The Regulatory Shift That Elevated PFNA and PFHxS
For most of the history of PFAS regulation, PFOA and PFOS dominated both regulatory attention and filter certification protocols. These two long-chain compounds — perfluorooctanoic acid (C8) and perfluorooctane sulfonic acid (C8) — were the subject of the earliest health-effects studies, the first EPA advisory levels, and the initial NSF/ANSI 53 PFAS reduction category. PFNA (perfluorononanoic acid, C9) and PFHxS (perfluorohexane sulfonic acid, C6) received comparatively less regulatory focus until the accumulation of epidemiological data linking them to thyroid function disruption, immune suppression, and developmental effects prompted EPA to act.
The EPA 2024 National Primary Drinking Water Rule established individual MCLs of 10 ppt for both PFNA and PFHxS, placing them in a tier of regulated compounds that water systems must now monitor and control. This regulatory change has direct implications for filter certification because neither compound is currently within the scope of NSF/ANSI 53 testing. A filter system certified under NSF/ANSI 53 for PFAS has no verified performance data for PFNA or PFHxS, creating a compliance gap for any facility relying on point-of-use or point-of-entry devices to meet the 2024 MCLs.
Chemical Properties That Differentiate PFNA and PFHxS from PFOA and PFOS
PFNA, as a C9 perfluorocarboxylic acid, is one carbon chain length longer than PFOA. This additional fluorinated carbon increases its hydrophobicity and strengthens adsorptive interactions with activated carbon surfaces. From a removal standpoint, PFNA generally performs comparably to or slightly better than PFOA on GAC and ion-exchange media. However, at the low concentrations now regulated (10 ppt), accurate measurement requires analytical methods capable of detecting sub-ppt concentrations, and filter certification must demonstrate reduction to below the MCL from realistic influent concentrations.
PFHxS presents a more challenging profile. As a C6 perfluorosulfonic acid, it is shorter-chain than PFOS (C8) but retains the sulfonate functional group. Its log Koc value is measurably lower than PFOS, reflecting weaker adsorption affinity for carbon-based media. In field studies of GAC systems processing water with mixed PFAS profiles, PFHxS frequently shows earlier breakthrough than PFOS, sometimes appearing in finished water at detectable concentrations while PFOS remains well below detection limits. This differential breakthrough behavior is the core technical reason why PFHxS-specific certification data cannot be inferred from PFOS performance data.
KCL’s Role in Compound-Specific PFNA and PFHxS Testing
Korea Conformity Laboratories (KCL) has established compound-specific test protocols for PFNA and PFHxS that go beyond the current NSF/ANSI 53 scope. Korean filter manufacturers seeking to document performance for these two regulated compounds have worked with KCL to generate challenge-test data using EPA Method 533 analytical methods, which provide detection limits of approximately 0.3 ppt for ionic PFAS including PFHxS. This testing infrastructure positions KCL-certified products with explicit PFNA and PFHxS data as technically superior candidates for facilities that must demonstrate compliance with the 2024 EPA MCLs.
For engineers specifying filtration systems in the United States or in jurisdictions adopting equivalent PFAS standards, KCL test certificates listing compound-specific PFNA and PFHxS effluent concentrations provide the evidentiary basis that domestic NSF certification currently cannot. The analytical rigor of KCL’s approach — using EPA-referenced methods, standardized challenge matrices, and independent laboratory verification — is directly compatible with the documentation requirements of EPA compliance programs.
Implications for Filter Procurement Specifications
Water utilities and institutional procurement officers responding to the 2024 PFAS rule need to update their filter certification requirements to explicitly reference PFNA and PFHxS. A specification that reads “NSF/ANSI 53 certified for PFAS” does not address these two newly regulated compounds. A more complete specification would require: (1) NSF/ANSI 53 certification for PFOA and PFOS reduction; (2) independent third-party test data (KCL, WQA supplementary, or equivalent) demonstrating PFNA reduction below 10 ppt from a representative influent concentration; (3) equivalent data for PFHxS using EPA Method 533 analytics.
The broader lesson is that the regulatory PFAS compound list has expanded faster than certification standard revision cycles. Procurement officers who specify only existing certification marks will find themselves holding products with unverified performance for the newest regulated compounds. Compound-specific test data, regardless of which accredited body generated it, is the technically defensible specification anchor for the current regulatory environment.