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4:2 FTS

4:2 Fluorotelomer Sulfonic Acid

3,3,4,4,5,5,6,6,6-nonafluorohexane-1-sulfonic acid

Also known as 4:2 FTS4:2 FTSA
4:2 Fluorotelomer sulfonic acid1H,1H,2H,2H-Perfluorohexanesulfonic acid3,3,4,4,5,5,6,6,6-nonafluorohexane-1-sulfonic acid4-2 fluorotelomer sulfonic acidFtS 4:2 fluorotelomer sulfonic acid 4:2FtSA 4:2 fluorotelomer sulfonic acid 4:21H,1H,2H,2H-Perfluorohexanesulphonic acidFluorotelomer sulfonate 4:2

A short-chain fluorotelomer sulfonic acid (4 perfluorinated + 2 alkyl carbons), formed as a degradation product of fluorotelomer-based aqueous film-forming foam (AFFF). Detected at low ppt levels in U.S. drinking water under UCMR 5.

Group
PFAS
Regulatory status
UCMR
Unregulated Contaminant Monitoring Rule
Top removal tech
Activated carbon

Primary concern: Immune and liver toxicity

Affected organ systems

Liver Immune Developmental Endocrine Affecting the body's hormone system — thyroid, adrenal, reproductive, or metabolic hormones. Thyroid

Health effects

Immunotoxin Suppresses or alters the immune system. Can increase risk of infection or reduce vaccine effectiveness. Endocrine disruptor Mimics or blocks the body's hormones (estrogen, thyroid, testosterone). Can affect development, reproduction, and metabolism at very low doses. Developmental toxin Causes harm during development before birth or in early childhood — birth defects, low birth weight, delayed growth, or learning problems later in life. Hepatotoxin Damages the liver. Effects range from elevated liver enzymes on a blood test to fatty liver disease and, with prolonged exposure, liver cancer.

Exposure routes

Drinking water Dietary Occupational In utero

Vulnerable populations

Pregnant Infants Children

Overview

4:2 Fluorotelomer sulfonic acid (4:2 FTS or 4:2 FTSA) is a short-chain fluorotelomer sulfonic acid consisting of a perfluorinated four-carbon tail (C4F9) bonded to a two-carbon CH2CH2- spacer terminating in a sulfonic acid head group. It belongs to the broader class of fluorotelomer sulfonates (FTSs) that arose as PFAS chemistry shifted away from long-chain compounds like PFOS in the early 2000s. 4:2 FTS rarely appears as an intentional commercial product; instead, it forms in the environment as a degradation intermediate of fluorotelomer-based aqueous film-forming foam (AFFF) and other fluorotelomer raw materials. EPA added it to UCMR 5 in 2021 to characterize occurrence in U.S. drinking water.

Pollution sources

Military AFFF Aqueous Film-Forming Foam — firefighting foam used at military bases and airports for fuel fires. The leading source of PFAS contamination in groundwater near former training sites. Industry Manufacturing

Sources

4:2 FTS enters drinking water primarily through historical use of fluorotelomer-based AFFF at military airfields, civilian airports, oil refineries, and fire-training facilities. Under aerobic and anaerobic soil conditions, the parent fluorotelomer thioether amido sulfonate (FtTAoS) and related precursors slowly biotransform into 4:2 FTS and other fluorotelomer sulfonate chain lengths. UCMR 5 detections so far cluster around AFFF source areas in Florida (Pensacola), Massachusetts (Canton), and New Hampshire (Jaffrey). Because FTSs are mobile in groundwater and resist further degradation, they migrate readily into source-water wells and surface intakes downstream of release sites.

Health risks

Toxicological data specific to 4:2 FTS are limited compared to PFOA and PFOS. The compound has not been formally classified as a carcinogen. However, structurally similar short-chain perfluoroalkyl sulfonates have been linked in animal and human studies to hepatotoxicity, thyroid hormone disruption, immune suppression, and developmental effects. Fluorotelomer sulfonates can also undergo further metabolic or environmental transformation into longer-lived perfluoroalkyl carboxylic acids, complicating risk assessment. 4:2 FTS is monitored under UCMR 5 but is not currently subject to a federal MCL.

Effective treatments

Activated carbon Granular or block carbon that traps organic contaminants as water flows through. The most common point-of-use filter media — handles chlorine taste, VOCs, pesticides, and many PFAS. Reverse osmosis A semipermeable membrane that pushes water through under pressure, leaving most dissolved contaminants behind. Removes a very broad range — PFAS, lead, arsenic, nitrate, salts — at the cost of slower flow and some wastewater.

Does not remove

UV Ultraviolet light that disrupts the DNA of bacteria, viruses, and protozoa as water flows past the lamp. Disinfects but doesn't remove chemicals or particles. Chlorination Adds chlorine or hypochlorite to kill bacteria and viruses. The most common disinfection method in US water systems; leaves a residual that keeps water protected through the distribution pipes. Boil Kills bacteria, viruses, and protozoa after a one-minute rolling boil. Does NOT remove chemicals like lead, nitrate, or PFAS — boiling actually concentrates them as water evaporates.

NSF certifications

NSF/ANSI 53 Health-related contaminants NSF/ANSI 58 Reverse osmosis systems

Water treatment

4:2 FTS is removed by granular activated carbon (GAC), anion exchange resin, and high-pressure membranes including reverse osmosis and nanofiltration, though short-chain fluorotelomers tend to break through GAC media earlier than their long-chain counterparts. Anion exchange resins selective for short-chain PFAS offer better long-term performance for this class. Conventional treatment processes — coagulation, sand filtration, disinfection, UV, and ozonation — do not remove 4:2 FTS. Boiling concentrates rather than removes it. NSF/ANSI 53 and NSF P473 certify point-of-use filters for PFAS reduction; NSF/ANSI 58 covers RO systems.

Gallery

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Chemistry + classification data sourced from public databases (PubChem, EPA CompTox, IARC monographs); 3D molecular models from 3d.nih.gov where available.