Which PFAS compounds carry enforceable federal drinking water limits, and what are the numbers?
Six: perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorohexane sulfonic acid (PFHxS), perfluorononanoic acid (PFNA), hexafluoropropylene oxide dimer acid (HFPO-DA, marketed as the GenX chemicals), and mixtures of the last three together with perfluorobutane sulfonic acid (PFBS), which are regulated through a Hazard Index rather than through a concentration of their own. The Environmental Protection Agency promulgated all of it in the PFAS National Primary Drinking Water Regulation published at 89 FR 32532 on 26 April 2024, effective 25 June 2024.
The enforceable limits sit at 40 CFR 141.61(c), stated in milligrams per liter. PFOA and PFOS are each 0.0000040 mg/L, which is 4.0 nanograms per liter, usually written as 4.0 parts per trillion. PFHxS, PFNA and HFPO-DA are each 0.00001 mg/L, which is 10 ng/L. The Hazard Index entry for the mixture of HFPO-DA, PFBS, PFHxS and PFNA is “1 (unitless).” PFBS carries no individual maximum contaminant level; it enters the rule only through the mixture calculation.
The non-enforceable health goals are separate and worth keeping distinct in any document that quotes a number. 40 CFR 141.50(a) sets the maximum contaminant level goal for PFOA and PFOS at zero; 40 CFR 141.50(b) sets it at 0.00001 mg/L each for HFPO-DA, PFHxS and PFNA and at “1 (unitless)” for the Hazard Index. A goal is a health target; the obligation is the maximum contaminant level.
One caution about vocabulary. The 2024 rule is a drinking water regulation under the Safe Drinking Water Act, governing public water systems. It is not the Environmental Protection Agency’s separate designation of PFOA and PFOS as hazardous substances under the Comprehensive Environmental Response, Compensation, and Liability Act, 89 FR 39124 (8 May 2024), whose cleanup consequences are covered at CERCLA liability and cost recovery.
How does the PFAS Hazard Index work?
The Hazard Index is an arithmetic sum of four ratios, and a system exceeds it when the sum is greater than 1. 40 CFR 141.61(c) states the calculation in the regulation itself: “Hazard Index = ([HFPO-DAwater ng/l]/[10 ng/l]) + ([PFBSwater ng/l]/[2000 ng/l]) + ([PFNAwater ng/l]/[10 ng/l]) + ([PFHxSwater ng/l]/[10 ng/l]).” Each denominator is a health-based water concentration: 10 ng/L for HFPO-DA, PFNA and PFHxS, and 2000 ng/L for PFBS.
Two consequences follow. A system can violate the Hazard Index without exceeding any individual maximum contaminant level, because three compounds at four to five nanograms per liter apiece sum past 1 while each sits below its own 10 ng/L limit. And PFBS matters only in the mixture: its 2000 ng/L denominator means it contributes little at the concentrations usually reported, but omitting it produces the wrong number.
The Hazard Index is also the part of the 2024 rule with the least settled future. It is among the provisions the Environmental Protection Agency proposed to rescind on 20 May 2026 at 91 FR 29413, and among those covered by the pending petitions in American Water Works Association v. EPA, No. 24-1188 (D.C. Cir.). Any exceedance analysis built on it should state the version of the rule relied on and the date. Whether the calculation survives is for the agency on the rulemaking record and for the D.C. Circuit on the petitions.
When do public water systems have to monitor for PFAS and comply with the limits?
Initial monitoring results are due to the state by 26 April 2027, and compliance with the maximum contaminant levels is required by 26 April 2029. Both dates are in 40 CFR 141.900(b), the deadline provision of 40 CFR part 141 subpart Z, the PFAS-specific subpart the 2024 rule created. It requires each system to report the results of initial monitoring to the state by 26 April 2027 and to meet the compliance monitoring requirements of 40 CFR 141.902 by the same date, and to meet the maximum contaminant level compliance requirements of 40 CFR 141.903 by 26 April 2029.
The rule reaches community water systems and non-transient non-community water systems. In the 20 May 2026 proposal at 91 FR 29425 the Environmental Protection Agency described the affected universe as the systems subject to the 2024 rule, together with the state and Tribal agencies responsible for drinking water regulatory development and enforcement, and quoted the definition of a public water system at 40 CFR 141.2 as one serving water for human consumption through pipes or other constructed conveyances with “at least fifteen service connections or regularly serves an average of at least twenty-five individuals daily at least 60 days out of the year.”
Private wells are outside all of it. The Safe Drinking Water Act regulates public water systems, so a household on its own well has no federal monitoring obligation and no federal limit applying to its water, whatever the sampling shows. In community matters that is usually why private well data exists only where somebody paid for it. What sampling has to satisfy before it is usable as evidence is covered at sampling and data quality.
What did the Environmental Protection Agency change after the 2024 PFAS rule was published?
Nothing yet, as a matter of enforceable law. The agency announced an intention and then published two proposals. In a news release dated 14 May 2025 it said “the agency will keep the current National Primary Drinking Water Regulations (NPDWR) for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS),” said it “plans to develop a rulemaking to provide additional time for compliance, including a proposal to extend the compliance date to 2031,” and announced “its intent to rescind the regulations and reconsider the regulatory determinations for PFHxS, PFNA, HFPO-DA (commonly known as GenX), and the Hazard Index mixture of these three plus PFBS.” A stated intent to propose is not a change to any obligation.
