverse osmosis) with PFOA/PFOS or PFAS named in the listing. NSF’s current protocol certifies a filter when it brings PFAS below 20 ppt, not 4.0 ppt. First step: get the utility’s PFOA and PFOS result, then buy only a unit whose certification listing names them.

If the utility’s PFOA and PFOS results already read below 4.0 ppt, you can leave now: every filter on this page is certified to a 20 ppt protocol, and none of them is tested to improve on a number the water already meets.

What does the utility’s PFAS result tell you?

The report gives a number per compound, usually in nanograms per litre (ng/L). One ng/L is one part per trillion, so the EPA limits read straight off the sheet.

What the report shows	What it means	Confirm with
PFOA and PFOS “not detected”	Below the laboratory’s reporting limit; check that limit is at or below 4.0 ppt	The reporting limit printed beside the result
PFOA or PFOS detected, below 4.0 ppt	Under the EPA MCL. A point-of-use filter is a preference, not a requirement	The next monitoring result; single samples move
PFOA or PFOS at 4.0 ppt or above	At or above the MCL. The system must reach compliance by April 26, 2029 under the 2024 rule, or April 26, 2031 if the May 2026 extension is finalised and granted	Whether the system has requested the extension; at 12 ppt or above EPA’s proposal requires interim mitigation
PFHxS, PFNA or HFPO-DA above 10 ppt, or Hazard Index above 1	Above a 2024 limit that EPA’s May 2026 companion proposal would rescind	The status of that proposal at the time you read the report
No report, because the water is a private well	No monitoring requirement applies	A laboratory PFAS panel, ordered with the well water test panel

Under the 2024 rule, public water systems have three years from publication, to 2027, to complete initial monitoring and public notification. Until then, ask the utility directly or search the state drinking water program’s PFAS results page for the system name.

What does 4.0 ppt mean?

A Maximum Contaminant Level (MCL) is the highest concentration a public water system may deliver. EPA’s final rule, published April 26, 2024 and effective June 25, 2024, sets the PFOA and PFOS MCL at 4.0 ppt and the Maximum Contaminant Level Goal, the non-enforceable health-based target, at zero.

Compound	2024 MCL	Status under EPA’s May 2026 proposals	Hazard Index component
PFOA	4.0 ppt	Kept; compliance date extendable to April 26, 2031	No
PFOS	4.0 ppt	Kept; compliance date extendable to April 26, 2031	No
PFHxS	10 ppt	Proposed for rescission	Yes
PFNA	10 ppt	Proposed for rescission	Yes
HFPO-DA (GenX)	10 ppt	Proposed for rescission	Yes
PFBS	No individual MCL	Proposed for rescission with the Hazard Index	Yes
Hazard Index (mixture)	1, unitless	Proposed for rescission	The index itself

The Hazard Index is a sum. Kentucky’s compliance guidance for the rule states the formula: each measured concentration divided by its health-based water concentration, which is 10 ng/L for PFHxS, PFNA and HFPO-DA and 2,000 ng/L for PFBS, added together. A running annual average above 1 is a violation.

EPA’s compliance-extension proposal, Federal Register document 2026-10086 of May 20, 2026, states that it does not change the PFOA and PFOS MCLs, which remain 4.0 ppt each. Its comment period closed July 20, 2026, the docket is EPA-HQ-OW-2025-1742, and nothing in it was final on September 12, 2026.

Which certification does a PFAS filter need?

A certification is a listing by NSF, IAPMO R&T, WQA, CSA or UL that names the standard, the model and each contaminant the model was tested against. The word PFAS on a box is not a certification; the listing is.

Standard	Scope	What it says about PFAS
NSF/ANSI 42	Aesthetic contaminants: chlorine, taste, odour, particulates	Nothing. A 42-only filter has no PFAS claim
NSF/ANSI 53	Health-effects contaminants, more than 50 claims including lead, VOCs and PFOA/PFOS	The PFOA/PFOS claim was first added in the 2019 edition; the 2023 edition added Total PFAS claims and tightened the combined effluent limit from 70 ppt to 20 ppt
NSF/ANSI 58	Reverse osmosis systems	The RO standard; NSF lists 53 and 58 as the two standards under which PFAS reduction is validated
NSF/ANSI 401	Up to 15 emerging or incidental contaminants: pharmaceuticals, pesticides, herbicides	Not a PFAS standard. NSF calls it a separate, additive certification
NSF/ANSI P473	NSF’s PFOA/PFOS reduction protocol, cited alongside 53 by manufacturers such as Aquasana	A listing that names P473 carries a PFOA/PFOS test in addition to the 53 claims

The threshold gap matters. NSF’s own PFAS page states that to make a reduction claim a filter must reduce PFAS to below 20 parts per trillion, and that its Joint Committee task groups are still working to align NSF/ANSI 53 and 58 with the EPA rule. A certified filter is therefore verified to below 20 ppt combined, which is under the MCL but is not a test to 4.0 ppt.

Which filter class fits?

Match the class to the certification it needs and to where the water is used. Every example below was opened on Amazon on September 12, 2026; the certification column is what the manufacturer or Pennsylvania DEP’s list states, not a measurement by this site.

