Organisation for Economic Co-operation and Development

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ENV/CBC/MONO(2021)25
English - Or. English
9 July 2021

ENVIRONMENT DIRECTORATECHEMICALS AND BIOTECHNOLOGY COMMITTEE

Reconciling Terminology of the Universe of Per- and Polyfluoroalkyl Substances:
Recommendations and Practical Guidance
Series on Risk Management
No.61

JT03479350
OFDE

This document, as well as any data and map included herein, are without prejudice to the status of or sovereignty over any territory, to thedelimitation of international frontiers and boundaries and to the name of any territory, city or area.

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ENV/CBC/MONO(2021)25  3OECD Environment, Health and Safety Publications
Series on Risk Management
No. 61

Reconciling Terminology of the Universe ofPer- and Polyfluoroalkyl Substances: Recommendations andPractical Guidance

Environment Directorate
ORGANISATION FOR ECONOMIC COOPERATION AND DEVELOPMENT
Paris 2021

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About the OECDThe Organisation for Economic Co-operation and Development (OECD) is an intergovernmentalorganisation in which representatives of 38 industrialised countries in North and South America, Europe
and the Asia and Pacific region, as well as the European Commission, meet to co-ordinate andharmonise policies, discuss issues of mutual concern, and work together to respond to international
problems. Most of the OECD’s work is carried out by more than 200 specialised committees andworking groups composed of member country delegates. Observers from several countries with special
status at the OECD, and from interested international organisations, attend many of the OECD’s
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located in Paris, France, which is organised into directorates and divisions.
The Environment, Health and Safety Division publishes free-of-charge documents in eleven different
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More information about the Environment, Health and Safety Programme and EHS publications isavailable on the OECD’s World Wide Web site (www.oecd.org/chemicalsafety/).

This publication was developed in the IOMC context. The contents do not necessarily reflect
the views or stated policies of individual IOMC Participating Organizations.
The Inter-Organisation Programme for the Sound Management of Chemicals (IOMC) wasestablished in 1995 following recommendations made by the 1992 UN Conference on
Environment and Development to strengthen co-operation and increase international coordination in the field of chemical safety. The Participating Organisations are FAO, ILO, UNDP,
UNEP, UNIDO, UNITAR, WHO, World Bank and OECD. The purpose of the IOMC is topromote co-ordination of the policies and activities pursued by the Participating Organisations,jointly or separately, to achieve the sound management of chemicals in relation to human healthand the environment.

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ENV/CBC/MONO(2021)25  5This publication is available electronically, at no charge.
Also published in the Testing and Assessment link
For this and many other Environment,
Health and Safety publications, consult the OECD’s
World Wide Web site (www.oecd.org/chemicalsafety/)or contact:

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16, FranceOECD Environment, Health and Safety Publications

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Acknowledgements: This report was prepared under the framework of the OECD/UNEP Global PFC
Group and developed with financial support of Switzerland. In particular, the report was prepared by awriting group of the following members, led by Zhanyun Wang (ETH Zürich, Switzerland) andbenefited from the review and input of other members of the group:
Robert C. Buck (Chemours), Andreas Buser (Swiss Federal Office for the Environment), Ian T. Cousins(Stockholm University, Sweden), Silvia Demattio (European Chemicals Agency), Wiebke Drost(German Environment Agency), Audun Heggelund (Norwegian Environment Agency), Olof Johansson(Swedish Chemicals Agency), Eeva Leinala (OECD), Koichi Ohno (National Institute forEnvironmental Studies, Japan), Grace Patlewicz (US EPA), Ann Richard (US EPA), Simone Schalles(German Environment Agency), Glen Walker (Australian Department of Agriculture, Water and theEnvironment), Graham White (Health Canada), with earlier contributions by Urs Berger (UFZ,
Germany), Stellan Fischer (Swedish Chemicals Agency) and Laurence Libelo (US EPA).
The report is published under the responsibility of the OECD Chemicals and Biotechnology Committee.

The OECD Per- and Polyfluoroalkyl Substances (PFAS) project has been produced with thefinancial assistance of the European Union. The views expressed herein can in no way be taken toreflect the official opinion of the European Union.

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Executive Summary
This report summarizes recent efforts by the OECD/UNEP Global PFC Group between June 2018 andMarch 2021 in reviewing the universe and terminology of per- and polyfluoroalkyl substances (PFASs)
to provide recommendations and practical guidance to all stakeholders with regard to the terminology
of PFASs. In particular, this report highlights (1) a revised PFAS definition to comprehensively reflect
the universe of PFASs and a comprehensive overview of the PFAS universe (Chapter 2), (2) practicalguidance on how to use the PFAS terminology (Chapter 3), (3) a systematic approach to characterizationof PFASs based on molecular structural traits to assist stakeholders, including non-experts, in making
their own categorization based on their needs (Chapter 4), and (4) areas in relation to the PFAS
terminology that warrant further development (Chapter 5). It should be noted that this report does notaddress the nomenclature and understanding of individual PFASs, including the sources of exposureand the actual composition of commercial products.
PFASs comprise a class of synthetic compounds that have attracted much public attention since the late
1990s and early 2000s, when the hazards and ubiquitous occurrence of two PFASs, perfluorooctanoic
acid (PFOA) and perfluorooctane sulfonic acid (PFOS), started to be reported and recognized. Since
then, research and risk management measures have expanded from these two PFASs to a wider range
of PFASs. Early communications used many different terminologies (e.g. per- and polyfluorinatedchemicals, perfluorinated organics, perfluorochemical surfactants, highly fluorinated compounds). In
2011, to unify and harmonize communication on PFASs, Buck et al. published a milestone paper,providing a first clear structural definition of PFASs and recommendations on names and acronyms forover 200 individual PFASs.
Currently, there is a growing interest by regulators and scientists across the globe to assess legacy andnovel PFASs. In 2018, the OECD/UNEP Global PFC Group prepared a new list of PFASs that may
have been on the global market. In total, a set of substances with over 4730 CAS numbers have been
identified, including substances that contain such fully fluorinated carbon moieties, but do not meet the
PFAS definition in Buck et al. (2011) due to a lack of a –CF3 group in the molecular structures. In
addition, recent advancement of non-target screening analytical techniques using high-resolution mass
spectrometry has enabled identification of many unknown substances in different environmental andproduct samples. The identification of these substances motivates the present work to reconcile theterminology of the universe of PFASs, including a renewed look at the PFAS definition in Buck et al.
(2011).
It is key to have a coherent and consistent logic behind the PFAS definition to adequately reflect allcompounds with the same structural traits, i.e. the PFAS universe. Building on the OECD 2018 PFAS
List and recent non-target screening studies, Chapter 2 first identifies four major gaps in the previous
PFAS definition by Buck et al. (2011) in representing the PFAS universe. Then, Chapter 2 recommends
a revised PFAS definition, with detailed elaboration on individual changes provided:
PFASs are defined as fluorinated substances that contain at least one fully fluorinated methyl or
methylene carbon atom (without any H/Cl/Br/I atom attached to it), i.e. with a few noted
exceptions, any chemical with at least a perfluorinated methyl group (–CF3) or a perfluorinatedmethylene group (–CF2–) is a PFAS.
The rationale behind the revision is to have a general PFAS definition that is coherent and consistent

