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Proposed Use of a Hazard Index for PFAS National Primary Drinking Water Regulation (NPDWR)

Jenny Phillips & Faith Morse | April 4, 2023

Preliminary Regulatory Determination and Proposed Rule

On March 29, 2023, The United States Environmental Protection Agency (EPA) proposed a National Primary Drinking Water Regulation (NPDWR) for six PFAS:

Proposed National Primary Drinking Water Regulation (NPDWR) Values
PFAS Compound Non-enforceable MCLG  (ppt) Enforceable MCL (ppt) HBWC (ppt)
PFOS Perfluorooctanesulfonic acid 0 4 N/A
PFOA Perfluorooctanoic acid 0 4 N/A
PFNA Perfluorononanoic acid  




PFHxS Perfluorohexanesulfonic acid 9
HFPO-DA (Gen X) Hexafluoro- propylene oxide dimer acid 10
PFBS Perfluorobutane sulfonic acid 2000
HBWC = Health-Based Water Concentration
HI = Noncancer Hazard Index = ∑ (concentration/HBWC)
MCL = Maximum Contaminant Level
MCLG = Maximum Contaminant Level Goal
N/A = Not Applicable; HBWC used in HI calculation

The EPA is proposing to establish non-enforceable health-based Maximum Contaminant Level Goals (MCLGs) and enforceable Maximum Contaminant Levels (MCLs) for these compounds. The proposal includes regulation of PFOA and PFOS as individual chemicals. The remaining four PFAS—PFNA, PFHxS, GenX and PFBS—are proposed to be regulated using a Hazard Index (HI) approach. The remainder of this post will focus on the HI approach as proposed. Click here for an overview of the proposed rule. Public comments are due May 30, 2023. 

Why a Hazard Index?

The Proposed MCL and MCLG for the four PFAS—PFNA, PFHxS, GenX and PFBS—considers their toxicity as additive. The EPA has proposed a HI of 1.0 as the MCL and MCLG for the four PFAS combined[1]. PFOS and PFOA are not proposed to be included in the HI as EPA has indicated they are both likely carcinogens.

The HI is a common approach used in risk assessment to evaluate the potentially increased noncarcinogenic risks associated with additive effects of chemical mixtures, and is determined through the equation shown below. When calculating the HI, a ratio known as the hazard quotient (HQ) is used. The HQ is calculated for each of the four PFAS—PFNA, PFHxS, GenX and PFBS—by dividing the concentration detected by an exposure metric (the health-based water concentration [HBWC] for the specific PFAS). The individual HQs for the four PFAS are summed to yield the HI. While a routine component of risk assessment, it is not a common approach for setting MCLs; in fact this is the first time it has been proposed.

Where GenXwater = monitored concentration of GenX

PFBSwater = monitored concentration of PFBS

PFNAwater = monitored concentration of PFNA

PFHxSwater = monitored concentration of PFHxS

For example, if the mixture contains the following levels of these four PFAS, the Hazard Index for that mixture would exceed the proposed MCL.

EPA stated in the Preliminary Regulatory Determination and Proposed Rule, that the HI is “a reasonable approach for estimating the potential aggregate health hazards associated with the occurrence of chemical mixtures in environmental media.” (USEPA, 2022). The Federal Register notice indicates the following toxicological endpoints:

Chemical Toxicological Endpoint per EPA (2023)
PFNA Developmental effects in mice. This is the ATSDR human equivalent dose (HED). Uncertainty factor (UF) of 300 applied to define a chronic reference dose (RfD) of 0.000003 milligrams per kilogram per day (mg/kg/day). The HBWC is 10 ppt.
PFHxS Thyroid effects in male rats. The reported no-observed-adverse-effect level (NOAEL) of 1 mg/kg/day was adjusted to calculate a HED. UF of 3000 applied to define a chronic RfD of 0.000002 mg/kg/day. The HBWC is 9 ppt.
PFBS Thyroid effect following gestational exposure of mice to the potassium salt of PFBS. UF of 300 applied to define a chronic RfD of 0.0003 mg/kg/day. The HBWC is 2000 ppt, and is consistent with the 2022 Health Advisory.
GenX Liver effects in female mice. Composite UF of 3000 applied to define a chronic RfD of 0.000003 mg/kg/day. The HBWC is 10 ppt, and is consistent with the 2022 Health Advisory.

As shown above, the selected toxicity endpoints are not identical (with exception of PFHxS and PFBs); however, EPA has stated, “PFHxS, HFPO-DA, PFNA, and PFBS, individually and in a mixture, may cause adverse human health effects on several biological systems including the endocrine, cardiovascular, developmental, immune, and hepatic systems”. EPA indicated that the HI approach is appropriate “because of the known and additive toxic effect and occurrence and likely co-occurrence in drinking water.” This approach differs from EPA risk assessment where the HI is typically summed across similar  toxicological endpoints. Further EPA states “given this high occurrence and co-occurrence likelihood and that adverse health effects arise as a result of both individual and PFAS mixtures” as justification for inclusion of the four noncarcinogenic PFAS compounds.

