PFAS Terminology and Nomenclature

Common PFAS terms
A word cloud containing some common PFAS terms. Made using Wordit.

Per- and polyfluoroalkyl substances (PFAS) are a ubiquitous emerging public health threat. PFAS terminology can be extremely confusing because it looks like an alphabet soup of similar acronyms. This article is the second of two on PFAS terminology. The first intended to provide an overview of PFAS structure (including what counts as PFAS), this one intends to help demystify PFAS terminology and nomenclature. These are part of a broader series on PFAS including their serendipitous discovery.


PFAS are a large chemical class generally produced in industrial processes which do not produce pure substances; often impurities are introduced to the end product. An isomer is a compound with the same chemical composition as another compound but a different chemical structure.  In the PFAS world the terms “branched” and “linear” refer to differing isomers. As a short hand, people often mean all isomers of a particular compound when they name it. PFAS can also be cyclic in which case the linear/branched nomenclature breaks down.

Linear and branched non-polymeric PFAS are typically composed of two parts: a hydrophobic as well as lipophobic fluorinated carbon backbone and a typically polar hydrophilic functional group.  The suffix philic means an affinity for and phobic means fear or aversion, the prefix hydro means water and lipo is for lipid or fat. 

Structural analogues are compounds similar to other compounds but differing in one aspect. Structural analogues are also known as congeners. An example could be homologues. PFAS with the same functional groups differing in the number of carbons and fluorine atoms in the backbone are known as homologue series. For example, perfluoroalkyl carboxylic acids with 4 fully saturated carbons in the backbone to 12 fully saturated carbons in the backbone would form a homologue series.  A homologue group includes all linear and branched carboxylic acids with the same number of carbons in the backbone. For example, the C8 carboxylic homologue group includes linear perfluoroalkyloctanoic acid (PFOA), isopropyl-PFOA, and 3-methyl-PFOA; there are 89 total theoretically possible C8 carboxylic homologue congeners although in addition to the linear form. The picture below shows some C8 carboxylic homologue congeners, while technically separate compounds for ease they are often referred to as a singular entity.

Examples of branched PFOA
Image courtesy of Battelle Memorial Institute particularly Craig Hutchings and Steve Helgen. While technically different compounds these are often considered together.

Naming Systems

There are 5 main systematic methods currently in use to name fluorinated compounds: the International Union for Pure and Applied Chemistry (IUPAC) nomenclature, the perfluoro nomenclature, code numbers, Chemical Abstract Service (CAS) nomenclature, and F-nomenclature systems. Due to strong similarities, fluorinated organic compounds tend to follow hydrocarbon naming conventions.

Basic Conventions

Many PFAS are acids and may exist as a protonated or anionic form as well as a mixture of both depending on the pH. The pKa values tend to be quite small and scientists generally refer to PFAS with acid functionalities as “acids” rather than carboxylates, sulfonates, or the correct terms even though the dissociated forms may be the only relevant forms. Scientists refer to both the protonated and ionized forms by the same acronyms.  Most perfluorinated acids are environmentally anions.  Some labs report results in the acidic form, some report anions, and some a mixture.  Many times, perfluorinated carboxylic acids are reported as acids (for example perfluorooctanoic acid instead of perfluorooctanoate) and perfluoroalkyl sulfonic acids as anions (for example perfluorooctane sulfonate instead of perfluorooctane sulfonic acid).  This has caused confusion.  The labs are really measuring the concentration of perfluorinated acid anions present; when the lab reports an acid the lab adjusts for the cation. Most sulfonate standards are prepared from salts and the mass adjusted which is why they are often reported that way.  This is especially true for PFHxS, PFOS, ADONA, 9Cl-PF3ONS and 11Cl-PF3OUdS. Section 7.2.3 of EPA Method 537.1 details how to convert to adjust for this.

IUPAC Nomenclature

IUPAC Nomenclature systematically establishes unambiguous, unique, uniform, and consistent compound names based on their chemical structure.  IUPAC Nomenclature can be used for all organic compounds and is not specific to PFAS.  IUPAC Nomenclature is quite literally a book containing many precedents and steps however, the general simplified steps for IUPAC Nomenclature are:

  1. Identify the longest carbon chain.  The longest carbon chain is also known as the “parent chain.”
  2. Identify any groups off the parent chain.
  3. Number the carbons in the parent chain so that the groups off the parent chain have the lowest number.
  4. Number and name the groups just counted.
  5. Put all the name components together in alphabetical order; prefixes such as di- are not used for alphabetical order.