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across compounds from the chemical structure point of view and is easily implementable for
distinguishing between PFASs and non-PFASs, also by non-experts. The decision to broaden thedefinition compared to Buck et al. is not connected to decisions on how PFASs should be grouped inregulatory and voluntary actions. Based on the revised definition of PFASs, Chapter 2 further illustrates(1) how PFASs fit into organofluorine compounds, (2) a comprehensive overview of PFAS groups,
their structural traits, examples and notes on whether common nomenclatures (including acronyms)
exist for them, and (3) some common synthesis routes of different individual or groups of PFASs.
As PFASs are a chemical class with diverse molecular structures and physical, chemical and biologicalproperties, it is highly recommended that such diversity be properly recognized and communicated ina clear, specific and descriptive manner. The term “PFASs” is a broad, general, non-specific term, which
does not inform whether a compound is harmful or not, but only communicates that the compounds
under this term share the same trait for having a fully fluorinated methyl or methylene carbon moiety.
In particular, Chapter 3 provides practical guidance to governments and other stakeholders on how to
use the PFAS terminology, starting from the distinction between the general definition and user-specificworking scopes of PFASs. In particular, the general definition of PFASs is based on molecular structurealone and serves as a starting and reference point to guide individual users to have a comprehensive
understanding of the PFAS universe and to keep the big picture of the PFAS universe in mind. At thesame time, individual users may define their own working scope of PFASs for specific activities
according to their specific needs by combining the general definition of PFASs with additional
considerations (e.g. specific properties, use areas). This report does not make any recommendation on
how working scopes should be set up, in terms of which factors to be considered (which depends highly
on specific local context), nor on PFAS grouping. However, when a working scope of PFASs is used,
this report highly recommends that users clearly provide the context and rationale for selecting their
PFAS working scope in order to provide transparency and avoid confusion by others. Further, the report
recommends to use and build upon existing common terminologies such as in this report, in Buck et al.
(2011) and common practices in organic chemistry as set by IUPAC and CAS, unless it is essential todeviate from existing naming conventions, in order to keep the consistence and coherence of the PFAS
terminology.
As users often define their own working scope of PFASs according to their specific needs, they need tocharacterize PFASs based on pre-defined traits and categorize them (e.g. whether a compound withcertain traits falls or does not fall into their working scope). However, given the high complexity anddiversity of PFASs, it can be a challenging task to characterize and categorize PFASs based on chemical
structures in a coherent and consistent manner, particularly for non-experts. In addition, different users
may have very different needs, and there is no single categorization/grouping system that can meet allneeds. Therefore, Chapter 4 provides a standardized approach for systematic characterization ofdifferent PFASs based on molecular structural traits that will allow stakeholders to make their own
categorization in a coherent and consistent manner. In addition to the manual application of the system
to characterize and categorize PFASs, the elements presented here may also be used as inputs fordeveloping cheminformatic tools that would allow automated characterization and categorization of
PFASs.
While this report makes advancement on several important points regarding PFAS terminology andpractical guidance of how to use the PFAS terminology, Chapter 5 also recognizes four areas thatwarrant further work, in order to facilitate clear and unambiguous communication of PFASs andUnclassified

ENV/CBC/MONO(2021)25  9beyond: (1) a centralized PFAS nomenclature database/platform; (2) further development ofcheminformatics-based tools for automated systematic characterizing and categorizing PFASs; (3)further work on the characterization and reporting of polymers; and (4) work on organofluorinecompounds other than PFASs including many fluorinated aromatics.

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List of Acronyms
ADONA

Ammonium 4,8-dioxa-3H-perfluorononanoate

Br

Bromine atom

CAS

Chemical Abstracts Service

CAS Nos.

Chemical Abstracts Service registry numbers

Cl

Chlorine atom

CTFE

Chlorotrifluoroethylene

ECHA

European Chemicals Agency

ETFE

Ethylene-tetrafluoroethylene copolymer

EU

European Union

FASAs

Perfluoroalkane sulfonamides

FASEs

Perfluoroalkane sulfonamidoethanols

FEP

Fluorinated ethylene propylene co-polymer

FPs

Fluoropolymers

FTABs

Fluorotelomer sulfonamide alkylbetaines

FTEOs

Fluorotelomer ethoxylates

FTIs

Fluorotelomer iodides

FT(MA)ACs

Fluorotelomer (meth)acrylates

FTOs

Fluorotelomer olefins

FTOHs

Fluorotelomer alcohols

FTSAs

Fluorotelomer sulfonic acids

HFCs

Hydrofluorocarbons

HFEs

Hydrofluoroethers

HFOs

Hydrofluoroolefins

HFP

Hexafluoropropylene

HFPO

Hexafluoropropylene oxide

HFPO-DA

Hexafluoropropylene oxide dimer acidH

Hydrogen atom

I

Iodine atom

ICCM

International Conference on Chemicals Management

InChI

International chemical identifier

InChIKeyA hashed version of the full InChI

ITRCInterstate Technology & Regulatory Council in the United States

IUPAC

International Union of Pure and Applied Chemistry

OBS

Sodium p-perfluorous noenoxybenzenesulfonate

OECD

Organisation for Economic Co-operation and Development

PACFs

Perfluoroalkanoyl fluorides

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PASFs

Perfluoroalkane sulfonyl fluorides

PCTFE

Polychlorotrifluoroethylene

PFA

Perfluoroalkoxyl polymer

PFAAs

Perfluoroalkyl acids

PFAIs

Perfluoroalkyl iodides

PFASs

Per- and polyfluoroalkyl substances

PFCAs

Perfluoroalkyl carboxylic acids

PFdiCAs

Perfluoroalkyl dicarboxylic acids

PFdiSAs

Perfluoroalkane disulfonic acids

PFECAs

Perfluoroalkylether carboxylic acids

PFEI

Perfluoroethyl iodide

PFESAs

Perfluoroalkylether sulfonic acids

PFHxS

Perfluorohexane sulfonic acid

PFOA

Perfluorooctanoic acid

PFOS

Perfluorooctane sulfonic acid

PFPAs

Perfluoroalkyl phosphonic acids

PFPEs

Perfluoropolyethers

PFPIAs

Perfluoroalkyl phosphinic acids

PFSAs

Perfluoroalkane sulfonic acids

PFSIAs

Perfluoroalkane sulfinic acids

PolyFCAs

Polyfluoroalkyl carboxylic acids

PolyECAs

Polyfluoroalkylether carboxylic acids

PolyESAs

Polyfluoroalkylether sulfonic acids

POPs

Persistent Organic Pollutants

POSF

Perfluoroctane sulfonyl fluoride

PPVE

Perfluoropropylvinyl ether

PTFE

Polytetrafluoroethylene

PVDF

Polyvinylidene fluoride

PVF

Polyvinyl fluoride

REACH

Registration, Evaluation, Authorisation and Restriction of Chemicals (EC 1907/2006)

SaMPAPs

Perfluorooctane sulfonamidoethanol phosphate esters

SFAs

Semifluorinated alkanes

SMILES

Simplified molecular input line entry specification

TFE

Tetrafluoroethylene

THV

Terpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride

UNEP

United Nations Environment Programme

VDF

Vinylidene fluoride

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Table of contents
1. Background, motivation and scope ...................................................................................... 13
2. Reconciling Terminology of the Universe of PFASs ........................................................... 16
2.1. The previous PFAS definition in Buck et al. (2011) ......................................................... 16
2.2. Gaps in the previous PFAS definition by Buck et al. (2011) ............................................ 16
2.3. A revised PFAS definition ................................................................................................ 18
2.4. A comprehensive overview of the PFAS universe ............................................................ 21
3. Practical Guidance on How to Use the PFAS Terminology ............................................... 25
3.1. Distinction between the General Definition and User-Specific Working Scopes of
PFASs ....................................................................................................................................... 25
3.2. Practical guidance on how to identify and use suitable PFAS terms ................................ 26
4. Systematic characterization and categorization of PFASs ................................................. 29
5. Areas for Future Work .......................................................................................................... 33
References ................................................................................................................................... 34

FIGURES
Figure 1. PFOA and examples of substances with similar molecular structures, but having functional groups(including single atoms such as hydrogen) on both ends of the perfluoroalkanediyl moiety.
16
Figure 2. PFOS, an example of a cyclic PFSA, and a shorter-chain homologue of the cyclic PFSA
17
Figure 3. 6:2 FTOH, and a 6:2 fluorotelomer iodide derivative with one aromatic ring in the functional group
17
Figure 4. 6:2 FTOH and two 6:2 fluorotelomer derivatives, and their corresponding fluorine contents
17
Figure 5. Examples of PFASs. The fully fluorinated methyl or methylene carbon atoms are highlighted in red.
19
Figure 6. Examples of compounds that are not PFASs due to a lack of fully fluorinated methyl or methylene carbon
atoms.
20
Figure 7. An example of side-chain fluorinated aromatics.
21
Figure 8. An illustrative scheme of how PFASs fit into the universe of organofluorine compounds
22
Figure 9. A comprehensive overview of PFAS groups, their structural traits, examples and notes on whether
corresponding common nomenclatures (including acronyms) exist.
23
Figure 10. An overview of some common synthesis routes of different individual or groups of PFASs based on
publicly accessible source
24
Figure 11. A visual guide to identify the best terms to use for a specific statement with four examples (increasinglevel of specificity illustrated with same colour within examples).
27

TABLES
Table 1. Examples of ambiguous statements and associated good practices of using more specific PFAS terminology
to refine these statements
27
Table 2. Molecular structure-based elements of a characterization system for PFASs.
30
Table 3. Examples using the proposed characterization system.
32

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1. Background, motivation and scope

