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?

As I understand it they discovered a long chain molecule which was highly inert and wouldn’t stick to anything. Which was a useful feature but you know makes it hard to attach to anything. So they created a similar smaller chain molecule which had a reactive tip but was still super stable. Unfortunately it’s also a bit amino acid like. So we ended up with a molecule which is very durable and accumulates in living things.

Then of course we produced it at industrial scale for decades flooding the entire planet with this stuff.

PFAS are many different molecules.

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.

Not biologically inert. And they bioaccumulate in humans. https://en.wikipedia.org/w/index.php?title=PFAS&section=10

As far as I understand the nomenclature, PFAS covers both the inert final products like Teflon and reactive intermediates, degradation products and reactants. It's a very broad category of chemicals.

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.

I think, if I’m reading correctly, that PFAS are the thing that PFXX stuff gradually breaks down into? https://pmc.ncbi.nlm.nih.gov/articles/PMC12028640/

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

Fluoroalkyl chemicals are only "inert and unreactive" in a relatively narrow sense of "wouldn't catch fire", "don't react with strong acids and bases", and similar.

They are plenty reactive in a sense of interacting with enzymes and other cellular machinery.

PFAS used to be considered totally inert but later research showed correlations between bad health effects and higher concentrations of PFOA and PFOS.

3M and DuPont knew since the 1970s and suppressed the information, not dissimilar to how tobacco and oil industries created disinformation about externalities.

Making PFAS and having to dispose of byproducts is the nasty part as far as I understand. There is also some kind of reaction that can happen where it will off gas nasty enough stuff to kill your pet bird if you overheat your pan.