The first reached the Federal Register on 20 May 2026 at 91 FR 29413, docket EPA-HQ-OW-2025-0654. Its summary states that the agency “is proposing to rescind its regulatory determinations to regulate four per- and polyfluoroalkyl substances (PFAS) … under the Safe Drinking Water Act,” together with “all associated regulatory provisions currently codified in the EPA’s 2024 PFAS National Primary Drinking Water Regulations (NPDWR) exclusive to these PFAS.” The stated ground is procedural rather than toxicological: the action “is necessary to correct the unlawful procedure under which these regulations were promulgated,” because the agency “proposed and finalized regulatory determinations and regulations for these PFAS simultaneously and in tandem,” which it now reads Safe Drinking Water Act section 1412(b)(1)(E) to forbid.
The second, published the same day at 91 FR 29425, docket EPA-HQ-OW-2025-1742, is narrower than the shorthand suggests, because it does not move the deadline for everyone. It proposes “a federal exemption pursuant to SDWA 1416, including 1416(f) and 1450(a)(1), that would extend the compliance deadlines for the MCLs for PFOA and PFOS from April 26, 2029, to April 26, 2031, for systems that apply,” structured as an “exemption by rule” for eligible systems in states, territories and Tribes that have not obtained primacy for subpart Z, each of which must request it and submit system-specific information. The proposal states that it “strictly deals with providing more time to comply with the PFOA and PFOS MCL standards; the underlying MCLs and health-based MCLGs remain unchanged.”
Comments on both closed on 20 July 2026, and the agency held a virtual public hearing on each proposal on 7 July 2026. As of 15 September 2026 neither proposal has been published as a final rule and the Code of Federal Regulations still carries the 2024 values. Quoting a proposed number as though it were the standard, or saying the deadline is 2031, describes a state of the law that does not exist.
Which parts of the PFAS drinking water rule are being litigated, and which are settled?
The 2024 rule is under review in the D.C. Circuit, and the Environmental Protection Agency said so in its own rulemaking document. The rescission proposal at 91 FR 29413 states: “The EPA’s final regulatory determinations and portions of the EPA’s NPDWR issued April 26, 2024 are the subject of pending litigation. Am. Water Works Ass’n, et al. v. EPA, et al., No. 24-1188 (D.C. Cir.).” Which provisions survive is for that court to decide.
What is settled should be stated narrowly. The PFOA and PFOS maximum contaminant levels of 0.0000040 mg/L are in the Code of Federal Regulations, the agency has said it intends to keep them, and it did not propose to change them in May 2026; the subpart Z dates are in force as written. Unsettled: the PFHxS, PFNA, HFPO-DA and Hazard Index package, facing both the rescission proposal and the pending petitions, and the 2029 date, which the agency proposes to make subject to a two-year exemption on application.
A separate PFAS rule went the other way in the same period, and the two should not be conflated. In Chamber of Commerce of the United States v. Environmental Protection Agency, No. 24-1193, argued 20 January 2026 and decided 18 August 2026, the D.C. Circuit denied the petitions challenging the designation of PFOA and PFOS as hazardous substances under the Comprehensive Environmental Response, Compensation, and Liability Act, 89 FR 39124 (8 May 2024): seven interest groups “representing industries that use, transport, or discharge PFOA and PFOS petitioned this court to overturn EPA’s decision. We deny those petitions.” The court added that the statutory finding that a release may present “substantial danger” has “real bite,” because “[t]he risk of harm must be both serious and real, not hypothesized.” That is a cleanup and reporting statute, not the drinking water limits.
One sentence of the same opinion is frequently quoted out of position. Describing the agency’s record, the panel wrote that “EPA based its determination on extensive studies that link PFOA and PFOS exposure to serious health conditions in humans, including cancer and developmental delays.” That is a court summarizing the basis of an agency rule. It is not a finding that any exposure caused any individual’s disease, and it does not do the work that general causation and specific causation require in a tort case. A health question about one’s own exposure belongs with a physician, not with a rule.
Can a state set a different PFAS drinking water limit, and does exceeding a limit prove harm?
A state can set a stricter limit; it cannot go below the federal floor while holding primary enforcement responsibility. The Safe Drinking Water Act conditions primacy on the state having “adopted drinking water regulations that are no less stringent than the national primary drinking water regulations promulgated by the Administrator” (42 U.S.C. 300g-2(a)(1)). Where the federal position contracts, the operative number in a given state may therefore be the state’s, and it may predate the 2024 federal rule. Name the state and cite its code; one state’s limit is not a national limit.
Primacy also explains the shape of the pending extension proposal. At 91 FR 29425 the agency noted that Safe Drinking Water Act sections 1416(a) and (b) authorize primacy states to grant exemptions, and that section 1416(f) gives the Administrator the same authority “under the same conditions and in the same manner” where a state has not obtained primacy. The proposed exemption by rule is aimed at non-primacy jurisdictions; in a primacy state the path to additional time is the state’s.
An exceedance is not proof that anyone was injured. A maximum contaminant level is an obligation imposed on a water system, not a threshold at which disease begins and not a finding about any individual. Exceeding it can establish a violation, trigger notification and monitoring obligations, and justify investigation; it does not establish exposure to a particular person, a dose, or medical causation. How regulatory numbers are properly offered, and the ways they are misused, is covered at regulatory standards as evidence, and the exposure side at exposure assessment.
Two omissions here are deliberate. This page states no view on whether any water system, manufacturer or defendant is responsible for any contamination, and it quantifies no loss: what a contaminated supply, a treatment obligation or a property is worth belongs to our Economic Damages Institute. Background on persistence, analytical methods and the AFFF docket sits at PFAS and AFFF. Every figure, citation and date above was read in a primary source on 15 September 2026 and describes the position on that date only.