The OECD/UNEP Global PFC1 Group was established to respond to the Resolution II/5
adopted at the second session of the UN International Conference on Chemicals
Management (ICCM 2) in 2009, which calls upon intergovernmental organizations,
governments and other stakeholders to “consider the development, facilitation andpromotion in an open, transparent and inclusive manner of national and international
stewardship programmes and regulatory approaches to reduce emissions and the content
of relevant perfluorinated chemicals of concern in products and to work toward global
elimination, where appropriate and technically feasible”. Further work on this resolution
was reaffirmed in Resolution III/3 adopted at ICCM 3 in 2012 noting that a significant
need remains for additional work to support implementation of Resolution II/5. This
report is prepared within the framework of the Group. For more details on the Groupand its work, see the OECD PFAS web portal (https://oe.cd/2M9).
This report summarizes recent efforts by the OECD/UNEP Global PFC Group between
June 2018 and March 2021 in reviewing the universe and terminology of per- andpolyfluoroalkyl substances (PFASs2) to provide recommendations and practicalguidance to all stakeholders (governments, industry, academia, civil society
organizations, etc.) regarding the terminology of PFASs. In particular, this reporthighlights (1) a revised PFAS definition to comprehensively reflect the universe of
PFASs and a comprehensive overview of the PFAS universe (Chapter 2), (2) a practicalguidance on how to use the PFAS terminology, from a general PFAS definition to userspecific working scopes to naming conventions of individual PFASs (Chapter 3), (3) asystematic approach to characterization of PFASs based on molecular structural traits
to assist stakeholders, including non-experts, in making their own categorization basedon their needs (Chapter 4), and (4) areas in relation to the PFAS terminology thatwarrant future work (Chapter 5). It should be noted that this report does not address thenomenclature and understanding of individual PFASs, including the sources of exposureand the actual composition of commercial products. It also does not addressorganofluorine compounds other than PFASs.
PFASs comprise a class of synthetic compounds that have attracted much publicattention since the late 1990s and early 2000s, when the hazards and ubiquitous
occurrence of two PFASs, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonicacid (PFOS), started to be reported and recognized. Since then, research and risk
management measures have expanded from these two PFASs to a wider range of PFASs[e.g. 3M’s voluntary global phase-out of C6–10 perfluoroalkane sulfonic acids (PFSAs),
PFOA and related chemistries in 2000–2002]. It is noted that early communications“PFCs” here refer to “per- and polyfluorinated chemicals”, and not to “perfluorocarbons”. As stated below,“per- and polyfluorinated chemicals” was a term commonly used before the term “per- and polyfluoroalkyl
substances” was recommended by Buck et al.. As it is part of the Group official name, it remains unchanged.
2This report uses the acronym “PFASs” for “per- and polyfluoroalkyl substances” as stated in Buck et al. (2011),
and its corresponding singular form “PFAS” refers to either a perfluoroalkyl or polyfluoroalkyl substance. It isnoted that there is a notion of using “PFAS” as the acronym for both the singular and plural forms. This report
does not make any recommendation to address this notion, as it is a trivial point that is difficult for everyone tocomprehend, particularly for non-PFAS experts and non-English native speakers. While recognizing that readers
may make their own decision which acronym they would use, this report encourages readers to always use theacronym consistently in their documents (for more details on practice guidance on how to identify and use the
PFAS terminology, see Section 3.2 below).
1

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used many different terminologies (e.g. per- and polyfluorinated chemicals,
perfluorinated organics, perfluorochemical surfactants, highly fluorinated compounds).
In 2011, to unify and harmonize communication on PFASs, Buck et al. published amilestone paper on a first comprehensive overview of PFASs detected in theenvironment, wildlife, and humans. It provided a first clear structural definition of
PFASs. A particular emphasis of Buck et al. (2011) was placed on long-chain
perfluoroalkyl acids [PFAAs, i.e., perfluoroalkyl carboxylic acids (PFCAs) with sevenor more perfluorinated carbons and PFSAs with six or more perfluorinated carbons]3,
substances related to the long-chain PFAAs, and substances intended as alternatives tothe long-chain PFAAs or their precursors4. In addition, Buck et al. (2011) provided alist of 42 families and subfamilies5 of PFASs and 268 selected individual compounds,
including recommended names and acronyms, structural formulas, and Chemical
Abstracts Service registry numbers (CAS Nos.).
Today, several long-chain PFAAs have been recognized as global contaminants of highconcern. For example, PFOS, its salts, and perfluorooctane sulfonyl fluoride (POSF6),
as well as PFOA, its salts, and PFOA-related compounds have been listed under the
Stockholm Convention on Persistent Organic Pollutants (POPs) for global actions. In
addition, the POPs Review Committee to the Stockholm Convention decided in 2019 torecommend that the Conference of the Parties to the Stockholm Convention consider
listing perfluorohexane sulfonic acid (PFHxS, C6 PFSA), its salts and PFHxS-related
compounds at its tenth meeting. In response to these actions, an industrial transition has
taken place to replace long-chain PFAAs and their precursors with alternative
chemicals, many of which are still PFASs, including short-chain PFAAs and theirprecursors as well as perfluoroalkylether-based substances (for examples, see Buck etal., 2011, Wang et al., 2013, 2016 and references therein). It is noted that there is a
growing interest by regulators7 and scientists across the globe to assess legacy and novel
PFASs other than long-chain PFAAs and their well-known precursors.
In particular, various efforts have been made to identify overlooked PFASs. In 2018,
the OECD/UNEP Global PFC Group prepared a new list of PFASs8 that may have been
Note that the definition of “long-chain PFAAs” here is based on the OECD definition(https://www.oecd.org/chemicalsafety/portal-perfluorinated-chemicals/aboutpfass/), and the definitions of“long-chain PFAAs” may differ by jurisdiction.
4
PFAA precursors refer to chemicals that can transform and form PFAAs in the environment and biota.
5Note that in the literature, some authors have used other taxonomy terminologies, e.g. “groups and subgroups”
instead of “families and subfamilies”. This report does not propose a new taxonomy terminology for PFASs, but
makes some practical guidance on how to use taxonomy terminologies (see Chapter 3 below).
6Note that the acronym “POSF” here is used in accordance with the recommendations by Buck et al. (2011),
whereas under the Stockholm Convention, another acronym “PFOSF” is used.
7
For example, five European Union (EU) member states have agreed to prepare a joint REACH restriction
proposal to limit the risks to the environment and human health from the manufacture and use of a wide range
of PFASs, and thus launched a public call for evidence in May 2020 with regard to substances that contain atleast one aliphatic –CF2– or –CF3 element. For more details, see https://echa.europa.eu/hottopics/perfluoroalkyl-chemicals-pfas. In addition, multiple PFASs other than long-chain PFAAs and theirprecursors are listed in ECHA’s Public Activities Coordination Tool (PACT) to be assessed by ECHA or EU
member states (https://echa.europa.eu/pact).
8
The Excel Spreadsheet version of the OECD 2018 PFAS list can be found athttps://www.oecd.org/chemicalsafety/risk-management/global-database-of-per-and-polyfluoroalkylsubstances.xlsx. In addition, several other entities have curated the OECD 2018 PFAS list into their databases,
with features such as an easier overview of chemical structures and links to other information, including the US
EPA CompTox Chemicals Dashboard (https://comptox.epa.gov/dashboard/chemical_lists/PFASOECD),
3

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on the global market using a systematic search of substances that have a –CnF2n– (n≥3) or –CnF2nOCmF2m− (n and m ≥1) moiety in different publicly accessible sources.
In total, a set of substances with over 4730 CAS Nos. have been identified, including
substances that contain fully fluorinated carbon moieties and are structurally similar toor related to commonly known PFASs [e.g. perfluoroalkyl dicarboxylic acids(PFdiCAs) to PFCAs], but do not meet the PFAS definition in Buck et al. (2011) due toa lack of a –CF3 group in the molecular structures (for more details, see Section 2.2).
Meanwhile, recent advancement of non-target screening analytical techniques using
high-resolution mass spectrometry has enabled identification of many unknown
substances in different environmental and product samples [e.g. H–(CF2CH2)n–CF2COOH by Newton et al. (2017)].
The identification of overlooked PFASs motivates the present work to reconcile theterminology of the universe of PFASs, including a renewed look at the PFAS definition
in Buck et al. (2011) (see Chapter 2). In light of these newly identified substances andbuilding on existing common terminology provided in Buck et al. (2011), this reportand others, this report also looks into practical guidance on how to use the PFAS
terminology, including uses of user-specific working scopes (see Chapter 3). In
addition, the OECD 2018 PFAS List and recent non-target screening studies show thecomplexity and diversity of the PFAS universe, resulting in challenges for non-experts
in conducting their own categorization of PFASs based on molecular structures.
Therefore, this report also looks into systematic approaches to characterization andcategorization of PFASs to assist stakeholders in making their own categorization basedon their needs (see Chapter 4). Further, this report highlights open questions in relation
to PFAS terminology for future consideration (see Chapter 5).

