I had significant levels of mercury in hair but also in urine. That was a "fortunate" exception - with chronic poisoning you may end up - no, you usally
do end up always testing with very low levels that don't raise any alarms.
Growing up as a child in East Germany we had lead pipes at home, but they had a thick layer of mineral build-up and even though I also heavily used DMSA, a prime chelator for lead, no significant amounts of lead ever showed up. Given that health came back - to levels not seen in decades - without us ever seeing much of lead, only mercury, I guess it must have been mostly mercury.
The only possible source is amalgam fillings in teeth, which I got in childhood in East Germany. I found a document that indicates that the kind of mixture they used there was different and was abandoned elsewhere long before, so that may be a contributing factor. Nevertheless, my jaw bones had lost their trabecular structure and were in a bad state in exactly the places where the amalgam fillings had been. This was found by the doctor who just wanted to do a regular injection into the buccal mucosa - and ended up deep inside the bone. Ooops, that's bad, he said. He injected DMPS, mercury chelator, and in the following months my jaw bones "came alive". In a positive sense. A year later he found the bone impenetrable everywhere he tried, it was penetrable by a simple needle without pressure in all four quadrants before that (I had had fillings in all quadrants, fortunately none very deep).
I also remembered that my East German dentist, after reunification asked me if she should replace some of the amalgam fillings with plastic fillings. I had no idea why and what and did not care, I always said "yes" to every suggestion a doctor ever made to me, I was completely unaware of any "amalgam controversy" until I had to do my desperate research of what was wrong with me. But now I remember that at exactly that time, just a year after I had started studying, I had suddenly and out of the blue developed severe, almost asthma-like allergy, had inexplicable problems in various joints, had terrible sleep issues, and a lot of other stuff. And I barely managed to pass the math classes (CS study) - I had always been a top math person and we had high-level math instruction at high-school (in year 3 of the CS study I discovered that the lecture was exactly about the same stuff that we had had in high-school from our over-achieving teacher). I had gotten "stupid", but I only realized it when everything cleared up under the chelation treatment and I've taken over 60 mostly STEM courses on Coursera and edX and now my brain works again. My dentist had removed the amalgam fillings with a normal drill and without any protection. When I did the research eight years ago even a fresh doctor in residency warned me not to have my remaining amalgam fillings removed - because of the danger of mercury being freed ("this is dangerous" was what she said), the drill creates vapor no matter how much water you use to cool it, and mercury vapor is the worst since it can penetrate all the surrounding tissues. When I had the remaining fillings removed it was done with special equipment and a chelating agent to wash the mouth after each removed filling.
There's a lot more - A LOT... after all, all those years of (essentially) experiments that hardly anyone ever performs. I've taken waaayyyy more chelators than even the doctors who think amalgam fillings cause poisoning and offer chelation treatments usually do, but since I have had success that could be clinically proven, and since I seem to be handling the whole thing very reasonably the doctor keeps filling my prescription. My conclusion, based on actual, sort of, "experiments" - on and off chelators, is that heavy metals have effects given FAR lower amounts than are clinically relevant today. I feel my conclusion is in line with science, given that we have reached the official conclusion for lead that the amount with zero effects is zero lead - and mercury is worse than lead (according to an NIH document I found). But you won't be able to detect those effects unless you try. The slow decline of health and mind is very subtle, I was "lucky" that I hit an extreme. I see soooo many people with issues I had, such as psoriasis, warts on the feet, colds that won't end, issues with eyes, etc. etc. - a looong list, that I have to wonder.... I DID take bio-chemistry, physiology, anatomy, etc etc. - at least a thousand ours of medical lectures and courses if not more over the last few years, so I am aware of the complexity. I just think that heavy metals contribute much more than is currently accepted. All you have to do to get (what I now think is) a significant amount of mercury is to eat lots of sea fish, especially the predators.