For example, C2F4 is tetrafluoroethene, shown below, is the simplest perfluorinated alkene (fully saturated carbon backbone with no functional groups).

Picture from Wikidata. There are two carbons and a double bond so it is named “ethene.” Tetrafluoro comes from the 4 fluorine atoms attached to the carbons.

Perfluoro Nomenclature

Perfluoro Nomenclature only applied to perfluoroalkyl substances.  It is commonly used to quickly identify fully fluorine saturated carbon compounds. For shorter substances using IUPAC naming conventions is still easy and understandable. Most chemists would immediately understand that hexafluorobenzene (C6F6 – shown below) has a cyclic fully saturated carbon backbone. However, for longer compounds such as pentadecafluorooctanoic acid it is difficult to readily tell which is the motivation for Perfluoro Nomenclature.  The proper IUPAC name for that compound is actually: 2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctanoic acid which is fairly cumbersome to use further motivating Perfluoro Nomenclature.  The pentadecafluoro refers to the 15 fluorine atoms, octanoic acid refers to a carboxylic acid functional group that is off the 8th carbon in a chain.  This compound is pictured under hexafluorobenzene.

Picture from Wikimedia. Hexafluoro denotes the 6 fluorine atoms coming off the carbons in the benzine ring. By Buck et al’s or one of the OECD’s PFAS definitions this compound would not be included however, it would be under Glüge et al’s definition. The preferred IUPAC name for this compound is 1,2,3,4,5,6-hexafluorobenzene.
Pentadecafluorooctanoic acid
Pentadecafluorooctanoic acid courtesy of PubChem.

Perfluoro Nomenclature uses the prefix perfluoro along with standard hydrocarbon nomenclature. This simplified naming system has been in place since the 1950s and is the most common naming system among practitioners. The acronyms developed from this system follow a simple pattern; they all start with the prefix PF for perfluoro then have a root based on the carbon backbone, and a suffix denoting the functional group.  The table below gives the roots for C1-C14 and the two most common functional group abbreviations are A for carboxylic acids as well as S for sulfonic acids.

Carbon backbone length naming roots
Table of roots for specific carbon backbone lengths.
Carboxylic and sulfonic functional groups
The carboxylic acid (left) and sulfonic acid (right) functional groups. The R represents some organic chain.

The acronym itself provides a well-defined structure for the PFAS compound.  For instance perfluorobutanesulfonic acid or PFBS is shown below.  The front letters PF stand for per-fluoroalkyl, the B stands for but-, -ane shows there are only single bonds in the carbon backbone which is implied, and the S is for sulfonic acid.  PFBA, PFHpA, and PFDA are also shown.

Perfluorobutanesulfonic acid or PFBS
Perfluorobutanesulfonic acid or PFBS. The sulfonic acid functional group is circled in dashed yellow, the carbons are in red, numbered so that the first carbon is nearest the functional group. By convention, the unlabeled connectors are carbon atoms. Carbon always forms 4 total bonds, if 4 bonds are not shown the carbon by general agreement is bonded to a hydrogen.
Examples of PFAS acronyms
The table shows the appropriate root color coded, carboxylic acid groups are circled in dashed blue and sulfonic acid groups are circled in dashed yellow. PFBS is perfluorobutanesulfonic acid, PFHpA is perfluoroheptanoic acid, and PFDA is perfluorodecanoic acid. PFBA is perfluorobutanoic acid however, butanoic is sometimes replaced by butyric acid in older works.

