How CheckPFAS works

01Data source

All contamination data on CheckPFAS comes from the EPA's Fifth Unregulated Contaminant Monitoring Rule (UCMR 5) occurrence dataset. This is a publicly available federal dataset that the EPA is required to release under the Safe Drinking Water Act. We do not conduct our own water testing.

• Testing period: January 2023 – December 2025
• Data released: January 2026
• Systems covered: ~10,000 public water systems serving more than 25 people
• Compounds tested: 29 PFAS compounds plus lithium
• Measurement type: Highest single-sample concentration at system entry points (LIMS/distribution entry point)

View the raw EPA UCMR 5 dataset ↗

02ZIP code mapping

The EPA UCMR 5 dataset is organized by Public Water System ID (PWSID) — not by ZIP code. We map each PWSID to the ZIP codes it serves using the EPA's Safe Drinking Water Information System (SDWIS) service area data, which records the geographic areas served by each utility.

This mapping is imperfect. Some ZIP codes span multiple water systems; some water systems serve partial ZIP codes. Where a ZIP is served by more than one system, we report all systems and display the highest risk level across any of them.

What's not covered: Private wells are not regulated under UCMR 5 and are not included in our data. Very small systems serving fewer than 25 people were not required to participate. Absence of data does not mean absence of contamination.

03Risk levels

Risk levels are computed per water system, then aggregated across systems for each ZIP:

Risk level thresholds reflect EPA's April 2024 MCL rulemaking. The PFOA and PFOS limits are stable; EPA announced a reconsideration of the four 10 ppt limits (PFNA, PFHxS, HFPO-DA, PFBS) in May 2025. We will update thresholds if and when revised MCLs are finalized.

04Filter recommendations

Filter recommendations displayed on HIGH and MODERATE risk ZIP pages are drawn from our independently maintained product database. Products are evaluated on:

• NSF/ANSI certification status (NSF 58 for RO systems, NSF 53/P473 for carbon block)
• Certified PFAS removal rate from manufacturer testing and independent lab data
• Price-to-performance value and 5-year total cost of ownership
• Installation effort and household suitability

Some filter links are affiliate links (primarily Amazon Associates). Affiliate relationships do not influence which products are recommended or how they are ranked. See our Affiliate Disclosure and full filter reviews for details.

05Build process & update frequency

CheckPFAS is a statically generated site built with Astro. All ~14,000 ZIP code pages and ~50 state pages are pre-rendered at build time from the data files. There is no live database — the published site is a snapshot of the data at the time of the most recent build.

The UCMR 5 dataset was released by the EPA in January 2026 and covers testing through December 2025. The EPA does not currently publish ongoing UCMR 5 updates; when the EPA releases revised or additional UCMR 5 data, we will rebuild and redeploy the site within 30 days of the release.

06Known limitations

• Results reflect entry-point measurements at the treatment plant. Aging distribution pipes, building plumbing, and on-site storage tanks can add contamination between the plant and your tap.
• The minimum reporting limit (MRL) for most PFAS is ~1.5–2 ppt. "Not detected" means below this floor, not literally zero.
• ~98% of all UCMR 5 test results came back below the detection limit — the dataset is pre-filtered to systems with detections on our ZIP result pages, but "NONE" ZIPs had their systems tested and came back clean.
• Not all water systems were required to participate (small systems, tribal systems, territories). Absence of a ZIP from our database does not mean its water is safe.
• Bottled water is regulated by the FDA, not the EPA, and is not covered by UCMR 5.