EWG vs EPA: Why Their PFAS Numbers Don't Agree

If you’ve looked up your ZIP code on CheckPFAS and on the Environmental Working Group’s PFAS tap-water tracker, you may have noticed the numbers don’t always match. The same water utility can show different detected concentrations, different “above safe level” warnings, and sometimes wholly different sets of compounds in the two systems.

This is not an error in either dataset. The two organizations use different methodologies to answer slightly different questions. Knowing what each is doing helps you read both more clearly.

What CheckPFAS shows (and where it comes from)

CheckPFAS is built entirely on the EPA’s Fifth Unregulated Contaminant Monitoring Rule (UCMR 5) occurrence dataset — the EPA’s federally-mandated, EPA-validated PFAS testing program covering all US public water systems serving more than 25 people.

Two things define UCMR 5 results:

• Federal regulatory dataset. UCMR 5 is the dataset the EPA uses to set future MCLs. Every value in it was collected under EPA chain-of-custody protocol by labs certified to run EPA Method 537.1 or 533. The dataset is the legal-grade source.
• Highest single-sample concentration. What we display for a system is the maximum reported concentration of each compound across the sampling period (January 2023 – December 2025). It’s the worst-case value the EPA recorded, not an average.

The numbers we compare against are the EPA’s enforceable MCLs (4 ppt for PFOA / PFOS, 10 ppt for PFNA / PFHxS / HFPO-DA, last three under reconsideration as of May 2025). When CheckPFAS says “above EPA limit,” we mean above the federally-enforceable MCL.

What EWG shows (and where it comes from)

The Environmental Working Group has tracked drinking-water PFAS contamination since the early 2010s. EWG’s tracker draws on multiple data sources — UCMR 3 (2013–2015), UCMR 5 (2023–2025), independent testing programs, state-level monitoring, and citizen-collected samples — and applies EWG’s own “health guideline.”

Two things define EWG’s framing:

• Multi-source dataset. EWG aggregates data from many years and many programs. A 2014 detection from UCMR 3 sits next to a 2024 detection from UCMR 5 in the same tracker view. This makes EWG a stronger longitudinal record but a weaker snapshot of current conditions.
• EWG’s own health guideline, not the EPA’s MCL. EWG’s recommended safe level for PFOA + PFOS combined is 1 part per trillion — four times stricter than the EPA’s 4 ppt PFOA limit. EWG defends this level by citing the National Academies of Sciences clinical thresholds and arguing that the EPA’s MCL doesn’t adequately protect the most vulnerable populations.

When EWG says “above safe level,” they typically mean above their 1 ppt advisory — not above the EPA’s MCL. A water system at 3 ppt PFOA is below the EPA limit (4 ppt) but above EWG’s guideline (1 ppt). Both numbers can be correctly displayed; they’re answering different questions.

Why the same ZIP can show different numbers

Three common reasons your local utility shows different values across the two trackers:

1. Different sampling dates

UCMR 5 was the first comprehensive federal PFAS sampling in nearly a decade. Before that, UCMR 3 (2013–2015) was the most recent federal dataset. EWG’s tracker may show a utility’s UCMR 3 result (older, possibly higher when industrial sources were active) or a state-program result from another year, while CheckPFAS shows the 2023–2025 UCMR 5 result. Same utility, different time periods, different numbers.

2. Different compound lists

UCMR 5 monitored 29 PFAS compounds plus lithium. UCMR 3 monitored only 6 PFAS. State programs vary widely. If EWG’s tracker shows a utility with PFAS detections that CheckPFAS doesn’t, it may be because EWG is reporting a compound UCMR 5 didn’t include.

3. Different aggregation

CheckPFAS shows the maximum single-sample concentration. EWG sometimes shows reported peaks, sometimes averages, depending on the source. If a system had one bad sample and four clean ones, EWG’s display may smooth that; CheckPFAS will flag the peak.

Which one should you trust?

Both. For different questions.

• “Does my water exceed the federal legal limit?” Use CheckPFAS. That’s the EPA MCL question, and CheckPFAS sources from the EPA’s regulatory dataset.
• “Has there ever been PFAS detected at my utility?” Use EWG. They aggregate multi-year data and capture historical detections UCMR 5 alone won’t show.
• “Is the EPA limit strict enough?” That’s a policy question both organizations are explicit about. The EPA argues its MCL reflects achievable treatment performance balanced against health risk; EWG argues the EPA’s number is a compromise that under-protects vulnerable populations. Both arguments have merit; the science is genuinely contested at the margins.

Where this site lands

CheckPFAS makes one editorial choice in this debate: we report what the EPA’s data shows, against the EPA’s published limits, because those are the only enforceable standards. We don’t apply EWG’s tighter advisory level on the site — not because we think they’re wrong, but because mixing advisory guidelines with regulatory data confuses the question. If a reader wants the EWG view, EWG’s tracker is the right tool.

That said, several of EWG’s substantive points apply to our data too:

• “Not detected” doesn’t mean zero. UCMR 5’s method floors at ~1.5–2 ppt; values below that are invisible.
• The EPA’s MCLs are policy thresholds, not biological safety thresholds. Below an MCL doesn’t mean “safe”; it means “below the level at which the EPA requires utility action.”
• Vulnerable populations — pregnant women, infants, immunocompromised people — may reasonably choose to filter even at detections below the federal MCL.

For the audit-grade clinical guidance, we point readers to the National Academies’ 2022 PFAS clinical report and the ATSDR’s PFAS clinical resource, both of which provide thresholds based on blood PFAS rather than tap-water concentrations.

The bottom line

EWG and the EPA are not in conflict; they’re describing two different facts about your water. EWG describes what’s been detected over time, evaluated against an advocacy-set health guideline. The EPA describes what was detected during the most recent federal sampling, evaluated against the federally-enforceable legal limit. Both are correct. Both can coexist.

When the two show different numbers for your address, the discrepancy is information, not an error. Reading the two together gives you a richer picture than either alone.