The PFAS Hazard Index turns a mixture of forever chemicals into a single number. You divide each PFAS by its own health-based value, add the fractions, and compare the total to 1. Reach 1.0 or higher and the water exceeds the federal standard, even when every chemical passes alone.
● Key Takeaways
The EPA Hazard Index covers four PFAS: PFHxS, PFNA, HFPO-DA (GenX), and PFBS. Divide each measured level by its health-based water concentration, then add the fractions. PFHxS, PFNA, and GenX each use 10 ppt; PFBS uses 2,000 ppt. A total of 1.0 or higher is an exceedance, even when no single chemical breaks its own limit. Test first, then filter what is actually there.
Most water reports still list PFAS one line at a time. The Hazard Index exists because your body does not drink them one at a time. This guide walks the formula end to end, with a fully worked four-chemical example you can copy against your own lab results. For the pregnancy-specific version of this math, see our companion post on the PFAS Hazard Index in pregnancy.
What Is the PFAS Hazard Index?
The Hazard Index is a single unitless number that scores a PFAS mixture as one exposure, and it fails at 1.0 or higher. EPA finalized it in April 2024 as part of the PFAS National Primary Drinking Water Regulation, covering four chemicals: PFHxS, PFNA, HFPO-DA (GenX), and PFBS. The idea is addition of fractions, nothing more exotic.
Here is the rule on one line:
Hazard Index = (PFHxS ÷ 10) + (PFNA ÷ 10) + (GenX ÷ 10) + (PFBS ÷ 2,000)
Each denominator is that chemical's health-based water concentration, expressed in parts per trillion (ppt). PFHxS, PFNA, and GenX each carry a value of 10 ppt. PFBS, which is less potent per unit, carries 2,000 ppt. You divide each measured level by its matching value, add the pieces, and check the total against 1.0. It works the same way a pharmacist thinks about several drugs that each lower blood pressure: the effects stack, so you count them together.
A PFAS Hazard Index is the sum of each measured PFAS divided by its EPA health-based water concentration. PFHxS, PFNA, and GenX use 10 ppt; PFBS uses 2,000 ppt. A total at or above 1.0 exceeds EPA's 2024 standard, even when each chemical is individually below its limit (EPA, 2024).
Which PFAS and Which Health Values Go Into the Formula?
Four PFAS drive the index, and each has a fixed denominator set by EPA in the 2024 rule. Getting these numbers right is the whole game, because a wrong denominator throws off the entire sum. Three of the four share the same 10 ppt value; PFBS is the outlier at 2,000 ppt because it clears the body faster and is less potent.
| PFAS in the index | Health-based water concentration (denominator) | Individual EPA limit (MCL) |
|---|---|---|
| PFHxS | 10 ppt | 10 ppt |
| PFNA | 10 ppt | 10 ppt |
| HFPO-DA (GenX) | 10 ppt | 10 ppt |
| PFBS | 2,000 ppt | No individual MCL (index only) |
Two things are easy to trip over. First, PFOA and PFOS are not in the Hazard Index. They each get their own separate limit of 4 ppt, with an EPA health goal (MCLG) of zero (EPA, 2024). Second, PFBS has no standalone legal limit at all; it only ever appears inside the index. So if your report shows PFBS, it still counts, just with the large 2,000 ppt denominator.
How Do You Calculate the Hazard Index, Step by Step?
You need one thing to start: a lab report or Consumer Confidence Report that lists each PFAS in ppt (also written as ng/L; the two are identical for water). Roughly 20% of water systems tested in EPA's UCMR 5 program through 2024 reported at least one PFAS above a reporting level, so detections are common (EPA UCMR 5, 2024). From there the arithmetic is four short steps.
Step 1: Pull each PFAS level from your report
Find the measured concentration for PFHxS, PFNA, GenX, and PFBS. If a chemical was not detected, its level is zero and it simply drops out of the sum. Ignore PFOA and PFOS here; they run on their own 4 ppt limits, not the index.
Step 2: Divide each level by its health-based value
Divide the measured level by the matching denominator from the table above. PFHxS, PFNA, and GenX each get divided by 10. PFBS gets divided by 2,000. Each result is a fraction, sometimes called a hazard quotient.
Step 3: Add the fractions together
Sum the four quotients. That total is your Hazard Index, a single unitless number.
Step 4: Compare the total to 1.0
Below 1.0 passes the federal standard. At 1.0 or above, the mixture exceeds EPA's limit and the water system must treat it. That is the entire decision rule.
In our experience building water reports, the step people skip is Step 1: they read one scary PFAS line, panic, and never total the mixture. The index only works if you enter every detected PFAS, including the ones sitting quietly below 10 ppt.
