Every node in the PubChem PFAS Tree (i.e., the blue numbers besides each category name in Figures ) or any classification browser in PubChem can be sent to PubChem Search by clicking on the numbers. A separate search window will open, which allows browsing and sorting of the results, the ability to interact with individual compound records, as well as the ability to save and combine searches (see Figure ) or send the content to Entrez for advanced search building and/or to browse in the classification browser (see Figure for example outputs). Each search query can then be downloaded in a variety of formats (see Figure ). It is also possible to upload custom lists to search via the PubChem landing page 20 (either pasting into the search bar, or via the "Upload ID list" option) or the PubChem Identifier Exchange . The download file contains a number of useful fields, a selection of which will be described here (for more information see the PubChem documentation 58 ). Several chemical identifier and structural information fields are included, such as names, synonyms (including CAS numbers where provided), the PubChem CID, the International Chemical Identifier (InChI) and the hashed form InChIKey , plus the Simplified Molecular Input Line Entry System (SMILES) . Several property fields are also given, including molecular formula, exact mass, molecular weight and octanol-water partitioning prediction (XlogP 66 ). Several additional fields help add context to the chemicals, including (at the time of writing; column header in brackets) the consolidated literature count (pclidcnt), patent count (gpidcnt), annotation categories (annothits), the count of annotation (annothitcnt), the date the CID was added (cidcdate), the names of the sources who deposited this structure (sidsrcname) and the deposition categories of the sources (depcatg). The annotation categories will be discussed more in the next paragraph; note that the columns, headers, and content are potentially subject to change. The patent and literature counts have been used for many years to help prioritise chemicals in non-target identification efforts , but as demonstrated in Figure and D, the distribution of the counts shown by the Chemical Stripes per chemical can also reveal interesting patterns, with the patent data often increasing earlier than the literature. This means that patent data could potentially be useful to find chemicals that are being used increasingly in industry (above the trend of other chemicals) before they are discovered through problematic emissions. It is possible to find recently added CIDs using the CID date (cidcdate). Since PubChem originated in 2004, this CID date will not always be an accurate reflection of the origin date of older chemicals. For older chemicals, the literature and patent dates can help build a more accurate history, as shown in Figure and D for PFOS, which was first added to PubChem in 2005, but was first mentioned in patents in 1913 and in the literature (within the collection available to PubChem) in 1981. The name of the depositors and the deposition category can help distinguish whether these chemicals come exclusively from patent literature or combinatorial libraries used for drug discovery, or whether these have been deposited by researchers, or the US EPA and so on. While these lists can be extremely long for well-known PFAS, these also tend to have substantial quantities of annotation, literature, and patent counts; the source information can help distinguish interesting entries among the long tail of matching chemicals with very little other data that potentially includes chemicals of high concern that have only just been discovered and documented.