Additionally, in EPA’s evaluation they considered use of a relative potency factor (RPF) approach which has been used for other chemical classes like polynuclear aromatic hydrocarbons (PAHs), but chose the HI approach as it “provides the most health protective endpoint for multiple PFAS in a mixture to ensure there would be no known or anticipated adverse effects on the health of persons”. EPA is requesting comment on the “merits and drawbacks” of the target-specific HI or RPF approach.

What do Drinking Water Systems Need to Do?

To determine the HI for these four PFAS, water systems would monitor and use the sampling results as inputs into the formula above. Water systems would use a calculator tool provided by EPA to easily determine their HI result. The tool performs the calculation for summing the HQ for each of the four PFAS.

Trigger Levels

Additionally, EPA is proposing use of a trigger level for the six PFAS compounds for which MCLs have been proposed. For the HI MCL approach for PFHxS, Genx, PFNA and PFBS, the EPA is proposing that if the HI is 0.33 or less (aka the trigger level), compliance monitoring could be less frequent (once or twice every three years, dependent on the size of the population served). EPA is requesting comment on establishing the trigger level at other levels for the HI, specifically 0.5. Additionally, EPA is requesting comment on the use of a sampling waiver up to nine years if (1) after one year of quarterly sampling, the HI is below rule trigger level or (2) when reduced monitoring is in place, the HI is below the trigger level after at least two consecutive three-year compliance period samples.

EPA is proposing that compliance will be determined based on the analytical results obtained at each entry point to the distribution system.

Some Considerations with the HI Approach

  • While commonly used in the assessment of noncarcinogenic risk, the use of an HI approach has not been used before as the basis for an MCL.
  • EPA is taking the stance that PFAS compounds will be evaluated cumulatively for noncarcinogenic PFAS; will it be their intention to add other PFAS to this HI?
  • In our experience, we do not often see the co-occurrence of GenX with the other three PFAS compounds used in the HI calculation (PFHxS, PFNA, PFBS).
  • The EPA has stated that a calculator will be available to every facility that is sampling PFAS; many facilities do not have staff that are comfortable with risk-based assessments and therefore, added cost for consulting may need to be borne by the facility.
  • Communication of this approach to the public could be challenging.

How You Can Prepare

  • Submit comments on the proposed rule by May 30, 2023 (within the 60-day comment period).
  • Develop a plan for how your entity will come into compliance with the rule.
  • There will likely be litigation associated with this proposed rule which will potentially delay the compliance dates – be aware of the timeline.
  • Consider conducting public outreach if your entity is affected by this proposed rule.

Overview of the Proposed MCLGs and MCLs for PFAS

Learn more about the final regulatory determination for contaminants on the Fourth Contaminant Candidate List. Our experts break down the criteria put forth to regulate PFOS and PFOA in drinking water and the implications of this proposed rule.

Read the Full Article

[1] The HI in this approach does not include the commonly utilized risk-based assumption of one significant figure. For this proposed approach, a HI of 1.2 would exceed the goal of 1 and be considered an exceedance. EPA is requesting comment on this approach of considering two significant figures (i.e., 1.2) instead of one significant figure (1.0).

Although the individual concentrations of the four PFAS may be below their respective HBWC, the summation of the HQs may result in an exceedance of the HI MCL or MCLG.

[2] USEPA. 2022. Transmittal of the Science Advisory Board Report titled, ‘‘Review of EPA’s Analyses to Support EPA’s National Primary Drinking Water Rulemaking for PFAS.’’ EPA–22–008. Available on the internet at:

Jenny Phillips, DABT, Director of Technical Development, VP

Jenny Phillips, Director of Technical Development, VP – Jenny Phillips leads the Technical Development Unit of TRC, focusing on Emerging Contaminants. She is an expert in human health and ecological risk assessment with a focus in risk communication and stakeholder discussions. Ms. Phillips also leads TRC’s Center for Research and Expertise which includes 30+ technical teams. Contact Ms. Phillips at

Faith Morse

Faith is an Environmental Intern working on TRC’s environmental risk team. She has experience with environmental fieldwork, laboratory research and risk evaluations. In college, she conducted research focusing on the use of biochar as an emerging remedial technology for contaminated marine environments. As a recent graduate from Western Washington University, she holds a bachelor’s degree in Environmental Science with an emphasis in toxicology. Faith can be reached at

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