I wonder if this has to do with limits being set below the detection limit of existing technology.
An off the shelf Agilent spectrometer with EPA approved methods can detect PFOA at .28 ng/L and PFOS at .78 ng/L:
https://www.agilent.com/cs/library/applications/application-...
This limit is not even the lowest EPA limit in drinking water. Radium is limited to ~5 picograms per liter, 1000x lower that the PFOA/PFOS limit. Dioxin is 0.03 ng/L.
https://www.epa.gov/ground-water-and-drinking-water/national...
> Drinking water health advisories (HAs) provide information on contaminants that can cause human health effects and are known or anticipated to occur in drinking water. *EPA's HAs are non-enforceable and nonregulatory* and provide technical information to drinking water system operators
The update was because of additional evidence of cancers. The EPA always acknowledged that .004 ppt is not reasonably detectable, but the point of an advisory is to note the level that is evidenced to be dangerous. Its a goal, and the new goal is "as good as possible". It should be obvious why the EPA should set requirements to reduce toxins while also noting what is a safe level.
https://www.epa.gov/system/files/documents/2022-06/technical...