NORMAN
Network(https://www.norman-network.com/?q=suspect-list-exchange)
and
PubChem(https://pubchem.ncbi.nlm.nih.gov/classification/#hid=101).
In addition, the US EPA CompTox Chemicals Dashboard also provides a number of other PFAS lists intended
to address different research and regulatory interests, including PFASSTRUCT that is compiled from all therecords with a structure assigned in the Dashboard using a pre-defined set of substructural filters and contains
over 8000 compounds, as of 23 November, 2020 (for more details including the list of substructural filters, see
https://comptox.epa.gov/dashboard/chemical_lists/PFASSTRUCT). Note that these lists may also include
substances that are not regarded as PFASs in accordance with the revised PFAS definition below.

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2. Reconciling Terminology of the Universe of PFASs

2.1. The previous PFAS definition in Buck et al. (2011)
In Buck et al. (2011), PFASs were defined as “the highly fluorinated aliphatic
substances that contain 1 or more C atoms on which all the H 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–” (i.e. must contain at least −CF3). The definition highlights the presence of atleast one fully fluorinated saturated carbon atom in the PFAS molecules.

2.2. Gaps in the previous PFAS definition by Buck et al. (2011)
It is key to have a coherent and consistent logic behind the PFAS definition to reflect
all compounds with shared structural traits, i.e. the PFAS universe. Building on theOECD 2018 PFAS List and recent non-target screening studies, this section identifies
gaps in the previous PFAS definition by Buck et al. (2011) in representing the PFAS
universe. Note that the gaps identified in this report are not exhaustive and additional
gaps in the PFAS definition may be identified in the future; therefore, an iterative
approach is guaranteed to ensure the consistency between the PFAS universe andterminology when new knowledge of gaps in the PFAS definition is generated.
Case 1: The fully fluorinated saturated carbon moiety9 is connected with functional groups on bothends, including having a single H/Br/Cl atom on one end. As such, it does not meet the structuralrequirement of “–CnF2n+1” in the previous definition. In the example of a1 in Figure 1, it is a PFdiCA
with a similar structure to PFCAs (e.g. PFOA in the example of A in Figure 1), but having carboxylic
groups on both ends of the perfluoroalkanediyl moiety. In addition, for the example of a2 in Figure 1,
it would meet the previous definition if the H atom was moved to a secondary carbon atom (i.e.
CF3CFHCF2CF2CF2CF2CF2COOH, a positional isomer).

Figure 1. PFOA and examples of substances with similar molecular structures, but having functionalgroups (including single atoms such as hydrogen) on both ends of the perfluoroalkanediyl moiety.

Furthermore, functionalized fluoropolymers and perfluoropolyethers10 (i.e. those thathave functional groups on both ends of the polymer backbone, e.g. Fomblin HC/P2
100011) do not meet the structural requirement of “–CnF2n+1” in the previous definition,
Note that a “saturated carbon moiety” means no unsaturated bonds occurring in the moiety, including doublebond (=), triple bond (≡) or aromatic rings, and thus, a saturated carbon moiety is always considered aliphatic.
9

According to Buck et al., fluoropolymers are “carbon-only polymer backbone with F directly attached tobackbone C atoms”, whereas perfluoropolyethers are “ether polymer backbone with F atoms directly attached”(i.e. having –C–O–C– moieties on the polymer backbone).
10(HO)2(O)PO–(CH2CH2O)n–CH2CF2–(OCF2)p–(OCF2CF2)q–OCF2CH2–(OCH2CH2)n–OP(O)(OH)2; Trier X,
Granby K, Christensen JH. Polyfluorinated surfactants (PFS) in paper and board coatings for foodpackaging. Environ Sci Pollut Res Int. 2011;18(7):1108-1120. doi:10.1007/s11356-010-0439-3
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ENV/CBC/MONO(2021)25  17

whereas their closely related analogues with only fluorine atoms on each end of thepolymer backbone would meet the previous definition.
Case 2: The substance is a fully fluorinated aliphatic cyclic compound which may or may not have afully fluorinated alkyl side chain. As such, it may not meet the structural requirement of “–CnF2n+1” inthe previous definition. For example, b1 in Figure 2 meets the previous definition, whereas its shorterchain homologue, b2 in Figure 2, does not meet the previous definition.

Figure 2. PFOS, an example of a cyclic PFSA, and a shorter-chain homologue of the cyclic PFSA

Case 3: The functional group contains an aromatic ring. Thus, it may not meet the term “aliphatic
substances” in the previous definition, although the example of c1 in Figure 3 is a derivative of 6:2
fluorotelomer iodide, i.e. a 6:2 fluorotelomer-based compound.

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

OH

F
OH

F

F
F

F

F

F

F

F

C. 6:2 FTOH, CAS No. 647-42-7

FF

c1. CAS No. 356055-76-0

Figure 3. 6:2 FTOH, and a 6:2 fluorotelomer iodide derivative with one aromatic ring in the functional group

Case 4: The description “highly fluorinated” in the previous definition is an ambiguous, problematic
term. It cannot and should not be literally translated to, e.g., the weight percentage of fluorine atoms inthe molecules, using three 6:2 fluorotelomer-based compounds as an example (see Figure 4):

C6F13C2H4OH (6:2 FTOH; CAS No. 647-42-7) has a fluorine content of 67.8 wt%,
C6F13C2H4SO2NHC3H6N(O)(CH3)2 used in Forafac® 1183 (CAS No. 80475-32-7) has afluorine content of 46.7 wt%, and
6:2 fluorotelomer ethoxylates [C6F13–(CH2CH2O)n–H, n = 0–13] in a commercial product(Frömel and Knepper, 2010) would have even lower fluorine content when n>4.
But they are all 6:2 fluorotelomer-based compounds and may act as precursors toperfluorohexanoic acid (PFHxA) in the environment and biota.

Figure 4. 6:2 FTOH and two 6:2 fluorotelomer derivatives, and their corresponding fluorine contents

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18  ENV/CBC/MONO(2021)25

2.3. A revised PFAS definition
Therefore, there is a need to revisit the previous definition in Buck et al. (2011) toaddress these gaps (i.e. the previous definition was not comprehensive enough andcontained ambiguous descriptions). A clear distinction of the logical relationship needs
to be made here: the intention of the revision of the PFAS definition is not to expand
the PFAS universe, but to comprehensively reflect it. More concretely, the rationale
behind the revision is to have a general PFAS definition that is coherent and consistentacross compounds from the chemical structure point of view and is easily
implementable for distinguishing between PFASs and non-PFASs, also by non-experts.
This revised PFAS definition reads,
PFASs are defined as fluorinated substances that contain at least one fully fluorinated methyl or
methylene carbon atom (without any H/Cl/Br/I atom attached to it), i.e. with a few noted
exceptions, any chemical with at least a perfluorinated methyl group (–CF3) or a perfluorinatedmethylene group (–CF2–) is a PFAS.

Both a perfluorinated methyl group and a perfluorinated methylene group are saturated
and aliphatic. Note that the carbon in a R–CF2–O– or R–CF2–Si– group (R ≠ H/Cl/Br/I)
is a perfluorinated methylene carbon. A perfluorinated methylene group may also be
represented as “>CF2”, where “>” denotes two single bonds. A fully fluorinated carbon
that is bound to the rest of the molecule by a double bond is a perfluorinated methylidene
carbon atom (=CF2). This distinction is important. Further, a perfluorinated methinecarbon moiety (>CF–) alone does not meet this revised PFAS definition.
It should be noted that this general PFAS definition is based only on chemical structure,and the decision to broaden this definition compared to Buck et al. (2011) is notconnected to decisions on how PFASs should be grouped and managed in regulatory
and voluntary actions. For further practical guidance on how to use this general PFAS
definition, see Section 3.1.
Figure 5 illustrates substances that are PFASs, and Figure 6 shows those that are not
PFASs. Note that tetrafluoroethylene (TFE, CAS No. 116-14-3, CF2=CF2) is not a PFAS
as both fully fluorinated carbon atoms are unsaturated; its longer-chain homologuehexafluoropropylene (HFP, CAS No. 116-15-4, CF2=CF–CF3) is a PFAS due to thepresence of a fully fluorinated methyl carbon atom (–CF3).

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ENV/CBC/MONO(2021)25  19

Figure 5. Examples of PFASs. The fully fluorinated methyl or methylene carbon atoms are highlighted in red.

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Figure 6. Examples of compounds that are not PFASs due to a lack of fully fluorinated methyl or
methylene carbon atoms.

