PFAS Hazard Index

The PFAS Hazard Index (HI) is a unitless ratio that measures the combined risk of four PFAS compounds in drinking water; when it reaches 1.0 or higher, the mixture exceeds a federal MCL and the system is in violation.

Where you'll see it on a CCR

The EPA's PFAS National Primary Drinking Water Regulation (89 FR 32532, April 26, 2024) established six MCLs: individual limits for PFOA, PFOS, PFNA, PFHxS, and HFPO-DA, plus the Hazard Index for mixtures. If a system detects any combination of the four HI compounds — PFHxS, PFNA, HFPO-DA, and PFBS — it must calculate and report the HI value. An HI at or above 1.0 is an MCL violation under 40 CFR § 141.61(c)(2) and must appear on the CCR as an exceedance. Initial compliance monitoring is required by April 26, 2027; MCLs become enforceable April 26, 2029.

The four PFAS in the Hazard Index

| Compound | Also known as | Individual MCL | |---|---|---| | PFHxS | Perfluorohexane sulfonic acid | 10 ng/L | | PFNA | Perfluorononanoic acid | 10 ng/L | | HFPO-DA | Hexafluoropropylene oxide dimer acid (GenX Chemicals) | 10 ng/L | | PFBS | Perfluorobutane sulfonic acid | None — HI only |

All four have assigned Health-Based Water Concentrations (HBWCs), which are the denominators in the formula. For PFHxS, PFNA, and HFPO-DA the HBWC matches the individual MCL. PFBS has no individual MCL, so its HBWC (2,000 ng/L) is used only in the HI calculation.

How the Hazard Index is calculated

Divide each compound's measured concentration by its HBWC, then sum the four fractions:

HI = (PFHxS / 10) + (PFNA / 10) + (HFPO-DA / 10) + (PFBS / 2,000)

All concentrations are in nanograms per liter (ng/L). The result is unitless. A value below 1.0 means the mixture is within the safe level. A value at or above 1.0 is an MCL violation.

Example: a system measures PFHxS at 5 ng/L, PFNA at 4 ng/L, HFPO-DA at 0 ng/L, and PFBS at 500 ng/L.

HI = (5/10) + (4/10) + (0/10) + (500/2000)
   = 0.50 + 0.40 + 0.00 + 0.25
   = 1.15  →  MCL violation

No single compound exceeds its individual limit, but together they exceed the mixture threshold.

Why PFBS only appears in the HI

EPA evaluated PFBS and determined its individual health risk at environmentally relevant concentrations is low enough that an individual MCL is not warranted. Its HBWC of 2,000 ng/L — 200 times higher than the limit set for PFHxS, PFNA, and HFPO-DA — reflects that lower relative potency. However, PFBS can still contribute meaningfully to mixture risk when the other three compounds are present near their individual limits. Including it in the HI captures that additive effect without overstating its standalone hazard.

Related terms

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