There simply is no feasible way to have "zero" of pretty much anything today though. The molecules are out there and there time flows only in one direction. So thresholds simply mean that above that is when something can be done about it. If environmental levels rise thresholds would have to rise too because what can you do if it's everywhere? No use having the option to sue someone, including government, when it's out of there hands. Things like mercury or lead in the environment are global (even higher levels can be found in the arctic -- https://cordis.europa.eu/article/id/122855-a-better-understa...).
So for better or worse, life on this planet will have to adapt. There will only be action on the edges, when it gets so bad that it cannot be ignored. All the many small long term effects can't even be properly studied, because what do you compare with when it's everywhere and it's impossible to isolate only one or few chemicals' effects?
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So, in summary, citing thresholds in a medical discussion is pretty much just a distraction at best. It's for discussions about what to do, or what can you do (legally). The values of the thresholds don't easily or directly translate to "it's safe". Of course they are taken into consideration when threshold levels are set, but only as one factor.
Mercury,Hg:
“ The EPA has established a limit of 2000 parts per trillion (ppt) of allowable mercury of drinking water. The Food and Drug Administration (FDA) has set a maximum permissible level of 1,000,000 part of methylmercury in a trillion parts of seafood.”
Lead, Pb: Flint water crisis had 15,000,000 ppt of lead, then after clean up it dropped to FDA ‘allowable’ 15,000 ppt (ideally would be zero).
Cadmium, Cd: EPA limits 5,000 ppt measurements of Cd in drinking water.
Arsenic, As EPA limit of 10,000 ppt in drinking water, short term exposure about 130,000 ppt can cause health effects.
Copper, Cu EPA limits of 1,300,000 ppt in tap water.
Elements functionally involved in human biology: “sodium (Na), potassium (K), magnesium (Mg), and calcium (Ca) that belong to main group of elements, and vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn),”
This doesn’t exist, but I would imagine that the ideal water would have the exact ppt levels of these elements needed by the human body and nothing else.