The rationale for making such changes is detailed as follows.
Change from “highly fluorinated aliphatic substances” to “fluorinated substances that
contain at least one fully fluorinated methyl or methylene carbon atom (without any
H/Cl/Br/I atom attached to it)”:First, the qualifier “highly” is removed from the definition, as it is not meaningful when the
fluorinated carbon chain can cleave from the substance to produce a new molecule that is more
highly fluorinated [see Section II in FOEN (2017) and references therein].
Second, the term “aliphatic” is removed from the definition. As shown in Case 3 in Figure 3,
aromatic ring(s) may be present as a part of the functional group connecting to a fully
fluorinated methyl or methylene carbon moiety. Using the previous definition by Buck et al.
(2011), such compounds would not be recognized as PFASs, whereas compounds with similar
structures but without aromatic ring(s) are recognized as PFASs. This may easily create
confusion as to when a substance is or is not a PFAS, particularly for non-experts. The changeof wording here is also to make the definition more straightforward. At the same time, the newwording “substances that contain at least one fully fluorinated methyl or methylene carbon
atom” means that this revised definition is still constrained to the key trait of having an aliphatic
fully fluorinated saturated carbon moiety and excluding those fluorinated aromatics that only
have fluorine directly attached to the aromatic rings. Overall, this revised definition includes
side-chain fluorinated aromatics [i.e. aromatics that have one or more aliphatic fully fluorinatedsaturated carbon moiety on the side chain(s) attached to the aromatic ring(s), an analogy to“side-chain fluorinated polymers”12 as in Buck et al. 2011] as PFASs; for examples, see c1 inFigure 3 and Figure 7 below.

In Buck et al. (2011), side-chain fluorinated polymers are defined as “nonfluorinated polymer backbone withfluorinated side chains”.
12

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ENV/CBC/MONO(2021)25  21

Figure 7. An example of side-chain fluorinated aromatics.

Change from “the perfluoroalkyl moiety CnF2n+1–” to “at least one fully fluorinatedmethyl or methylene carbon atom (without any H/Cl/Br/I atom attached to it)”:This change is to accommodate those that have functional groups on both ends of the fully
fluorinated saturated carbon moieties (Case 1) and those that have cyclic structure(s) at the endof the fully fluorinated saturated carbon moieties (Case 2).
In addition, two more specific descriptions are made here to make the definition clearer. First,
the term “methyl or methylene carbon atom” is added to describe the fully fluorinated saturated
carbon moiety, which was not clear from the description “that contain only 1 or more C atoms
on which all the H substituents … have been replaced by F atoms”, but only implicitly
mentioned in the description “in such a manner that they contain the perfluoroalkyl moiety
CnF2n+1–”. Second, adding “without any H/Cl/Br/I atom attached to it” highlights that the
carbon atom is considered non-fully fluorinated, when a H/Cl/Br/I atom is attached to it.

2.4. A comprehensive overview of the PFAS universe
Based on this revised definition of PFASs, a first scheme can be drawn to illustrate how
PFASs fit into organofluorine compounds (see Figure 8). It can be seen that besides
PFASs, there are many other organofluorine compounds, including (1) fluorinatedaliphatic substances that do not have a fully fluorinated methyl or methylene carbon
atom [e.g. trifluoromethane (HFC-23) and difluoromethane (HFC-32)], (2) fluorinatedaromatic substances with no side chain(s) (e.g. hexafluorobenzene, CAS No. 392-563), and (3) fluorinated aromatic substances with non-fluorinated side chain(s) (e.g.
pentafluorobenzoic acid, CAS No. 602-94-8). These other organofluorine compounds
are beyond the scope of this report, and future work on them is encouraged.
Looking at the PFAS universe, it is a highly complex chemical class with compounds
having diverse functional groups attached to the fully fluorinated saturated carbon
moiety/-ies. Figure 9 provides a comprehensive overview of PFAS groups, theirstructural traits, examples and notes on whether common nomenclatures (including
acronyms) exist for them, building on Buck et al. (2011) and the OECD 2018 List.
Figure 10 illustrates some common synthesis routes of different individual or groups of
PFASs based on publicly accessible sources. It should be noted that, while Figures 9
and 10 aim to be comprehensive, they are by no means exhaustive. For more information
on individual PFAS groups (e.g. major compounds in the group, synthesis routes, majoruses, regulatory status, environmental occurrence, etc.), readers may consult the PFAS
Fact
Cards
published
on
the
OECD
PFAS
Web
portal:https://www.oecd.org/chemicalsafety/portal-perfluorinated-chemicals/.

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other fluorinated ALIPHATIC
substances that do not have a fully fluorinatedmethyl or methylene carbon atomorganofluorinecompounds

fluorinated ALIPHATIC
substancesF

fluorinated AROMATIC ring(s)withfluorinated ALIPHATIC side chain(s) that have afully fluorinated methyl or methylene carbon atom
(non-)fluorinated AROMATIC ring(s)withfluorinated ALIPHATIC side chain(s) that do NOT have afully fluorinated methyl or methylene carbon atom
fluorinated AROMATIC ring(s)withnon-fluorinated ALIPHATIC side chain(s)HFC-32*, CAS No. 75-10-5
R-12*, CAS No. 75-71-8

F

F

F

HFC-23*, CAS No. 75-46-7

Per- and Polyfluoroalkyl Substances (PFASs)
e.g. R− CnF2n+1 or R− CnF2n− R’, n ≥1

fluorinated ALIPHATIC substances that have afully fluorinated methyl or methylene carbon atom
non-fluorinated AROMATIC ring(s)withfluorinated ALIPHATIC side-chain(s) that have afully fluorinated methyl or methylene carbon atom

fluorinated AROMATIC
substancesF(for examples, see Figure 5 above)

F

F

F
F
F

F

F

F

CAS No. 378-98-3

F
F

F
F

F

F
F

CAS No. 434-64-0

CAS No. 458-87-7
CAS No. 127399-61-5

CAS No. 602-94-8

fluorinated AROMATIC substances
without
a side chain
CAS No. 392-56-3

Substances that are not PFASs and are not addressed in this report* HFC-32, HFC-23 and R-12 are not PFASs, despite the presence of moieties such as − CF2− or − CF3, because not all H on the fluorinated carbon atom are replaced by F,i.e., they do not have a fully fluorinated carbon atom.

Figure 8. An illustrative scheme of how PFASs fit into the universe of organofluorine compounds

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ENV/CBC/MONO(2021)25  23

Figure 9. A comprehensive overview of PFAS groups, their structural traits, examples and notes on whether corresponding common nomenclatures (including acronyms)
exist.

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Figure 10. An overview of some common synthesis routes of different individual or groups of PFASs based on publicly accessible source

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ENV/CBC/MONO(2021)25  25

3. Practical Guidance on How to Use the PFAS Terminology

As shown above, PFASs are a chemical class with diverse molecular structures (e.g.
neutral, anionic, cationic or zwitterionic; with or without aromatic rings; non-polymers
or polymers; low molecular weight or high molecular weight) and thus diverse physical,
chemical and biological properties (e.g. involatile or volatile; water soluble or water
insoluble; reactive vs. inert; bioaccumulative or non-bioaccumulative). Therefore, it ishighly recommended that such diversity be properly recognized and communicated ina clear, specific and descriptive manner. The following sections aim to provide practicalguidance to governments and other stakeholders on how to use the PFAS terminology,
starting from the distinction between the general definition described here and userspecific working scopes of PFASs. An overarching rationale behind the practicalguidance is to use and build upon existing common terminologies such as in this report,
in Buck et al. (2011) and common practices in organic chemistry as set by IUPAC and
CAS, unless it is essential to deviate from existing naming conventions in order to keep
the consistence and coherence of the PFAS terminology.

