From my colleague: "The highly fluorinated chemicals that we discussed are organic compounds, meaning that they contain carbon atoms. They are called highly fluorinated or per- and polyfluoro alkyl substances because they have many fluorine atoms, and the carbon atoms are entirely or often bound to fluorine (There may be highly fluorinated chemicals that aren't strictly PFASs, but that's a minor point). These highly fluorinated chemicals have unique chemical properties that give them their characteristic grease and water resistance (typically one end of the molecule is more soluble in oil, and the other end is more soluble in water, similar to other surfactants). The concerns about PFAS toxicity stem from the properties of the whole molecule and not just because they contain fluorine."