ZeroPM Webinar: Are there really 6 million PFAS in PubChem? The increasing concerns about poly and perfluoroalkyl substances (PFAS) and calls for action upon them as a class has spurred intense debates on how to define and enumerate the “PFAS Chemical Space”. There are now >50 PFAS lists openly available, including the OECD PFAS list of ~4700 PFAS (ENV/JM/MONO(2018)7) and the US EPA PFASMASTER list of >12000 PFAS. However, searching the large open chemical collection PubChem (114 million chemicals, Feb. 2023) reveals that >6 million entries match the latest OECD PFAS definition where PFAS “contains at least one alkyl CF2 group” (ENV/CBC/MONO(2021)25). This webinar will introduce listeners to the new classification browser in PubChem designed to help navigate these incredible numbers, the “PFAS and Fluorinated Compounds in PubChem Tree” (“PubChem PFAS Tree” for short). The current version contains six main sections: OECD PFAS definition (>6 million PFAS), organofluorine compounds (>19 million compounds), other diverse fluorinated compounds, OECD PFAS by chemistry (>7 million PFAS including salts and mixtures), several PFAS collections (from CompTox, NORMAN-SLE, NIST, OntoChem and PubChem) and finally regulatory collections. We will walk listeners through the PubChem PFAS Tree and the many features it offers to help users explore the PFAS space in PubChem and look forward to lively discussions with the audience afterwards. A recording of this webinar is available here on the ZeroPM YouTube channel!