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Does “Not Detected” Mean Your Water Is PFAS-Free?

A water report that says “not detected” can sound like an all-clear. For PFAS, that conclusion goes further than the result supports. A new study shows how treating results below laboratory reporting limits as zero can make national drinking-water estimates look lower than a statistical model suggests.

The quick answer

A “not detected” PFAS result does not prove your water is PFAS-free. Check the report’s definition: a result below a reporting limit means the test did not produce a reportable concentration at or above that threshold. It does not establish zero, but it also does not prove a harmful concentration is present.

Three things to know

  • A reporting limit reflects measurement and reporting capability, not a health-based safety threshold.
  • The new findings estimate concentrations below reporting limits. They are not new measurements of those concentrations.
  • A national estimate cannot tell you the concentration in your own glass of water.

What does “below the reporting limit” actually mean?

The EPA’s UCMR 5 monitoring program collected drinking-water results for 29 PFAS and lithium. For each PFAS, the program set a minimum reporting level, or MRL, to make results from different laboratories more consistent.

A result marked “< MRL” falls below that reporting threshold. It is not the same as a measured zero. The EPA also notes that individual laboratories may be able to measure concentrations below the program’s MRL.

Just as importantly, the EPA says these reporting limits are not based on health-effects information. A reporting limit and a health-based limit answer different questions: what can be reliably reported, and what concentration is considered acceptable under a particular health or regulatory framework?

This distinction does not mean a utility is hiding results. The uncertainty is recorded in the qualifier. The problem comes when a later calculation replaces that uncertainty with zero.

What the new PFAS study found

In a study published October 23 in Cell Reports Sustainability, Gehendra Kharel and colleagues analyzed 1.86 million PFAS measurement records covering 9,991 community water systems.

Instead of assigning zero to every below-limit result, the researchers used statistical models to estimate possible concentrations in that unmeasured range. Their main analysis covered 25 PFAS with data supporting the estimates, not every chemical in the PFAS family.

Under their main model, treating below-limit results as zero captured only 44% of the population-weighted estimated concentration across those 25 PFAS. For the six PFAS included in the EPA’s 2024 drinking-water regulatory framework, it captured 50%.

Those figures compare two ways of summarizing the monitoring data. They are not percentages of unsafe homes, contaminated people, or missed health effects.

The researchers compared several statistical approaches. Zero substitution produced lower estimates across accepted comparison models, but the size of the gap depended on the model used. Their deposited analysis notes also flag that some chemicals had very few detected results, making the assumptions used to estimate the rest especially important. The direction of the finding is more persuasive than treating one headline number as an exact national truth.

Does that mean your water has twice as much PFAS?

No. The Phys.org coverage, supplied by Cell Press, describes drinking water as potentially containing double the reported PFAS levels. That is a national modeling result, not a correction factor you can apply to your water report.

The study did not retest every below-limit sample with a more sensitive laboratory method. It cannot establish the actual concentration in each sample, prove that every below-limit sample contains PFAS, or determine your household’s health risk.

The paper’s public summary is explicit that the analysis does not determine regulatory compliance or health risk. It is about interpreting monitoring data, not diagnosing the safety of a particular tap.

What you can do with your own water report

Start with local information rather than the national headline.

Look for the actual qualifier. Is the result a number, “not detected,” or “below the reporting limit”? Read the report’s explanation rather than assuming these labels are interchangeable.

Ask which PFAS were tested and at what limits. A result describes the chemicals and thresholds covered by that test. It is not a guarantee about the entire PFAS family. Your utility can explain its results and whether newer sampling or treatment information is available.

If you’re considering filtration, check the exact claim. The EPA recommends looking for independently certified PFAS-reduction claims. Verify the specific model, replacement cartridge, and chemicals covered. A general certification badge does not establish every contaminant-reduction claim.

Follow replacement and maintenance instructions. Certification supports a defined performance claim, not a promise of PFAS-free water under every condition. This study did not test household filters or endorse a product.

PFAS and microplastics are different problems

PFAS are a family of chemicals; microplastics are plastic particles. A filter’s evidence for one does not automatically establish performance for the other. If you want to address both, check both claims separately. Our guide to filtering microplastics from drinking water covers the particle side of that decision.

Our take

“Not detected” is useful information, but it needs its reporting limit beside it. This study gives a good reason to avoid treating an uncertain low-level result as a known zero. It does not give us a reason to assume every glass of tap water is unsafe.

For your kitchen, the useful next step is a clearer understanding of your local results, followed by a treatment decision based on specific evidence if one is warranted.

Reviewed October 24, 2026. This article explains research and water-report terminology; it is not a household water-safety assessment.

Sources

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