3.1. Distinction between the General Definition and User-Specific Working Scopesof PFASs
It should be noted that the revised definition of PFASs in Section 2.3 refers to a generaldefinition of PFASs that is coherent and consistent across compounds based on chemical
structure and is easily implementable for distinguishing between PFASs and nonPFASs, also by non-experts. It does not include any minimal or maximal chain length
requirements, or any other considerations beyond chemistry. It also does not conclude
that all PFASs have the same properties, uses, exposure and risks.
While this general definition of PFASs may be viewed as too broad, encompassing
thousands or more compounds, for anyone to address all of them at once, it serves as astarting and reference point to guide individual users to have a comprehensive
understanding of the PFAS universe and to keep the big picture of the PFAS universein mind. At the same time, individual users may define their own PFAS working scopefor a specific activity according to their specific needs by combining this generaldefinition of PFASs with additional considerations (e.g. specific properties, use areas).
For example, the US Interstate Technology & Regulatory Council (ITRC)13 used aworking scope of “CnF2n+1” (n>2) in making its own PFAS fact sheets. Another example
is the working scope used in compiling the OECD 2018 PFAS List, namely –CnF2n– (n≥3) and –CnF2nOCmF2m– (n and m ≥1). Also, the addition of criteria such as
bioavailability and persistence in Gore Fabrics’ Goal and Roadmap14 for Eliminating
PFCs of Environmental Concern may be regarded as a way of setting working scopes.
This report does not make any recommendation on how a working scope should be set
up regarding which factors to consider (which depend on specific local context)15, nor
13

The latest version of the fact sheet on naming conventions of PFASs is from April 2020: https://pfas1.itrcweb.org/fact_sheets_page/PFAS_Fact_Sheet_Naming_Conventions_April2020.pdf
14Here it refers to the version published on January 31, 2017, which can be found at:https://drive.google.com/file/d/0BxvQ_I44P_9eeTlwYUJCekhLNlE/view
15
Future work compiling various existing practices of defining working scope under different context may bebeneficial to provide further guidance to governments and other stakeholders on this matter.

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on PFAS grouping16. However, when a working scope of PFASs is used, this reporthighly recommends that users clearly provide the context and rationale for selecting
their PFAS working scope in order to provide transparency and avoid confusion byothers.

3.2. Practical guidance on how to identify and use suitable PFAS terms
The term “PFASs” does not inform whether a compound is harmful or not, but only
communicates that the compounds under this term share the same trait for having a fully
fluorinated methyl or methylene carbon moiety. In addition, particularly for PFASs
without an assigned CAS No., a lot of parallel and often non-intuitive acronyms are
employed, potentially prohibiting effective communication and creating barriers forsynthesizing knowledge. This section aims to provide practical guidance on how to
identify and use suitable terms to foster communication around PFASs with the aim ofbeing accurate, precise, understandable by others, and consistent.
First, it is strongly recommended that the PFAS terminology be used in a clear, specificand descriptive manner. It should be noted that “PFASs” is a broad, general, nonspecific term, which should only be used when talking about all the substances includedin the PFAS definition described here (or the user should clearly define the scope of
which substances are being referred to as PFASs in the documents they prepare).
Otherwise, it would introduce ambiguity and even factual error in the statements (asoccurred sometimes in past literature). For example, not all PFASs are surfactants, andthus, a statement “PFASs are surfactants” is factually inaccurate. Table 1 highlights
examples of ambiguous statements, which when are overgeneralized may lead toambiguity, and factual inaccuracies and miscommunication in some cases. Therefore, it
is recommended that users always ask the following two questions when drafting astatement: (1) Am I referring to all PFASs or not? (2) If not, what term(s) would mostly
clearly describe the substance(s) that my statement is referring to? There could be
multiple ways by users to locate the right levels of terms that are clear, specific anddescriptive for specific statements, by combining and ordering traits such as polymericvs. non-polymeric, PFAAs vs. PFAA precursors, or side-chain fluorinated polymers vs.
fluoropolymers vs. perfluoropolyethers. Figure 11 shows different levels of PFAS terms
and their respective characteristics in terms of clarity and specificity, along with
examples; one may either start from Level 1 (most general) and move downwards (withthe question of whether it is specific enough), or Level 5 (most specific) and move
upwards (with the question of whether it can be further generalized), to locate the rightlevel of terms for a specific statement. Table 1 also includes examples of good practice
to refine ambiguous statements using more suitable terms. Furthermore, individual
PFASs need to be named in a clear, specific and descriptive manner.

16

In a recent scientific article, various grouping strategies for PFASs were reviewed and the motivations,advantages and disadvantages for each approach were discussed; for more details, see Cousins et al. 2020.
Environmental Science: Processes & Impacts, 22, 1444–1460, https://doi.org/10.1039/D0EM00147C

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Table 1. Examples of ambiguous statements and associated good practices of using
more specific PFAS terminology to refine these statements
Examplesofambiguousstatements (which may also resultin factual inaccuracy in somecases)

Examples of good practices of using the PFAS terminology to avoid errors andreduce ambiguity(1) Using more specific PFAS
terms

(2) Adding qualifiers (less favorable than(1), as it remains quite ambiguous)

PFASs were investigated in human
milk.

C4–C14 PFCAs were investigated inhuman milk.

15
non-polymeric
PFASs
investigated in human milk.

PFASs are used to make protective
coatings on common householdproducts.

Fluorotelomer-based side-chain
fluorinated polymers are used tomake protective coatings oncommon household products.

A number of polymeric PFASs are used
to make protective coatings on common
household products.

PFASs are relatively ubiquitous intheenvironment
atlow
concentrations.
(factually inaccurate)

PFCAs are relatively ubiquitous intheenvironment
atlow
concentrations.

A number of PFASs are relatively
ubiquitous in the environment at low
concentrations.

PFASs are water repellent, oil,grease and dirt repellent surfactants.
(factually inaccurate)

Many perfluorooctane sulfonyl
fluoride-based derivatives arewater-, as well as oil-, grease- anddirt-repellent surfactants.

A number of PFASs are water-, as well as
oil-, grease- and dirt-repellent surfactants.

speciﬁcity

Most
GENERAL

Most
SPECIFIC

were

Explanations

Examples of best terms to be used

Level 1

When describing all chemicals with at least a
perﬂuorinated methyl (–CF3) or methylene group (–CF2–)- per- and polyﬂuoroalkyl substances (PFASs)

Level 2

When describing groups of PFASs that are separated bysimple traits (e.g. perﬂuoroalkyl vs. polyﬂuoroalkyl chain;
ﬂuoroalkyl vs. ﬂuoroalkylether chain; ﬂuoroalkyl(ether)chain and/or functional group being polymeric vs. nonpolymericA-2. perﬂuoroalkyl non-polymers
B-2. perﬂuoroalkyl non-polymers
C-2. polyﬂuoroalkyl substances (including both non-polymers and polymers)
D-2. ﬂuoropolymers (i.e. PFASs that have a polymeric ﬂuoroalkyl chain as the
backbone)

Level 3

When describing groups of PFASs that share the sameor similar structural components (including deri vatives
from the same parent compounds)

A-3. perﬂuoroalkyl acids (PFAAs, e.g. CnF2n+1–acidic groups)
B-3. perﬂuoroalkyl acids (PFAAs, e.g. CnF2n+1–acidic groups)
C-3. n:2 ﬂuorotelomer-based substances (C nF2n+1–C2H 4–R, R = any groups),
including side-chain ﬂuorinated polymers (R = polymeric)D-3. polytetraﬂuoroethylene [PTFE; [R–(CF2)n–R’, R, R’ = any groups]

Level 4

When describing a group of PFASs that belong to thesame homologue series with different perﬂuorinatedcarbon chain lengths

A-4. perﬂuoroalkyl carboxylic acids (PFCAs, CnF2n+1–COOH)
B-4. perﬂuoroalkane sulfonic acids (PFSAs, C nF2n+1–SO 3H)
C-4. n:2 ﬂuorotelomer alcohols, (n:2 FTOHs, CnF2n+1–C2H 4OH)
D-4. non-functionalized PTFE [F3C–(CF2)n–CF3]

Level 5

When describing indi vidual substances (identiﬁers suc h
as names, CAS numbers, molecular formula,
InChI(Key), SMILES, etc.)

A-5. perﬂuorooctanoic acid (PFO A, C7F15–COOH)
B-5. perﬂuorooctane sulfonic acid (PFOS, C 8F17–SO 3H)
C-5. 8:2 ﬂuorotelomer alcohol (8:2 FTOH, C6F13–C2H 4OH)
D-5. speciﬁc PTFE products [F3C–(CF2)n–CF3; X<n<Y; X,Y = integers]

Figure 11. A visual guide to identify the best terms to use for a specific statement with four examples(increasing level of specificity illustrated with same colour within examples).

Second, if users are not sure about how to name a specific compound, it is recommended
to first check whether a common nomenclature (including a common acronym) alreadyexists, e.g., in Buck et al. (2011), Barzen-Hanson et al. (2017)17, this report and otherstudies, before creating their own naming conventions. For example, for CAS No. 67839-7, a common name “8:2 fluorotelomer alcohol” and a common acronym “8:2 FTOH”
already exist, and should be used instead of other synonyms.

