PFAS Hazard Index: How the EPA Mixture Limit Works, and Its Status

PFAS rarely show up alone. The Hazard Index is the 2024 rule’s way of limiting a mix of four of them, even when each one on its own is under its individual limit. It only matters for you if your water report or test lists PFHxS, PFNA, HFPO-DA or PFBS. PFOA and PFOS aren’t part of it; they have their own 4.0 ppt limits.

How is the Hazard Index calculated?

Divide each PFAS level by its health-based water concentration (the level below which no harm is expected over a lifetime), then add the results. The EPA’s Hazard Index fact sheet works an example: one quarter’s sample with 5 ppt HFPO-DA (5 ÷ 10 = 0.5), 5 ppt PFBS (5 ÷ 2,000 = 0.0025) and 3 ppt PFHxS (3 ÷ 10 = 0.3) scores about 0.8. Compliance uses the average of four quarterly scores; in that example, the average was 0.7, under the limit of 1.

Why use an index instead of separate limits?

Because effects can add up. The EPA says research shows chemical mixtures can have additive health effects even when each chemical is at a lower level. PFBS has no individual limit at all: the 2024 rule notes it hasn’t been seen above its 2,000 ppt level on its own, but it often occurs alongside the others. In the EPA’s national UCMR 5 monitoring, 66% of sampling locations with any PFAS detected had more than one.

Is the Hazard Index still in effect?

Yes, for now. On May 18, 2026, the EPA announced a proposal to rescind the limits and health goals for PFHxS, PFNA, HFPO-DA and the Hazard Index mixture, published in the Federal Register on May 20. The EPA’s stated reason is procedural: it says the decisions to regulate these PFAS and the limits themselves were issued at the same time, which it now reads the Safe Drinking Water Act as not allowing. Comments closed on July 20, 2026. As of October 10, 2026, the EPA still describes it as a proposed rule and we found no final rule in the Federal Register, so the 2024 limits, with compliance due in 2029, still stand. Parts of the 2024 rule are also being challenged in court.

What should I do with my numbers?

Start with your utility’s results. The EPA’s home filter guidance says that if PFAS were measured below federal limits, a filter may not be useful to you. Our PFAS guide covers how to check and when a certified filter makes sense, and our news story tracks the proposal.