> Where I live the norm is 20 ng/L

How did they come to this?

Seems like a reasonable measure would be to look at a native person, from ones lineage, having a relatively average pre-industrial day.

If lineage doesn't matter, and all humans are the same, you could probably just go sample some tribe members that still lives mostly outdoors, outside of developed society.

And, regardless, I'll listen to my doctor.