Just because a concentration is low doesn’t mean it’s safe. Water with 0.7 mg/L of Po-210 is lethal.

You can put an amount of it in a 55 gallon drum that is not visible

It’s a natural substance

Fluoride is in fact safe at the amounts that the FDA regulates but saying it’s a small concentration or that it’s natural are not the reasons it’s safe. It’s the hundreds of peer reviewed research articles that show that it’s safe