A key factor seems to be the body's propensity to produce metallothionein [1], proteins that bind metallic compounds in the body, either to metabolise and utilise them (in the case of beneficial metals like zinc, copper, iron, selenium etc), or to remove them (in the case of toxic metals like lead, mercury, aluminium, cadmium etc).
This propensity seems to be determined by genetic predisposition and/or genetic expression in response to environmental stimuli.
This is why, in a group of people subjected to similar levels of environmental lead exposure (e.g., soldiers in the same military unit, children in the same town with contaminated water), only some will exhibit symptoms.
Here's my own experience:
I have not done any military service as an adult, nor used firearms more than a handful of times. (I participated in army cadets in high school but it was little more than boy-scout training - camping, survival skills, hiking, etc).
But I did spend a few years in my late teens/early 20s working in an electronics factory soldering circuit boards. I grew up in the last few years of leaded car fuel still being commonplace. I also grew up in an old house that may (or may not) have had lead water pipes.
For much of my life, I've experienced a complex set of vague, but moderately debilitating symptoms. Nothing apparently life-threatening, but always frustrating and often painful.
In childhood it manifested as respiratory problems, learning/cognition difficulties (including modest academic and sporting performance even though I tried hard at both and was often assessed as being of high natural ability), social difficulties, anxiety and mood instability, tinnitus, and digestive issues.
In adulthood it progressed into conditions such as depression, chronic fatigue syndrome, endocrine problems, painful muscle tension in the back, head, neck and hip regions, dizziness/confusion/fuzzy vision at times, among other things that have come and gone along the way.
I happened to do a home urine test just this morning, something I do from time to time. It indicated the presence of lead in my urine, which has routinely been the case for the past year or more.
I've also sent hair samples to a commercial testing lab every 6-24 months over the past 10 years, which have consistently suggested accumulations in my body of lead and other metals including aluminium, cadmium, mercury and others.
People may react with skepticism that home urine tests and commercial hair tests are of any clinical validity.
Fair enough.
But, as the linked article explains, you just can't easily get this stuff considered, tested and diagnosed by mainstream doctors.
I've seen multiple mainstream doctors over the years, and the best they can ever offer is anxiety or anti-depressant medication. They'll run tests on all the standard stuff - cholesterol, Vitamin D, iron... that's usually about it. Everything always shows up close enough to normal and they tell you you're fine and send you on your way.
But despite doctors' assurances, I haven't felt fine, and my ability to live a normal life, compared to most other people I observe, has not been fine.
So I do the only thing that seems to be available to me, which is home urine tests and commercial-lab hair tests. And they tell me something thing that makes sense when I read things like the linked article; there's just too much lead in my body.
In the 10+ years since this possibility became apparent and I started doing these kinds of tests, I've done a lot of work to get my physiology to a state where it could remove the metals from my system. I've considered doing chelation therapy but I understand that carries its own risks (i.e., removing important nutrients and moving toxic metals to areas where they can cause more harm), and that you're better off just supporting your body to detoxify at its own pace.
It seems to be working, slowly but surely. That would be why the lead is showing up in my urine now, after not doing so in the first few years; as alluded to in the article, it's been accumulated in my bones and is now coming out.
My health, and life, is steadily improving. People keep saying I'm looking healthier and functioning better. My fitness keeps improving. My muscle tension and pain keeps subsiding. My work performance and career seems to be on the up.
But boy, it's been tough to get through, and all the more difficult through the lack of mainstream medical recognition, and from the ridicule that inevitably comes your way as soon as you start talking about testing methods and healing techniques that are outside the mainstream.
That's probably the toughest part. I'm a scientifically-minded person from a family of mainstream science and medical professionals, yet due to the hand I've been dealt, in the absence of any other options, I've been forced to venture outside the mainstream, and have found myself being the subject of ridicule and scorn at times.
Anyone who has read this far might want to think about that next time you mock someone for seeking help from non-mainstream practitioners like naturopaths, acupuncturists, chiropractors, etc. You probably have little idea of just how much difficulty and frustration they've been through before winding up there, or what undiagnosed underlying issue has caused their predicament.
I know I'm not alone in having these challenges, and I know plenty of people have it a lot worse than me - particularly those lacking the financial means and family+social support that I’ve had.
With that in mind, it's high time that we started taking this problem seriously.
Knowing what I know now, it wouldn't surprise me if it turned out that lead and other metals toxicity is an explanation for a huge amount of the debilitating illness that mainstream medicine is currently unable to diagnose and treat.