17 In the Supporting Information, Barzen-Hanson et al. developed a simplified, manual IUPAC-based naming system for the

PFASs that they identified in their non-target screening. For more details, see Barzen-Hanson et al. 2017. Environmental
Science & Technology. 51(4), 2047–2057. https://doi.org/10.1021/acs.est.6b05843

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Third, acronyms are often necessary in communicating PFASs to avoid writing very
long names all the time; however, the same acronym may refer to different full names ordifferent forms of the same substance (e.g. the parent acid, the anion form, and various salt
forms), depending on context and personal understanding. To avoid confusion, it isrecommended that acronyms be spelled out when being mentioned for the first time in
the text and used consistently throughout the text.
Fourth, while chemical names and associated acronyms are the most common chemical
identifiers being used, it is also recommended that other more specific identifiers suchas CAS No., SMILES (simplified molecular input line entry specification), InChI(international chemical identifier), InChIkey (a hashed version of the full InChI) and/or
structural formula18 are provided for possibilities of cross-checking. This may also be
useful in reporting the chemical identities of PFASs that have been registered as
substances of unknown or variable composition, complex reaction products, or
biological materials (UVCBs, e.g., CAS No. 69991-67-9 = 1-propene, 1,1,2,3,3,3hexafluoro-oxidized, polymd.)8.

18

These identifiers may be found and verified using online databases, such as the CAS Common Chemistry(https://commonchemistry.cas.org), ChemSpider (http://www.chemspider.com), NORMAN Suspect List
Exchange(https://www.norman-network.com/?q=suspect-list-exchange),OECD
eChemPortal(https://www.echemportal.org/echemportal/),
PubChem(https://pubchem.ncbi.nlm.nih.gov/classification/#hid=101), SciFinder (http://scifinder.cas.org) and US EPA
CompTox Chemicals Dashboard (https://comptox.epa.gov/dashboard/chemical_lists/PFASOECD).

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4. Systematic characterization and categorization of PFASs

As users often define their own working scope of PFASs according to their specificneeds (see Section 3.1), they need to characterize PFASs based on molecular structures(and other considerations) and then categorize them by comparing characterization traits
against specific needs (e.g. whether a compound falls or does not fall into their working
scope). For example, the recent listing of PFOA and PFOA-related compounds under
the Stockholm Convention requires regulators across the world to be able to identify
PFOA-related compounds from a pool of PFASs.
However, given the high complexity and diversity of PFASs, it can be a challenging
task to characterize and categorize PFASs based on their chemical structures in a
coherent and consistent manner, particularly for non-experts. Detailed challenges may
include needs of specialized chemistry knowledge (e.g. on transformation), different
interpretations of structural traits by users for different groups of PFASs, and potential
for human errors including oversights and typing errors (Sha et al. 2019). In addition,different users may have very different needs, and there is no single
categorization/grouping system that can meet all needs.
Therefore, this section provides a standardized system for systematic characterizationof different PFASs based on molecular structural traits that will allow stakeholders tomake their own categorization in a coherent and consistent manner. Molecular structurebased elements of such a characterization system are provided in Table 2, with someexamples of applications given in Table 3. For example, if someone would like to havethe grouping of linear PFCAs, they would just need to search for molecules with theright characterization traits as defined in Table 3: under “fluorinated carbon chain (A)”,having “alkyl”, “perfluoro”, “linear”, “saturated”, “non-polymeric”; under “functionalgroup B”, having “COOH” and “non-polymeric”; and under “stoichiometry between A
and B”, having “1:1”. The system is flexible for future refinement including possible
addition of new elements as needed and also applications to new groups of PFASs as
identified.
In addition to manual application of the system to characterize and categorize PFASs,
the elements presented here may also be used as inputs for developing cheminformatic
tools that would allow automatized characterization and categorization of PFASs, as
demonstrated in Sha et al. (2019).
In that study, an algorithm was developed to systematically parse a PFAS molecule into
three fragments namely CnF2n+1–X–R, where CnF2n+1– refers to the fluorinated carbon
moiety and –X–R refers to functional group moiety. X was used to identify whether a
PFAS molecule falls into the target group of perfluoroalkane sulfonyl fluoridederivatives (where X = SO2), perfluoroalkanoyl fluoride derivatives (where X = CO),n:1 fluorotelomer-based compounds (where X = CH2 and R does not have a CH2 or CH
moiety connecting with X), or n:2 fluorotelomer-based compounds (where X =CH2CH2). The algorithm was applied to a set of 770 PFASs from the OECD 2018 PFAS
List and identified PFASs from the target four groups as intended. The algorithm wasalso able to identify PFASs that were mis-categorized in the OECD 2018 PFAS List, as
the original categorization was done manually.
The algorithm developed in Sha et al. (2019) serves as a proof-of-concept, and thus has
its limitations in terms of its purpose (i.e. to identify whether a PFAS falls into one of
the four target groups) and function (e.g. it cannot handle PFASs with more than one

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functional group moieties). However, it shows the potential of such cheminformatics
approaches, which can be expanded using the elements provided here for systematiccharacterization and categorization of PFASs in a coherent and consist manner,particularly for non-PFAS experts. It needs to be noted that tools proposed here that
integrate the concept presented in Sha et al. (2019) and the proposed elements of acharacterization system is one way of developing cheminformatics-based tools for
systematic characterization and categorization of PFASs. Depending on the needs, there
may also be other ways of doing so, including adding other elements into consideration(e.g. a ToxPrints approach that also considers structures related to adverse outcomes19)
or implementing in other ways (e.g. using Markush structures to annotate existing
lists20). An outlook of future developments is provided in the next section.
Table 2. Molecular structure-based elements of a characterization system for PFASs.
PFASs may beparsed into thefollowing two
structural parts

Molecular structurebased elements to beconsidered

Fluorinated carbon alkyl vs. alkylether
chain (A)perfluoro vs. polyfluoro

Note

Whether the fluorinated carbon chain is carbon only or has oxygenlinkage(s) between fluorinated carbons
e.g., –CnF2n– vs. –CnF2n–O–CmF2m–
Whether all hydrogen on the fluorinated carbon chain are replaced byfluorine (i.e. perfluoro) or not (i.e. polyfluoro)e.g., H–C2F4–, Cl–C2F4–, CF3CF2–C2H4–C2F4–C2H4–, CF3CF2–CH2–
CF2–CH2–CF2–, etc. = polyfluoro

linear vs. branched vs. Whether the fluorinated carbon chain is linear, branched or cyclic
cyclice.g., –C6F13 vs. –C3F6CF(CF3)2 vs. –cyclo(C6F12)
saturated vs. non-saturated Whether there is any unsaturated bond (a double or triple bond) in the
fluorinated carbon chaine.g., –CF2CF2- vs. –CF=CF–19

For an example, see https://figshare.com/articles/presentation/PFAS_Toxprints_A_Hierarchical_StructureBased_Categorization_Method_for_Characterization_of_Per-_and_Polyfluoroalkyl_Substances/12834329.
Currently, the US EPA is preparing a manuscript on this approach, including means for applying it.
20

A Markush structure is a generic type of description of chemicals used to summarize a potentially very large
set of closely related chemicals in a single condensed representation. It may consist of a “core” chemical structure
and a list of possible substituents attached to it, with four substituent options: substituent variation (allowing
different substituents at a position), position variation (allowing different attachment points for a substituent),
frequency variation (allowing substituents to occur multiple times) and homology variation (using genericexpressions covering many specific substituents like “alkyl”). For more details, see, e.g., Geyer P. 2013. World
Patent Information, 35(3), 178–182, https://doi.org/10.1016/j.wpi.2013.05.022.
The US EPA CompTox Chemicals Dashboard uses “Markush structures” to organize its PFAS list. In brief, theDashboard has curated 112 PFAS Markush structures with unique DTXSIDs assigned (e.g. DTXSID80893896
HOOC–(CF2)n–COOH for perfluoroalkyl (linear) dicarboxylic acids, i.e. homology variation). Each PFAS
Markush structure is considered a generalized substance or “parent ID” that can be associated with one or many“child IDs” within the Dashboard (e.g. DTXSID80893896 are linked to 12 linear perfluoroalkyl dicarboxylic
acids with different fluorinated carbon chain lengths in the Dashboard). For more details, see
https://comptox.epa.gov/dashboard/chemical_lists/EPAPFASCAT.