Should it be necessary to refer to a specific salt for a perfluorinated compound it is customary to write the salt’s common abbreviation in front of the standard perfluorinated abbreviation then drop the acid from the name.  For instance, the ammonium salt of PFOA, ammonium perfluorooctanoate, is often abbreviated APFO. While not strictly included in the perfluorinated system, some polyfluoroalkyl compounds can degrade into perfluorinated compounds. A polyfluoroalkyl compound is one where the carbon backbone is not fully fluorinated. An important subset of polyfluoroalkyl substances are fluorotelomers. Fluorotelomers are substances made from the most common industrial process for perfluorinated compounds, telomerization. In telomerization a transfer agent, known as a telogen, reacts with a polymerizable taxogen (monomer) to produce a telomer.  Generally in commercial production, perfluoroethyl (pentafluoroethyl) iodide as a telogen reacts with tetrafluoroethylene oligomers as taxogens, most commonly tetrafluoroethylene, to produce telomer perfluoroalkyl iodide polymers.  The perfluoroalkyl iodide polymers are then converted into other substances.

Picture from the Big Chemical Encyclopedia showing pentafluoroethyl iodide reacting with tetrafluoroethylene oligomers to produce perfluoroalkyl iodide polymers.

Similar to perfluorinated acids, fluorotelomer substances are typically named for their functional group.  Fluorotelomers often are abbreviated FT followed by their functional group.  There are often numbers preceding the abbreviation such as 8:2. The first number represents the total perfluorinated carbon atoms and the second number the unsaturated carbons atoms.

8:2 FTOH
By convention the unlabeled connectors are carbon atoms. Carbon atoms always have 4 bounds and as this field was developed from hydrocarbon chemistry the hydrogens are typically not shown unless part of a functional group. The carbon atom labeled “2” in green for instance has two hydrogens attached to it. An alcohol group is a hydroxyl connected to an aliphatic carbon.

Code numbers

Systematic names are fantastic in precision but can be difficult to use.  Unless you work with compounds frequently, memorizing the prefix structures and extra rules, which were glossed over in this article can represent an annoying time commitment.  Additionally, it can be error prone especially to infrequent or new users.  Industrial PFAS manufacturers such as ICI Fluoropolymers, ISC, DuPont, and 3M developed inhouse shorthands for PFAS.  As the inhouse systems were not meant for the general public this initially caused confusion. In Organofluorine Chemistry dichlorodifluoromethane (CF2Cl2) is used as an example to illustrate this. Dichlorodifluoromethane was known as Freon-12 by DuPont, Arcton-6 by ICI, and Isceon-122 by ISC. By 1957 a standardized code number system based on DuPont’s system was adopted and formalized as The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 34 which was last updated in 2019. DuPont’s system was first developed by Albert Henne, Thomas Midgley, and Robert McNary in 1929.


Dichlorodifluoromethane (CF2Cl2), better known as Freon-12, was formerly a popular chlorofluorocarbon halomethane refrigerant now banned under the Montreal Protocol. It is now only allowed as a fire suppressant in submarines and aircraft. Picture from the Gas Encyclopedia by Air Liquide.

In the ASHRAE system for pure compounds, a letter designates the primary purpose of the compound, R for refrigerant, P for aerosol propellant, S for solvent, and the numbers are based on the chemical formula.  Sometimes the letter is replaced with a  trade name such as Freon.  For blends, numbers are assigned sequentially based on ASHRAE data review completion date. For an ASHRAE designation, the first digit from the right is the number of fluorine atoms.  The second digit from the right is the number of hydrogen atoms plus one.  The third digit is the number of carbon atoms minus one; if this is a zero then that number would be omitted. The fourth digit is the number of unsaturated carbon-carbon bonds, in saturated compounds this number is omitted. If an upper-case B or I appears the number immediately following it is the number of bromine or iodine atoms respectively. There are often lowercase letters following the numeric designations.  The first appended letter is for substitutions on the central carbon atom, an x represents a chlorine substitution, y for fluorine, and z for hydrogen. The second appended letter designates substitutions on the terminal methylene carbon, =CCl2 is -a, =CClF is -b, =CF2 is -c, =CHCl is -d, =CHF is -e, and =CH2 is -f. Isomers are also designated by a Z or an E appended to the name for “zusammen” or “entgegen” respectively. Zusammen is German for together and entgegen is opposite; they represent cis and trans isomers respectively. 

