I've never understood one vital thing - if PFAS is by nature totally inert and unreactive, how is it harmful? If you drank a glass of the stuff, what would happen?
Then of course we produced it at industrial scale for decades flooding the entire planet with this stuff.
For example PTFE is a large molecule with strong bonds, and as a consequence isn't very reactive and likely safe.
On the other hand, perfluoroalkyls such as PFOA have the same shape as fatty acids, so they bind to the same places such as in the liver, which makes them grave health hazards.
Many precursors used for making PFAS are also toxic, so for example, even if PTFE is safe, manufacturing it isn't.
I'm not sure what "biologically inert" means specifically. Are you saying there are biological chemicals that actually do interact with this stuff? A single example would help me understand.
Inert doesn't really say anything about toxicity, it's not directly related to that. The opposite is though, pretty much any strongly reactive chemical is dangerous or toxic in some way since it will react with stuff humans are made from.
With PFAS the inert example is also usually Teflon. That is also a solid polymer, so not many individual molecules. There isn't much you body could do to process a macroscopic chunk of Teflon, so you'd almost certainly just excrete it.
Edit: Nevermind, Wikipedia makes it pretty clear that even the non-broken-down PFAS are totally unsafe, evil things which we knew were dangerous since the 70s and did nothing about until recently
My understanding is that the bigger danger is e.g. a Teflon-producing plant than the final Teflon products (assuming the Teflon isn't damaged and heated too much). Because the plant has to handle the reactive ingredients, and those can leak into the environment.
They are plenty reactive in a sense of interacting with enzymes and other cellular machinery.
Maybe sci-hub has a copy of the full paper. Not sure.
As briefly as possible, and therefore glossing over many many details, the toxic effects are mainly due to cell membrane perturbation, cell membrane transport disruption, and binding to hydrophobic protein cavities (thus disrupting the usual function of these cavities).
3M and DuPont knew since the 1970s and suppressed the information, not dissimilar to how tobacco and oil industries created disinformation about externalities.