Parts per trillion are small, but can still be bad for you. And this is an area of active study, the old threshold of 70 ppt was dropped a thousandfold recently:

https://www.hsph.harvard.edu/news/features/stricter-federal-...

The Wolverine shoe leather tannery up the road dumped their ScotchGuard contaminated scraps/waste in the swamps behind a farm owned by a family friend back in the 70s and 80s. Now my well water is contaminated with a few parts per trillion of PFAS, and the PFAS levels in my blood are significantly higher than the threshold of proven harm.