Cyclic compounds (R300 series), ethers, inorganic fluids (R700 series), and miscellaneous organic compounds (R600 series) have additional rules.  The series are specified so the 000 series are methane based compounds, 100 are ethane based, 200 are propane based, and 300 are cyclic. The 400 series are zeotropes and 500 are azeotropes. The 600 series are other organic compounds. The 700 series are inorganic refrigerants such as ammonia, also known as R-717. The 1000 series which are unsaturated organic compounds such as the new R-1234yf refrigerant. R-1234yf is used for about half of all automobiles built after 2018 and it breaks down into short chained perfluorinated carboxylic acids; specifically, trifluoroacetic acid.

2,3,3,3-Tetrafluoropropene also known as HFO-1234yf and R-1234yf.  It has a greenhouse gas (GHG) warming potential less than that of carbon dioxide. R-134a, which R-1234yf  helped replace, has a GHG warming potential of 1,430 as a comparison. Picture from Wikipedia.
The compound on the left is R-1234ye(E) and the compound on the right is R-1234ye(Z).
The compound on the left is R-1234ye(E) and the compound on the right is R-1234ye(Z).  The part which makes the compound an isomer is circled in dotted blue lines. R-1234 has other possibilities and chemical structures as well which can be further specified by the letters following the numbers. As an example R-1234yc is CH2FCF=CF2.

There are other code systems in use that are more field dependent as well.  For instance, chlorofluorocarbons have a numbering system as do halogenated fire extinguishers.  The Halon numbering system is in comparison very easy; there are always four numbers and left to right the digits represent carbon, fluorine, chlorine, and bromine atoms present in the molecule.  For instance, Halon-1211 (CF2ClBr also known as Freon-12B1) and Halon-1301 (CF3Br which would also be known as R-13B1).  On 1 January 1994 the United States banned the import and production of halons 1211 and 1301 under the Clean Air Act to comply with the Montreal Protocol. These halons are still allowed to be used and recycled halons may be purchased.  Both Halon 1211 and 1301 have Class A, B, and C ratings. Halon-1211 is a “streaming agent,” and more commonly used in hand-held extinguishers because it discharges mostly as a liquid stream.  Halon-1301 is a “flooding agent” and discharges mostly as a gas, allowing it to penetrate tight spaces and behind obstacles making it ideal for enclosed spaces commonly found in aircraft.  The United States owns about 40% of global halon-1301 supply and it will likely stay in use for many years worldwide.  Halon-1301 is critical as it was formerly the only material usable in enclosed spaces where humans were present. 

CAS nomenclature

CAS stands for Chemical Abstract Service (CAS) and it is a division of the American Chemical Society. A CAS number is a unique identifier assigned by CAS to every chemical substance. It includes isotopes, mixtures, alloys, and substances which have unknown or variable composition. CAS numbers do not contain information about the substance structure unlike all previously discussed naming systems, simplified molecular-input line-entry system (SMILES), or the IUPAC International Chemical Identifier (InChI) system.

The same PFAS chemical in different forms will have differing chemical properties because of structural differences.  For example, PFOS (perfluorooctanesulfonic acid) in its acid form (C8HF17O3S) has a CAS number of 1763-23-1, its potassium salt (C8F17KO3S) 2795-39-3, and its ammonium salt (C8H4F17NO3S) 29081-56-9.


F-nomenclature was accepted by the American Chemical Society as an alternative to perfluoro nomenclature because some early fluorochemistry pioneers disliked perfluoro nomenclature. JH Simons in particular disliked perfluoro nomenclature. Simons invented the electrochemical fluoridization processes which was the first industrial process for producing perfluorinated compounds.

In F-nomenclature perfluoropentanoic acid (PFPeA – a C5 fully saturated carboxylic acid) would be F-pentanoic acid. As an aside, C5H10O2 which is pentanoic acid, is also known as valeric acid because it is the cause of the unpleasant smell in valerian.

Picture of valerian by Ivar Leidus.  The unpleasant smell these flowers give off is caused by pentaonic acid.

Some elements of this naming system have survived.  For instance, the symbol -R which indicates an organic backbone sometimes is written -RF to indicate a perfluoro backbone. Also, placing an F internal to a benzene ring structure is a shorthand saying that all the bonds off the ring not otherwise indicated are fluorine instead of hydrogen.  A forerunner to F-nomenclature used a phi (φ) but was generally similar to F-nomenclature. 

