State Water Resources Control Board Affirms Intent to Adopt Maximum Contaminant Level for PFHxS

By Amilia Glikman, Lobbyist

Overview

On May 27, 2025, the State Water Resources Control Board (“State Board”) affirmed its intent to adopt a maximum contaminant level (MCL) for perfluorohexane sulfonic acid (PFHxS) in a written request asking the Office of Environmental Health Hazard Assessment (OEHHA) (“Request”) to develop a public health goal (PHG) for the contaminant.[1] In its Request, the State Board describes PFHxS as a “six-carbon fluorocarbon with a functional group that acts as an anionic surfactant that can be used in numerous commercial products to offer water- and stain-repellent properties and in fire-fighting foams,” with adverse growth and developmental impacts, particularly in fetuses, infants, and young children. In 2022, the State Board established a notification level for PFHxS of 3 parts per trillion (ppt) and a response level of 20 ppt based on recommendations from OEHHA.[2] In its Request, the State Board reported “1,609 sample points with detections of PFHxS at or above 2 ppt. Based on these results, 130 public water systems that serve a population of approximately 9.7  million people are impacted with PFHxS (highest detection per sample and per system).”

Background

The State Board’s Division of Drinking Water (DDW) foreshadowed the development of an MCL for PFHxS during a February 19, 2025 Board Meeting on the State Board’s 2025 drinking water priorities.[3] During the meeting, per- and polyfluoroalkyl substances (PFAS) were confirmed as the State Board’s highest priority drinking water standards for 2025,[4] and DDW discussed its efforts to develop information to support California-specific MCLs for PFAS. As one example, DDW noted its collaboration with the California Department of Public Health on the California Regional Exposure (CARE) Study on PFAS, which evaluated correlations between exposure to PFAS in drinking water and blood serum levels. According to DDW, study participants served by water systems that had PFHxS detections had 25 percent higher serum levels than study participants served by water systems with no detections of PFHxS.[5]

The CARE Study on PFAS involved approximately 879 participants, 54 percent of whom lived in the Southern California region.[6] The study found that PFAS were detected in over 99 percent of study participants, with, on average, seven different PFAS detected in each person. The study found that drinking water was significant source of exposure to perfluorooctanic acid (PFOA) and PFHxS, and that participants with measurable levels of these PFAS in their drinking water had higher corresponding blood levels.

CARE Study results were also analyzed in Environmental Science and Technology in May 2025, with the publication authors concluding that “drinking water remains an important source of exposure to PFOA and PFHxS in study participants. [7] “The largest effect for drinking water was observed for PFHxS, with participants linked to detectable levels of PFHxS in their PWS having 0.64 ng/mL (95% CI: 0.058, 1.23) higher levels of PFHxS in serum, compared to those without detectable levels in water.” The authors also found “significantly higher serum concentrations in participants residing in water service areas with detectable levels of PFOA and PFHxS compared to those residing in service areas without detectable levels (Table 2), suggesting that relatively low concentrations in drinking water can lead to elevated levels in serum.”[8]

What’s Next?

The timeline for OEHHA’s development of a PHG for PFHxS is unclear, as is the timeline for DDW’s development of a proposed MCL for PFHxS and the State Board’s adoption of an MCL. Further complicating matters are the politics surrounding the regulation of PFAS substances in drinking water. At the federal level, USEPA announced on May 14, 2025, that it intends to rescind and reconsider the federal MCL for PFHxS,[9] although it is unlikely that USEPA’s announcement will deter the State Board’s adoption of a California-specific MCL for PFHxS. At the state level, Assembly Bill (AB) 794 would require the State Board to adopt the current federal MCL of 10 ppt for PFHxS (and the other PFAS substances and mixtures for which federal MCLs presently exist) no later than December 31, 2026, through an expedited rulemaking process even if USEPA formally rescinds the federal MCL. Nothing in AB 794 would prohibit the State Board from subsequently adopting a more stringent MCL for PFHxS through a formal rulemaking process. The adoption of an MCL by the State Board of less than 10 ppt is highly likely given the current 3 ppt notification level for PFHxS.

Questions?

For questions about the State Board’s adoption of an MCL for PFHxS or the State Board’s process for adopting MCLs more generally, as well as OEHHA’s process for publishing PHGs, contact KP Public Affairs’ environmental regulatory advocacy professionals: Amilia Glikman (aglikman@ka-pow.com), Jeff Sickenger (jsickenger@ka-pow.com), and Olivia Wright (owright@ka-pow.com). For more information about Amilia, Jeff, and Olivia, visit KP Public Affairs’ website at https://www.kppublicaffairs.com/team.

[1] State Board, Notification Level Issuance (Oct. 31, 2022); see also State Board, Drinking Water Notification Levels.

[2] OEHHA recommended a notification level of 2 ppt based on the noncancer effects of PFHxS, the lowest level at which PFHxS could be reliably detected in drinking water using available and appropriate technologies. At the time, there were insufficient data to evaluate the cancer effects of PFHxS. See OEHHA, Notification Level Issuance, PFHxS (Mar. 2022).

[3] State Board. “State Water Resources Control Board Meeting” (Feb. 19, 2025). YouTube. https://www.youtube.com/watch?v=yS61Nr7wq90.

[4] State Board Resolution No. 2025-0006 (Feb. 19, 2025).

[5] According to Biomonitoring California, the CARE Study on PFAS involved approximately 879 participants, 54 percent of whom lived in the Southern California region. See Biomonitoring California, California Regional (CARE) Study for more information, including the CARE Study at a Glance and The California Regional Exposure (CARE) Study 2018 - 2020.

[6] See CARE Study at a Glance, supra, but see Pennoyer et al., infra, which notes 700 participants. 

[7] Pennoyer et al. (2025). Exposure to Legacy Per- and Polyfluoroalkyl Substances from Diet and Drinking Water in California Adults, 2018–2020. Environ. Sci. Technol. doi: 10.1021/acs.est.4c11872.

[8] The authors did question, however, whether these findings may be the result of the longer half-life for PFHxS, which is estimated to be five to eight years, as compared to other PFAS substances.

[9] On April 10, 2024, USEPA announced a federal MCL for PFHxS, as well as federal MCLs for five other PFAS substances and mixtures. See USEPA, Final PFAS National Primary Drinking Water Regulation. In its May 14, 2025 announcement, USEPA stated that it would retain the federal MCLs for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS), but rescind and reconsider the federal MCLs for PFHxS, perfluorononanoic acid (PFNA), hexafluoropropylene oxide dimer acid (HFPO-DA), and the Hazard Index mixture of these three PFAS and perfluorobutanesulfonic acid (PFBS).