PFAS · CAS 1763-23-1
PFOS in Tap Water
PFOS is a PFAS "forever chemical" closely related to PFOA, historically used in stain- and water-repellent products and firefighting foam.
Substance identity
- Scientific name
- Perfluorooctanesulfonic acid
- Family
- PFAS
- CAS number
- 1763-23-1
This guide explains the compound. Only a water-system record or household test can tell you whether it appears in your water.
- Federal reference
- 4 ng/L (MCL); MCLG of 0Enforceable maximum contaminant level
- Health-based goal
- 4 ng/L (0.004 µg/L) enforceable federal limit (MCL), effective 2029US EPA PFAS National Primary Drinking Water Regulation (2024)
- Measurement context
- Stated in the recordReported here from EPA UCMR5 occurrence monitoring. A non-detect reflects the method reporting limit, not a guarantee of zero. PurityMap publishes this substance per water system on individual system and city-report pages, where the source dataset provides a value.
- Cities on record
- 3,737US cities in PurityMap's published record where PFOS appears
A reference is a published threshold, not a safety line drawn by PurityMap. Where a result sits against it is a finding; whether that constitutes a violation is a separately published compliance field.
What is PFOS?
PFOS is a PFAS "forever chemical" closely related to PFOA, historically used in stain- and water-repellent products and firefighting foam.
Also known as: forever chemical
Where PFOS comes from
Firefighting foam and industrial/consumer product manufacturing; highly persistent.
- Firefighting foam
- Industrial discharge
- Legacy stain-resistant treatments
Most affected: Infants, pregnant people, and immunocompromised individuals.
How PFOS is measured
Reported here from EPA UCMR5 occurrence monitoring. A non-detect reflects the method reporting limit, not a guarantee of zero. PurityMap publishes this substance per water system on individual system and city-report pages, where the source dataset provides a value.
Why context mattersHow PurityMap reads a PFOS result
Evidence exceeds a reference level. Used sparingly and only when warranted.
This substance was found in the monitoring record. Being present is not the same as being above a reference level, and this signal does not say which.
Somebody looked and the laboratory reported nothing above its reporting limit. This is a result, and it is not the same as nobody looking.
No sampling for this substance has been reported for this system. Nothing is known either way, and an empty row is not a clean one.
Where the numbers come from
- A laboratory result for one sampling point
Collected under a monitoring programme, analysed by an approved method, and reported with its own detection limits.
- Read against a published reference
The reference comes from the regulator, with its own effective date. PurityMap does not set thresholds and does not adjust them.
- Attached to a system's Water Profile
Where it contributes to the published assessment alongside the evidence quality of the record it came from.
The full evidence chain →
Where PFOS is found across the U.S.
These are the places in PurityMap’s published record where PFOS appears on a water system’s monitoring results. It reflects the systems and sampling we track, not a national prevalence estimate.
+39 more states on record. Open any state above to see its cities.
Treatment capability, if a finding establishes a need
The formats that commonly hold a verified reduction claim for PFOS are Reverse osmosis, Activated carbon (GAC), Anion exchange. Holding the format is not holding the claim — the certification record is what is checked.
Boiling does not remove a dissolved chemical compound and can concentrate it. Many pitcher and refrigerator filters carry no verified claim for a compound like this at all.
Whether a system-level measurement or a household-tap measurement is the relevant scope depends on how the compound enters the water — a published characteristic of the record, not an assumption drawn here.
A finding establishes the need, the need names the capability, the capability requires a verified certification. No product appears before that sequence completes.
Filter Finder →Frequently asked questions
What is PFOS?
PFOS is a PFAS "forever chemical" closely related to PFOA, historically used in stain- and water-repellent products and firefighting foam.
What is the limit for PFOS in drinking water?
The enforceable federal limit is 4 ng/L (MCL); MCLG of 0. The health-based goal is 4 ng/L (0.004 µg/L) enforceable federal limit (MCL), effective 2029. PurityMap reads every measurement against its stated benchmark and never sets its own.
Where does PFOS in tap water come from?
Firefighting foam, Industrial discharge, Legacy stain-resistant treatments.
Why does PFOS matter?
PFOS is linked in studies to immune, thyroid, liver, and developmental effects and to certain cancers. Like PFOA, it bioaccumulates, and the enforceable limit is set near the lowest measurable level.
How do you remove PFOS from water?
Treatment technologies commonly certified to reduce PFOS include Reverse osmosis, Activated carbon (GAC), Anion exchange. Match the exact certified claim to your water before buying — holding a format is not the same as holding the claim.
Where is PFOS found in the US?
In PurityMap's published record, PFOS appears in 3,737 cities across 51 states, with 656 above the federal limit. This is the scope of our record, not a national prevalence estimate.
Sources and methodology
Reference values on this page come from the governed contaminant record. Local occurrence claims stay on the linked water-system evidence pages, where sampling scope, period and source remain visible.
How PurityMap analyzes water data →