PFAS in Drinking Water May Be Underreported: What Homeowners Should Do
PFAS in U.S. drinking water may be underestimated when results below laboratory reporting limits are counted as zero. An October 2026 study found a substantial gap between that approach and statistical estimates. For homeowners, the useful next step is to check the exact PFAS results, reporting limits and sampling dates for their water supply.
PFAS stands for per- and polyfluoroalkyl substances, a family of manufactured chemicals often called “forever chemicals” because many persist in the environment.
Headlines about “twice as much” deserve a closer look. The research concerns how existing monitoring data are summarized. It gives homeowners a reason to ask better questions about a water report; it cannot measure the water coming from an individual kitchen faucet.
Published November 11, 2026 · Research published October 23, 2026. Hero image is an illustrative scene.
What did the new PFAS drinking-water study find?
Counting below-limit results as zero captured 44% of the primary model’s population-weighted estimate for 25 PFAS. The study, published in Cell Reports Sustainability, reanalyzed EPA monitoring data rather than collecting new household samples.
The study at a glance
- Dataset: EPA’s January 2026 UCMR 5 release, containing approximately 1.86 million PFAS test records.
- Main comparison: 25 PFAS across 9,991 community water systems, weighted by population served.
- Missing precision: About 98% of individual test results were below reporting limits.
- Headline result: The zero-substitution estimate was 44% of the primary modeled estimate.
Sources: TCU’s research summary and the original paper.
Expressed another way, 1 ÷ 0.44 is approximately 2.3 times. That calculation explains the headline, but it applies to a modeled aggregate across the study’s systems and chemicals. It is not a multiplier to apply to your home’s test result.
The magnitude changed with statistical assumptions, although the tested approaches consistently placed zero substitution below their estimates. The paper does not establish an individual household’s exposure, health risk or regulatory compliance. Those limits matter as much as the headline number.
Does “below the reporting limit” mean no PFAS?
A result below a reporting limit leaves the exact concentration unresolved. EPA’s UCMR 5 minimum reporting levels reflect laboratory capability; they are not health thresholds. A lab’s “ND” notation also needs its accompanying definition and detection or reporting limit.
| What you see | What to check |
|---|---|
| < MRL or a “less than” value | The reporting threshold for that compound. Keep the less-than symbol when recording the result. |
| ND / Not detected | The laboratory’s definition, method and applicable limit. |
| 0 in a summary average | Whether below-limit results were assigned zero during the calculation. |
EPA explains that its UCMR 5 comparison averages use zero for results below the minimum reporting level. If every included result is below that level, the displayed average can be zero. Read the underlying sample rows alongside the average. See EPA’s definitions and calculation notes.
A simple example: how an average becomes zero
Hypothetical example, not results from any community: Imagine four samples of one PFAS, each reported as “less than 4 ng/L.” Replacing all four with zero produces an average of zero.
One possible set of concentrations beneath that threshold would be 0.5, 1.0, 1.5 and 2.0 ng/L. Their average would be 1.25 ng/L. Other values are possible; the reports alone do not reveal which values are correct.
The example shows why “below a limit” and a measured zero are different. It is not a method for estimating your water’s PFAS level.
What should homeowners do after seeing this news?
Start with the water system that serves your address, then work toward a treatment decision using its actual results. A national finding cannot settle a local question.
Confirm your provider
Check your water bill for the utility name and public water system ID. Use Clearhouse’s local water reports to find relevant reports, then confirm the supplier. A county or ZIP code may contain more than one water system.
Ask for the latest PFAS sample results
Request individual compound names, concentrations, units, sample dates, sampling locations and reporting limits. Ask whether treatment has changed since the samples were collected. The national study used a January 2026 data release, so its snapshot may differ from your provider’s latest information.
Keep the units and result type together
Record whether the number is one sample, a range or an average. In drinking-water reporting, 1 ng/L is numerically equivalent to 1 part per trillion (ppt); 0.004 µg/L equals 4 ng/L. For help with the rest of the report, use our water-quality report reading guide.
Identify what is still unknown
If you have a private well, your neighbor’s municipal report does not characterize it. Follow EPA’s private-well guidance and ask your state environmental agency about laboratories offering PFAS testing, the compounds covered, reporting limits and sampling instructions. Start with our well-water testing checklist.
Match any treatment claim to the result
Write down the PFAS compounds you want to reduce and the taps you want treated. Then check the exact filter model’s evidence, operating conditions and replacement requirements. A useful shortlist starts with those details.
EPA recommends contacting your water provider and state environmental or health agency for local information and recommended actions. Its home-filter guidance is a useful starting point.
Is the reporting limit the same as the EPA drinking-water limit?
Reporting limits describe measurement capability; drinking-water standards serve a regulatory purpose. EPA lists maximum contaminant levels of 4.0 ppt each for PFOA and PFOS. Compliance is assessed using running annual averages at sampling points under the applicable monitoring requirements.
As checked November 11, 2026, EPA describes the 2031 extension for eligible PFOA/PFOS systems and changes to several other PFAS provisions as proposals. Use the current EPA regulation page for rule status. A research estimate or an isolated UCMR result does not by itself establish a violation.
Which water filters can reduce PFAS?
Activated carbon, specific ion-exchange resins and reverse osmosis can reduce PFAS when the system is designed and maintained for that purpose. EPA identifies these treatment approaches, but the technology name alone does not establish an individual product’s performance.
For a drinking-and-cooking goal, a filter at the kitchen tap may be worth evaluating. Whole-house treatment needs evidence for its larger flow, capacity and installation configuration. Our whole-house filtration vs. under-sink reverse osmosis guide explains how those treatment locations differ.
Use the certification directories linked by EPA. Check the exact model and its specific PFAS reduction claim under NSF/ANSI 53 or NSF/ANSI 58, along with the replacement element, rated flow, service life and test endpoint. A certification for another contaminant does not establish PFAS reduction.
A reduction certification also should not be expanded into a promise that every PFAS will be absent or that treated water will meet a particular concentration in every home. For a detailed verification walkthrough, read how to check a filter’s PFAS claims.
For Clearhouse shoppers: Evaluate PFAS separately from chlorine taste, sediment and hardness. Our standard whole-house filter should not be treated as a verified PFAS solution without evidence for the exact model and configuration. Bring your report and treatment goals into the selection process.
What should you ask your water provider?
Ask for the original results and the context needed to interpret them. You can copy this request:
Please share the latest PFAS results for the water system serving my address, including the system ID, compound names, sample dates, sampling locations, units and laboratory reporting limits. Please explain any ND or less-than entries and how below-limit values are handled in averages. Has treatment or the water source changed since sampling, and what actions do you recommend for customers?
Save the reply with your report. If a result remains unclear, ask the provider or laboratory to explain the specific row before comparing products.
Sources and methodology
Reviewed November 11, 2026. This guide draws on the original research, TCU’s summary and EPA guidance. Clearhouse did not collect new samples. The worked example is illustrative, and the 2.3-times figure is calculated from the study’s reported 44% comparison.
- Kharel and colleagues: Censoring-aware models quantify systematic underestimation of PFAS occurrence in US drinking-water monitoring, Cell Reports Sustainability, October 23, 2026.
- Texas Christian University: study summary, October 23, 2026.
- EPA: UCMR 5 Data Finder and definitions.
- EPA: PFAS drinking-water regulation and proposal status.
- EPA: PFAS in Private Wells.
- EPA: Reducing PFAS in Your Drinking Water with a Home Filter.
- EPA: Identifying Drinking Water Filters Certified to Reduce PFAS.