There's a story of a university lab where research in hydroponics was being conducted. Various irreproducible results were found over a period of months. Upon closer investigation, it was determined that a stock solution, purportedly of Mg, had in fact been prepared by a graduate lab assistant using salts of Mn. That's the stuff you gotta watch.
The problem with Paracelsus' line of reasoning in this situation is that Pb bioaccumulates; the LD50 is relatively high, but the threshold for daily exposure over a period of months or years is not. For people who handle lead routinely on an occupational basis, this is a serious problem, and is the reason why blood tests are used in places like the battery industry. This is where you could run into problems with the DM who stacks lead on the boat twice a day over a period of years.