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.

If anyone wants to read up on this. One case is Dordrecht in the Netherlands, former Dupont now called Chemours. Not a pretty story....