Other less common nomenclature systems

In the 1970s, J.H. Simons, an early fluorochemistry pioneer, advocated for placing “for” before the final syllable in standard hydrocarbon nomenclature to convey it was perfluorinated. Organofluorine Chemistry gives the following examples: CF4 as methforane and the perfluorinated version of ethene, CF2=CF2 would be ethforene. 

Conclusion and Key Take Aways

Most PFAS naming systems are designed to reveal information about the basic chemical structure or function and to facilitate communication.  In industrial settings where not many PFAS are used simultaneously it can be easier to use codes to cut down on potential errors common in systematic names.  It is important to understand the basic names within your specific use.

This article is one of several on PFAS. You can read about PFAS discovery, structure, or all my articles on PFAS.


PFAS Structure

Common PFAS uses
This image shows some common uses of PFAS. Image courtesy of Carollo Engineers; particularly Cayla Cook and Eva Steinle-Darling.

Per- and polyfluoroalkyl substances (PFAS) are a ubiquitous emerging public health threat. PFAS terminology can be extremely confusing because it looks like an alphabet soup of similar acronyms. This article is the first of two on PFAS terminology. This one intends to provide an overview of PFAS structure, the second one intends to help demystify PFAS terminology and nomenclature. My previous article on PFAS was on Dr. Roy Plunkett’s discovery of PTFE, a tetrafluoroethylene fluoropolymer.


The EPA’s comptox database lists over 9,252 PFAS; this is too large a class to name on individual bases. A systematic naming convention has evolved so that much information is conveyed with just the name. There are 5 main ways to name fluorinated compounds: the perfluoro nomenclature, F-nomenclature, code numbers, the International Union for Pure and Applied Chemistry (IUPAC) nomenclature, and Chemical Abstract Service (CAS) nomenclature systems. Additionally, fluorine chemistry as a field has existed long before those five systems were created and naming was not really standardized until Buck et al tried to harmonize conventions in 2011. The idea of this article is to provide an overview of common PFAS groupings, the second article will go into the five naming conventions.


PFAS is a general nonspecific name encompassing a group of substances.  There is no clear agreed upon definition of what exactly counts as PFAS.  The Organization of Economically Developed Countries (OECD), the United States Environmental Protection Agency (EPA), and the World Health Organization (WHO) have all used various definitions.  In the paper meant to harmonize conventions, Buck defined PFAS as “highly fluorinated aliphatic substances that contain 1 or more carbon atoms on which all the hydrogen substituents (present in the nonfluorinated analogues from which they are notionally derived) have been replaced by F atoms, in such a manner that they contain the perfluoroalkyl moiety CnF2n+1–.” This definition was the first clear PFAS definition. A moiety means part of a molecule. The OECD/United Nations Environment Program (UNEP) pointed out that Buck’s definition left out many substances commonly recognized as PFAS such as perfluorodicarboxylic acids and recommended the –CnF2n– moiety instead, although there is still debate surrounding this. Buck’s definition was updated by Glüge et al and informed the OECD/UNEP definition.  Glüge’s definition expands Buck’s to cover perfluorocarbon chains with functional groups on both ends, aromatic substances with perfluoroalkyl moieties on side chains, and fluorinated cycloaliphatic substances.

Historical Note

As mentioned in the PFAS discovery article, PFAS were initially investigated to replace sulfur dioxide, methyl chloride, ammonia, and other extremely dangerous early refrigerants. This may jog your memory; the first major public PFAS concerns were mainly focused on perfluorocarbons under the 1987 Montreal and 1997 Kyoto Protocols.  The Montreal Protocol aimed to limit chlorofluorocarbons.  The Kyoto Protocol aimed to limit 7 greenhouse gas (GHG) categories and stated that climate change is real and driven by anthropogenic emissions. The GHG categories included perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs).

Kyoto Protocol countries by agreement status
Photo courtesy of Wikimedia Commons Green: Annex B parties with binding targets in the second period, Purple: Annex B parties with binding targets in the first period but not the second, Blue: non-Annex B parties without binding targets, Yellow: Annex B parties with binding targets in the first period but which withdrew from the Protocol, Orange: Signatories to the Protocol that have not ratified, Red: Other UN member states and observers that are not party to the Protocol.

