Per- and polyfluoroalkyl substances (PFAS) are of concern because of the ir high persistence (or that of their degradation products ) and their impacts on human and environmental health that are known or can be deduced from some well-studied PFAS . Currently, many different PFAS (on the order of several thousands) are used in a wide range of applications , and there is no comprehensive source of information on the many individual substances and their functions in different applications. Here we provide a broad overview of many use categories where PFAS have been employed and for which function; we also spec ify which PFAS have been used and discuss the magnitude of the uses. Despite being non-exhaustive, our study clearly demonstrate s that PFAS are used in almost all industry branches and many consumer products. In total, more than 200 use categories and subc ategories are identified for more than 1400 individual PFAS. In addition to well-known categories such as textile impregnation, fire -fighting foam, and electroplating , the identified use categories also include many categories not described in the scientif ic literature , including PFAS in ammunition, climbing ropes, guitar strings, artificial turf, and soil remediation. We further discuss several use categories that may be prioritised for finding PFAS -free alternatives . Besides the detailed description of us e categories, the present study also provides a list of the identified PFAS per use category , including their exact masses for future analytical studies aiming to identify additional PFAS . 2 Environmental Significance Statement Per- and polyfluoroalkyl substances (PFAS) are a large group of more than 4700 substances that are used in a wide range of technical applications and consumer products. Releases of PFAS to the environment have caused large -scale contamination in many countries. For an effective m anagement of PFAS, an overview of the use areas of PFAS, the functions of PFAS in these uses, and the chemical identity of the PFAS actually used is needed. Here we present a systematic description of more than 200 uses of PFAS and the individual substance s associated with each of them (over 1400 PFAS in 2 total). This large list of PFAS and their uses is intended to support the identification of essential and non-essential uses of PFAS. 3 Introd uction Per- and polyfluoroalkyl s ubstances (PFAS) are a class of thousand s of substances1,2 that have been produced since the 1940s and used in a broad range of consum er produ cts and industrial applications.3 Based on concerns regarding the high persistence of P FAS4 and the lack of knowledge on properties, uses, and toxicological profiles of many PFAS currently in use, it has been argued that the production and use of PFAS should be l imited.5 However, there are specific uses that make an immediate ban of all PFAS impractical. Some speci fic uses of PFAS may currently be essential to health, safety or the functioning of today's society for which alternatives so far do not exist. On the other hand , if some uses of PFAS are found to be non-essential, they could be eliminated without having t o first find alternatives that provide an adequate function and performance. To determine which uses of PFAS are essential and which are not, the concept of “essential use ,” as defined under the Montreal Protocol , has recently been further developed for PFAS , including illustrative case studies for several major use categories of PFAS .6 PFAS are costly to produce (e.g. fluorosurfactants are 100 -1000 times more expensive than conventional hydrocarbon surfactants per unit volume7) and therefore are often used where other substances cannot de liver the required performance ,1 or where PFAS can be used in a much smaller amount and with the same performance as a higher amount of a non -fluorinated chemical. Examples are use s that operate over wide temperature ranges or uses that require extremely stable and non - reactive s ubstances. The C -F bonds in PFAS lead to very stable substances, a feature that also makes the terminal transformation products of PFAS very persistent in the environment. Furthermore, the perfluoro carbon moieties in PFAS are both hydrophobic and oleophobic , making many PFAS effective surfactants or surface protectors.8 PFAS -based fluorosurfactants can lower the surface tension of water from about 72 mN/m9 to less than 16 mN/m , which is half of what is attainable using hydrocarbon surfactants .8,10 Likewise , the surfaces of fluorinated polymers have about half the surface tension compared to hydrocarbon surfaces . For instance, a close -packed, uniformly organized array of trifluoromethyl ( -CF3) groups creates a surface with a solid surface tension as low as 6 mN/m .11 Due to these and other desirable properties, PFAS are used in many different applicati ons. A good overview of the range of uses of PFAS as surfactants and repellents is provided in the monograph by Kissa (2001).3 It lists 39 use categorie s, mostly derived from paten ts, and describe s the function s of PFAS in these use categories . However, the work by Kissa (2001) was published nearly 20 years ago, focused on fluorosurfactants and repellents, and it is not clear which of the se uses are still relevant today. In addition to Kissa (2001) ,3 there are a few other monographs and a number of peer - reviewed scientific articles and reports that have looked into the uses of PFAS .8,12 –22 While these articles and reports provide useful information, each of them focuses on the uses of a specific PFAS group (in specific use categories). This is also the case for the review s from the Persistent Organic Pollutants Review Committee (POPRC) , the focuses of which are on perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PF OS), perfluorohexane sulfonic acid (PFHxS), their precursors, and the PFAS that may be or have been introduced as replacements for these PFAS.23–29 The FluoroCouncil30 has provide d additional informat ion on use s of PFAS . However, the information is rather generic with limited details about specific uses and substances . Hence, a comprehensive overview th at summarizes major current uses is missing. 3 The present paper , together with the Appendix and the Electronic Supplementary Information (ESI), aims to provide a broad , but not exhaustive overview of the uses of PFAS and associated individual substances (note that a working definition of PFAS is used here to define the scope of PFAS considered in this study, which is provided in the Method s section below ). The paper address es the following points: i) In which use categories have PFAS been employed and for which function s? ii) Which PFAS have been – and are still – used in a certain category ? iii) What is the extent of the uses in certain parts of the world ? Within the European Union (EU) , there are discussions underway for restrict ing PFAS to those uses that are essential ,31 and extensive information on many PFAS uses will be needed in this context. The present work also aims to support this process by showing in which specific applications PFAS are used , and in which function s, as a first step toward differentiating essential and non -esse ntial uses of PFAS . 4 Methods 4.1 Which PFAS are address ed? A first clear definition of PFAS was provided by Buck et al. (2011).1 They defined PFAS as aliphatic substances containing the moiety -CnF2n+1 within their structure, where n is at least 1. The OECD/UNEP Global PFC Group noted that many substances, containing other perfluorocarbon moieties (e.g. -CnF2n-), were not commonly recognized as PFAS according to Buck et al. (2011) , e.g. perfluorodicarboxylic acids.2 Considering their structural similarities to commonly recognized PFAS with the -CnF2n+1 moiety, the OECD/UNEP Global PFC Group proposed to also include substances that contain the moiety -CnF2n- (n ≥ 1) as PFAS.2 The present study is in line with this proposal in several, but not all, respects . In con trast to the definition by Buck et al. (2011) , the present study also includes i) substances where a perfluorocarbon chain is connected with functional groups on both ends, ii) aromatic substances that have perfluoroalkyl moieties on the side chains, and iii) fluorinated cycloaliphatic substances. More specifically, the present study focuse s on polymeric PFAS with the -CF2- moiety and non - polymeric PFAS with the -CF2-CF2- moiety . It does not include non -polymeric substances that only contain a -CF3 or -CF2- moiety, with the exception of p erfluoroalkylether s and per - and poly fluoroalkylether -based substances . For these two PFAS groups , substances with a -CF2OCF 2- or - CF2OCFHCF 2- moiety are also included. 4.2 Literature sources The present inventory was started with the risk profiles and risk management evaluations for PFOA , PFOS, PFHxS and their related compounds to obtain an overview of uses of these chemicals .23–29 Reports and books that address fluoro surfactants and fluoropolymers in general were also included .3,8,12,16,20,21,32 –43 Literature specific to certain use categories was retrieved for more information either on the substances used , or to understand why PFAS are, or w ere, necessary for a given use. All specific