EPA legal limit
10 ppt
Maximum Contaminant Level
EWG health guideline
1 ppt
Science-based, stricter target
Health effects
PFHxS is associated with thyroid hormone disruption, immune suppression, and developmental effects. It accumulates in the blood and thyroid gland and has a longer biological half-life than shorter-chain PFAS (estimated 7–8 years in humans). Animal studies show effects on thyroid function, liver, and the immune system. It is part of the EPA Hazard Index rule alongside PFNA, HFPO-DA, and PFHpA.
Where it comes from
Legacy and ongoing industrial use as a PFOS replacement in firefighting foam (AFFF), metal plating, textile treatments, and some food packaging. Military installations and airports with AFFF use are major contamination sources. Also found downstream from industrial manufacturing.
How it's regulated
EPA MCL: 10 ppt (effective April 2024 under PFAS NPDWR). Note: EPA announced in May 2025 a proposal to rescind the PFHxS individual MCL while retaining the PFOA and PFOS MCLs. The 10 ppt limit remains in effect as of 2025 but is under regulatory review. EWG health guideline: 1 ppt.
The EPA vs EWG gap
The legal limit (10 ppt) is 10× higher than the EWG health guideline (1 ppt). Water can be legally compliant while still exceeding the science-based threshold.
How to filter pfhxs
Not all filters address pfhxs. Look for independently certified filters — NSF International certification means the removal claim has been independently verified.
PFAS including PFHxS are not removed by water ionization. Reverse osmosis (NSF 58 certified) is the most reliable removal method.
Frequently asked questions
What is pfhxs?
PFHxS is a six-carbon sulfonic acid PFAS compound used as a replacement for PFOS in industrial applications including firefighting foam, metal plating, and surface treatments. It is one of the most widely detected PFAS in US drinking water, found in systems serving 71 million people across 46 states.
What are the health effects of pfhxs?
PFHxS is associated with thyroid hormone disruption, immune suppression, and developmental effects. It accumulates in the blood and thyroid gland and has a longer biological half-life than shorter-chain PFAS (estimated 7–8 years in humans). Animal studies show effects on thyroid function, liver, and the immune system. It is part of the EPA Hazard Index rule alongside PFNA, HFPO-DA, and PFHpA.
Is pfhxs regulated in drinking water?
EPA MCL: 10 ppt (effective April 2024 under PFAS NPDWR). Note: EPA announced in May 2025 a proposal to rescind the PFHxS individual MCL while retaining the PFOA and PFOS MCLs. The 10 ppt limit remains in effect as of 2025 but is under regulatory review. EWG health guideline: 1 ppt.
Where does pfhxs come from?
Legacy and ongoing industrial use as a PFOS replacement in firefighting foam (AFFF), metal plating, textile treatments, and some food packaging. Military installations and airports with AFFF use are major contamination sources. Also found downstream from industrial manufacturing.
How do I remove pfhxs from tap water?
The most effective methods for removing pfhxs are: Reverse osmosis (RO), NSF 58 certified RO system, Activated carbon block filter. Look for NSF-certified systems — independent certification confirms removal claims have been verified.
Is pfhxs in your water?
Enter your ZIP code to see the measured level in your specific utility.