Four sections of the PubChem PFAS Tree are collated by running custom-designed PERL scripts (available on GitLab ) over the entirety of PubChem on a weekly basis, since the chemical content of PubChem updates daily, and annotation content weekly. The "OECD PFAS definition" section contains all discrete chemicals (excluding salts and mixtures) fulfilling the 2021 OECD PFAS definition 3 quoted above (hereafter termed an "OECD PFAS"), while the "PFAS breakdowns by chemistry" section contains all discrete chemicals, including salts and mixtures, that are an "OECD PFAS" . Figure of the OECD Monograph ENV/CBC/MONO(2021)25 3 also included a breakdown of organofluorine content into several aliphatic and aromatic categories; this structure is reflected in the "Organofluorine compounds" section of the PubChem PFAS Tree (see Figure ). Over 100,000 fluorinated compounds in PubChem did not fit into the categories set out in the OECD Monograph, either because fluorine was connected to non-carbon atoms or due to the presence of non-organic elements (or both). These cases were separated into the "Other diverse fluorinated compounds" section, which was broken down into these two subsections (see Figure ). A more detailed description of the contents of each section and how this is constructed is contained in the PubChem PFAS Tree documentation .