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polymericpolymericvs.

non- Whether the fluorinated carbon chain is polymeric or non-polymeric
e.g.
using
the
OECD
definition(http://www.oecd.org/env/ehs/oecddefinitionofpolymer.htm)[Note: this may require additional consideration, e.g. whether aminimum perfluorocarbon moiety chain length of 20 would be
required]21

chain length of the e.g., for perfluoroalkylether-based substances, the total length offluorinated carbon chain
perfluoroalkylether moieties including both carbon and oxygen atoms
will be counted, and additional information on the number of oxygen
atoms will be provided as supplementary information, similarly to what
is in the OECD 2018 list.
Functional group (B) types and structures of As there is no common classification system of functional groups, here
functional groups
a simplified scheme is proposed that is intended to distinguish thosereactive and non-reactive (or those not so reactive) groups undernatural conditions, which can be used to differentiate e.g. PFAAs and
PFAA precursors.
1. Non-reactive groups (or those not so reactive)
1.1. H, Cl, Br
1.2. N, P
1.3. COOH
1.4. SO3H
1.5. PO3H2
2. Reactive groups
2.1. I
2.2. SO2H – sulfinic acids
2.3. PO2H
2.4. CH2–R – possibly n:1 fluorotelomers
2.5. CH2CH2–R – possibly n:2 fluorotelomers
2.6. CO–R (other than COOH) – alkanoyl fluoride-derivatives
2.7. SO2–R (other than SO2OH) – sulfonyl fluoride-derivatives
2.8. CmH2m+1, OCmH2m+1, CmH2m-1
3. Others (which may be refined in future work)
polymericpolymericstoichiometry
between A and B

vs.

non- Whether the non-fluorinated functional group is polymeric or nonpolymeric,
e.g.
using
the
OECD
definition(http://www.oecd.org/env/ehs/oecddefinitionofpolymer.htm)[Note: this may require additional consideration of additional qualifier,e.g. whether a minimum chain length of 20 would be required]

How are fluorinated carbon 1:0 = no functional groupchain(s) connected with
non-fluorinatedcarbon 1:1/1:2/1:3 = one fluorinated carbon chain connected with 1/2/3
chain(s)/functional groups? functional group(s)2:1 = two fluorinated carbon chains connected with one functionalgroup, e.g. PFPIAs

21

In many jurisdictions, a polymer is defined as a substance that has over 50 percent of the weight consisting ofpolymer molecules and the amount of polymer molecules presenting the same molecular weight must be lessthan 50 weight percent of the substance. A polymer molecule is defined as a molecule that contains a sequenceof at least 3 monomer units, which are covalently bound to at least one other monomer unit or other reactant.
Thus, a mixture of 8:2, 10:2 and 12:2 fluorotelomers (each 33%) can theoretically be regarded as a polymer.

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Table 3. Examples using the proposed characterization system.
Possible elements tobe considered

Example 1:
Linear PFCAs

Example 2:
PFOA precursors

Example 3:
ADONA

Example 4:
6:2 FT-acrylate polymer

Example 5: PTFE with –COOH on each end

alkyl vs. alkylether

Alkyl

Alkyl

Alkylether

Alkyl

Alkyl

vs.

Perfluoro

Perfluoro

Polyfluoro

Perfluoro

Perfluoro

linear vs. branched vs.
cyclicLinearLinear + BranchedLinearLinearLinearsaturated vs. nonFluorinatedsaturatedcarbon chain
polymeric vs. non(A)polymericSaturated

Saturated

Saturated

Saturated

Saturated

Non-polymeric

Non-polymeric

Non-polymeric

Non-polymeric

Polymeric

1–20>=7 (in the case of when A
and B connects via a carbon
atom);>=8 (in the case of when A
and B connects via otheratoms other than a carbon
atom)

6 + 2O

6XX

1.3 COOH

2 Reactive groups

1.3 COOH

2.5 CH2CH2–R –possibly n:2 fluorotelomers

1.3 COOH

Non-polymeric

Non-polymeric; polymeric

Non-polymeric

PolymericNon-polymeric

1:1

1:1

1:1

n:1

1:2

perfluoro
polyfluoro

chain length

Nonfluorinatedfunctionalgroup (B)types and structures offunctional groups
polymeric vs. nonpolymeric

How are fluorinatedcarbonchain(s)Connection
between A and connected with nonfluorinatedcarbonBchain(s)?

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5. Areas for Future Work

While this report makes advancement on several important points regarding the PFAS
terminology and practical guidance of how to use the PFAS terminology, it also recognizes
that the following four areas warrant further work within the field of PFASs (i.e. areas one
and two) and beyond (i.e. areas three and four), in order to facilitate clear and unambiguous
communication of PFASs.
First, a centralized PFAS nomenclature database/platform may be considered. With thefurther advancement and application of non-target screening methods, many moreunknown PFASs are expected to be discovered in the future. Such a centralized
nomenclature database/platform can help foster the use of harmonized names and acronyms
for the same compounds. It can also help to link different common names and acronyms
that have been used over time to specific substances.
Second, further development of cheminformatics-based tools for automated systematiccharacterizing and categorizing PFASs would advance the field. A solely structure-based
approach proposed in the report (i.e. Chapter 4) may serve as one starting point for possible
joint development of an open source tool by experts from different online
databases/platforms so that such a tool may be compatible for different online
databases/platforms. In addition, as cheminformatics is a fast-developing field, future work
may be conducted to monitor, assess and communicate which cheminformatics tools aredeveloped for which purposes.
Third, further work on the characterization and reporting of polymers should be considered,as well as assessment of their properties. The current definitions of polymers in many
jurisdictions originate from the OECD definition of polymer that was developed in the early
1990s, and in some cases, substances containing a significant fraction of low-molecularweight molecules may be identified as polymers, as indicated in Footnote 21. This may
impact how individual substances are registered (or not registered) and subsequent
regulatory requirements of safety information. Thus, chemical compositions in substancesthat are identified as polymers may warrant a closer look, particularly in terms of their lowmolecular-weight content, based on lessons learned in the past three decades. In addition,
the current reporting of many polymers are often rather ambiguous (e.g. a polymer may be
named as a co-polymer of three monomers A, B and C without indicating how themonomers are connected and in which molecular ratios, reaction schemes and molecularweight range of individual compositions, which could have implications on assessing thefate, behavior and risks of specific polymer products). Thus, future international efforts areneeded to look into ways to improve the understanding of polymer structures including
access to necessary information, focusing on polymeric PFASs or on polymers in general.
Fourth, as shown in Figure 8, there are many groups of organofluorine substances otherthan PFASs. Future work could also look into these compounds, including the terminology
of many fluorinated aromatics.

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References

Barzen-Hanson KA, Roberts SC, Choyke S, Oetjen K, McAlees A, Riddell N, et al. 2017. Discovery of 40
classes of per- and polyfluoroalkyl substances in historical aqueous film-forming foams (AFFFs) and
AFFF-impacted
groundwater.
EnvironSci
Technol
51(4),
2047–2057.
https://doi.org/10.1021/acs.est.6b05843
Buck RC, Franklin J, Berger U, Conder JM, Cousins IT, De Voogt P, et al. 2011. Perfluoroalkyl andpolyfluoroalkyl substances in the environment: terminology, classification, and origins. Integrated
Environmental Assessment and Management 7(4), 513–541, https://doi.org/10.1002/ieam.258
FOEN (Swiss Federal Office for the Environment). 2017. Additional information in relation to the risk
management
evaluation
of
PFOA,
its
salts,
andrelatedcompounds.
http://www.pops.int/TheConvention/POPsReviewCommittee/Meetings/POPRC12/POPRC12Followup/P
FOAComments/tabid/5950/Default.aspx. (submitted on 31 May 2017).
Newton S, McMahen R, Stoeckel JA, Chislock M, Lindstrom A, Strynar M. 2017. Novel polyfluorinatedcompounds identified using high resolution mass spectrometry downstream of manufacturing facilities
near Decatur, Alabama. Environ Sci Technol 51(3), 1544–1552. https://doi.org/10.1021/acs.est.6b05330.
Sha B, Schymanski EL, Ruttkies C, Cousins IT, Wang Z. 2019. Exploring open cheminformatics
approaches for categorizing per- and polyfluoroalkyl substances (PFASs). Environmental Science:
Processes & Impacts 21(11), 1835–1851. https://doi.org/10.1039/C9EM00321E.
Wang Z, Cousins IT, Scheringer M, Hungerbuehler K. 2013. Fluorinated alternatives to long-chain
perfluorocarboxylic acids (PFCAs), perfluorosulfonic acids (PFSAs) and their potential precursors.
Environ Int, 60, 242. https://doi.org/10.1016/j.envint.2013.08.021.
Wang Z, Cousins IT, Berger U, Hungerbühler K, Scheringer M. 2016. Comparative assessment of theenvironmental hazards of and exposure to perfluoroalkyl phosphonic and phosphinic acids (PFPAs and
PFPiAs): Current knowledge, gaps, challenges and research needs. Environ Int, 89, 235.
https://doi.org/10.1016/j.envint.2016.01.023.

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