Unfortunately, early 2000 environmental scientists adopted the PFC acronym for PFAS instead of the perfluorocarbons identified in the Kyoto Protocol. There is still quite some confusion caused by this. While PFC is used in turn of the millennium PFAS literature it should be avoided now.

General Organofluorine Concepts

Carbon (C) bonded to fluorine (F) is the class basis for PFAS.  The C-F bond is extremely persistent and the key factor for the problems PFAS causes as well as its desirable technical properties. Carbon can form up to 4 bonds with other atoms.  The first broad distinction is between polymers and non-polymers. A polymer is a molecule made up of repeating subunits called monomers. Polymers often serve as the feedstock to produce other PFAS chemicals, the most widely recognized fluoropolymer is polytetrafluoroethylene (PTFE) which Dr. Roy Plunkett discovered on 6 April 1938.

Structure of PTFE repeating monomer
Picture courtesy of Wikipedia Commons. This depicts PTFE. The brackets show the monomer tetrafluoroethylene, the n symbolizes that this is a repeating structure.

Functional groups often serve as the basis for classification systems. A functional group is a moiety made of a specific structure which causes characteristic chemical reactions which are similar no matter what the rest of the molecular composition is. PFAS chemicals can also exist in multiple states such as acids, anions, cations, and salts which have important implications for their physical and chemical properties. Anionic forms are the environmentally prevalent form and the general form most authors refer to. Some legislation, particularly European Registration, Evaluation, Authorisation, and Restriction of Chemicals (REACH), defines substance to include impurities and stabilizers of the main PFAS compound.

Broad non-polymeric structure

Non-polymeric PFAS include perfluorinated and polyfluorinated substances.  When carbon is bound only to fluorine except for functional groups it is called perfluorinated and the compound is saturated with F. When some carbon bonds include things other than functional groups or fluorine it is called polyfluorinated and is considered unsaturated with respect to F; hydrogen is the most common other bond. As PFAS stands for “per- and polyfluoroalkyl substances” and no single compound can be both unsaturated and saturated with respect to fluorine simultaneously “PFAS” is by default a plural acronym.  Both per- and poly-fluoroalkyl substances are subdivided into classes where the functional groups behave relatively similarly.  Two large classes are fluoroalkyl acids and fluoroalkane sulfonamides. These large classes are then subdivided again based on functional groups/moieties.  For example, perfluoroalkyl acids are further subdivided into carboxylic, sulfonic, sulfinic, phosphoric, phosphinic, and other acid types.  There are about 425 common moiety-based subdivisions within PFAS.

PFAS categorizations
Picture courtesy of The Interstate Technology Regulatory Council. This photo goes over some broad PFAS categories; while only 5 moiety based groups are shown here there are currently over 425 acknowledged nonpolymer moiety classification groups.

In 2017, Wang et al published an iconic chart covering the number of papers published on various PFAS groups reproduced below.

Number of peer reviewed articles between 2002 and 2016 for various PFAS.
Picture from Wang et al. This shows some broad helpful PFAS classifications. PFCA stands for perfluorinated carboxylic acids, PFSA for perfluorinated sulfonic acids, PFPA for perfluorinated phosphoric acids, PFPiA for perfluorinated phosphinic acids, PFECA for perfluoropolyether carboxylic acids, PFESA for perfluoropolyether sulfonic acids, and PASF for perfluoroalkane sulfonyl fluorides. Examples of groups left off this classification include perfluoroalkyl iodides (PFAI) and many others. The articles mentioned cover 2002 to 2016.

Conclusion and Key Take Aways

There is no clear definition on what PFAS encompass, even within regulatory organizations. There are about 9,252 PFAS substances which are broadly divided into polymeric and non-polymeric substances.  Non-polymeric PFAS subclasses are based on carbon chain saturation with fluorine into full saturated (perfluoroalkyl) or non-fully saturated (polyfluoroalkyl) groups. Those groups are subdivided based on moieties/functional groups so that compounds which behave similarly are classified together. An upcoming article will cover the five common classification systems for individual compounds.

Other articles in the series include PFAS Discovery

Further Reading on PFAS classifications