Filter class	Certification to look for	Verified example	What it does not cover
Pitcher or dispenser	NSF/ANSI 53, PFOA/PFOS named, listed by IAPMO or WQA	ZeroWater 10-Cup : IAPMO listing names lead, PFOA/PFOS, chromium, mercury; 5-stage ion exchange. Brita Everyday Elite 10-Cup : Elite (OB06) filter, PFOA/PFOS under 53, no P473 claim	Only the water you pour. ZeroWater reviewers report short filter life on high-TDS water
Under-sink carbon block	NSF/ANSI 53 with PFOA/PFOS, ideally P473 as well	Aquasana Claryum 3-Stage Max Flow : Aquasana states certification to NSF/ANSI 42, 53, 401 and P473	One tap. The listed ASIN is a bundle with three spare cartridges
Under-sink reverse osmosis	NSF/ANSI 58, plus 42 and 53 for the carbon stages	Waterdrop G3P800 : IAPMO R&T to NSF/ANSI 58, 42, 53 and 372; 800 gallons per day, 3:1 pure-to-drain	The listing markets PFAS reduction through the RO membrane; no separate PFOA/PFOS ppt figure is broken out on the page. Needs a power outlet under the sink
Whole-house (point of entry)	NSF/ANSI 53 at the household flow rate	No ASIN verified. Aquasana states its OptimH2O whole-house unit reduces 98% of PFOA/PFOS	Rare and expensive; size it with the whole-house filter flow rate calculator before pricing it

Pennsylvania DEP’s list, dated August 26, 2026, is the shortest route to a certified under-sink unit: it names A.O. Smith, Brondell Coral UC300, Aquasana, Multipure, Amway eSpring and Kohler Aquifer alongside the pitchers, each with its certifying body.

On RO, read the listing twice. iSpring’s RCC7 family and the Waterdrop above are certified to NSF/ANSI 58 for the membrane’s dissolved-solids claim; their PFAS reduction is a manufacturer statement riding on the RO mechanism unless the certificate names PFOA/PFOS.

Before you buy anything

Do not buy on the strength of the word PFAS. Open the NSF, IAPMO or WQA listing for the exact model and find PFOA/PFOS or Total PFAS under NSF/ANSI 53 or 58; if the listing shows only 42 and 401, the filter has no PFAS claim. Then admit the flaw in the cheapest route: a ZeroWater pitcher carries the IAPMO PFOA/PFOS listing for the least money, and its ion-exchange filter is the one reviewers replace most often on high-TDS water.

What should you do first?

Get the PFOA and PFOS numbers: the utility’s monitoring result, or a laboratory PFAS panel on a private well.
Compare each to 4.0 ppt, and note the reporting limit if the result reads “not detected”.
Pick the class from the table by where the water is used: one pitcher, one tap, or the whole house.
Open the certification listing for the exact model and find PFOA/PFOS or Total PFAS under NSF/ANSI 53 or 58.
Replace cartridges on the interval in the certification listing, which names a rated capacity.

Frequently asked questions

Does a Brita or ZeroWater pitcher remove PFAS?

Only the pitchers whose own certification listing names PFOA/PFOS under NSF/ANSI 53. Pennsylvania DEP's August 2026 list of certified point-of-use units includes ZeroWater pitchers and dispensers in 10-, 22-, 32- and 40-cup sizes, and Brita Denali and Tahoe pitchers. A plain NSF/ANSI 42 pitcher is certified for chlorine taste and odour only, and the PFAS word on the box is not a certification.

What is the difference between NSF/ANSI 53 and NSF/ANSI 58 for PFAS?

NSF/ANSI 53 is the health-effects standard for carbon block, ion exchange and pitcher filters; PFOA/PFOS is one of its more than 50 reduction claims. NSF/ANSI 58 is the standard for reverse osmosis systems. NSF says filters certified to either standard with a PFAS claim are the ones validated to reduce PFAS, and both currently certify to a 20 parts per trillion effluent, not the EPA's 4.0 ppt.

Is the EPA limit for PFOA and PFOS still 4.0 ppt after the May 2026 proposal?

Yes. EPA's proposed compliance-extension rule, published in the Federal Register on May 20, 2026, states that it does not change the PFOA and PFOS MCLs, which remain 4.0 ppt each. It would move the compliance date from April 26, 2029 to April 26, 2031 for systems that request it. The comment period closed July 20, 2026, and the rule was not final on September 12, 2026.

Does NSF/ANSI 401 mean a filter removes PFAS?

No. NSF/ANSI 401 covers up to 15 emerging or incidental contaminants such as pharmaceuticals, pesticides and herbicides, and NSF describes it as a separate, additive certification. A filter certified to 42 and 401 alone has no PFAS claim. Look for NSF/ANSI 53 or 58 with PFOA/PFOS named in the listing, or NSF/ANSI P473, NSF's PFOA/PFOS protocol.

How do I find out whether my tap water has PFAS?

Ask the utility for its PFAS monitoring results, or search the state drinking water program's PFAS page for the system name. EPA's 2024 rule requires public water systems to complete initial monitoring and public notification within three years of publication, so by 2027 every system will have a reported PFOA and PFOS number. A private well has no report: order a laboratory PFAS panel.