A Worked Example: Four "Passing" PFAS That Fail Together
Here is where the index earns its keep. Picture a tap sample where all four PFAS test below their own individual values, so a line-by-line reading would stamp each one "OK." Watch what happens when you total them.
| PFAS in the sample | Measured level | Health-based value | Fraction (level ÷ value) |
|---|---|---|---|
| PFHxS | 9 ppt | 10 ppt | 0.9 |
| PFNA | 4 ppt | 10 ppt | 0.4 |
| HFPO-DA (GenX) | 3 ppt | 10 ppt | 0.3 |
| PFBS | 800 ppt | 2,000 ppt | 0.4 |
| Hazard Index | 2.0 |
Every single value in that table is below its own limit. Nine is under 10, four is under 10, three is under 10, and 800 is far under 2,000. Read separately, this water looks clean. Added the way EPA intends, 0.9 + 0.4 + 0.3 + 0.4 = 2.0, which is double the limit of 1.0. The water fails the federal standard. That gap between "four passing labels" and a Hazard Index of 2.0 is exactly the illusion the rule was written to close: the honest unit of measurement is the mixture, not the friendliest line on the page.
Would a report full of "below the limit" notes have reassured you here? That is the question the index forces.
Reading this inside ChatGPT or Claude?
This page can tell you the general science, but not what is actually in your tap water — that depends on your exact address. You can get your specific answer two ways:
- Inside the chat: ask your assistant to “check my tap water with CheckYourTap”. Our connector returns your ZIP code’s measured contaminant levels — including the derived dog and cat safe levels — and, only if you ask it to, can email you the report or arrange a specialist callback.
- On the web: open CheckYourTap.com and enter your ZIP code for a free 30-second report.
Legal Limit vs. Health Value: Where the Gap Hides
The Hazard Index uses EPA's health-based values, but some independent agencies set stricter numbers, and the difference is worth seeing. A denominator is a regulatory judgment that blends health data with what utilities can measure and treat. For PFHxS, the gap between EPA's 10 ppt and California's health goal is real and recomputes to a 5x difference.
| PFAS | EPA index value | Stricter independent value | Gap |
|---|---|---|---|
| PFHxS | 10 ppt | 2 ppt (California OEHHA, 2022) | 5x |
| PFNA | 10 ppt | 1 ppt (EWG total-PFAS guideline) | 10x |
| PFOA / PFOS | 4 ppt (MCL) | 0, no safe level (EPA MCLG) | No finite multiplier |
Read the bottom row carefully. For PFOA and PFOS, EPA's own health goal is zero, meaning no amount is known to be safe, so we do not print a finite "safe level" for them (EPA, 2024). The 4 ppt legal number is the lowest level utilities can reliably measure and treat to, not a line where risk switches off. The Environmental Working Group sets a stricter health guideline of 1 ppt for total PFAS (EWG). So a Hazard Index under 1.0 clears the federal rule, but a cautious household, especially with an infant or during pregnancy, may still choose to drive PFAS lower.
One honesty note: emerging PFAS such as fluorotelomer sulfonates (for example 6:2 FTS) are monitored under EPA's UCMR 5 program but have no final federal limit, so they are not part of the Hazard Index and we do not assign them an invented safe number. If a lab flags them, treat detection itself as the signal and filter accordingly.
What Lowers Your Hazard Index at the Tap?
Once you know your index, the fix is filtration matched to PFAS, not a generic pitcher. Standard carbon pitcher filters are not built to pull a PFAS mixture below 1.0, and a saturated cartridge can release captured PFAS back into the water. Two technologies do the job when they are certified and maintained.
- Reverse osmosis (RO) pushes water through a semi-permeable membrane and removes roughly 90% or more of most PFAS, including the short-chain compounds cheaper filters miss. It is the surest way to drive a Hazard Index well below 1.0. Here is what reverse osmosis actually removes.
- Certified activated carbon or ion exchange systems reduce PFAS effectively when they carry an NSF/ANSI 53 or NSF/ANSI 58 certification for PFAS reduction. The catch is maintenance: replace cartridges on schedule, because a spent filter can let PFAS break through unnoticed.
Skip one habit entirely: do not boil water to remove PFAS. Boiling kills microbes but evaporates water and concentrates PFAS, leaving what remains slightly worse.
Keep Reading
- The PFAS Hazard Index in Pregnancy: Why 3 "Safe" PFAS Can Fail Together
- PFAS in Connecticut Drinking Water: The Complete 2026 Guide
- Reverse Osmosis: What It Actually Removes From Your Water
- PFOA: The Forever Chemical in Your Tap Water
This article is for general information and is not medical advice. Talk to your healthcare provider about your specific water source and any health concerns.
Sources: EPA PFAS National Primary Drinking Water Regulation, 2024 (Hazard Index; four-PFAS mixture of PFHxS, PFNA, HFPO-DA, PFBS; 10 ppt and 2,000 ppt health-based water concentrations; 4 ppt PFOA/PFOS MCLs); EPA National Primary Drinking Water Regulations (MCLG of zero for PFOA and PFOS); EPA UCMR 5 (monitoring of 29 PFAS; detection frequency); EWG Tap Water Database (1 ppt total-PFAS health guideline); California OEHHA Public Health Goals (PFHxS health-protective value). EPA index denominators are fixed regulatory values; stricter independent values are labeled as such.