Whether they might be "harmful" to the kidneys on the way out is almost moot since they are very harmful while still inside the body. There are two places where heavy (toxic) metals exist in the body.

There are both "intravenous chelation therapy" and "oral chelation." IV chelation uses the artificial amino acid, EDTA, only, to remove. This is not well understood by most medical doctors since there is no possibility that any drug could remove metals or reduce free radical activity. Vitamins and various natural substances can neutralize free radicals, but drugs mostly just create them, not stop them. You could have many thousands of these "pieces" of metal in your body -- this did not exist a few hundred years ago. One is free floating in fluids, mostly blood, and the other is "stuck" into some tissue -- these are tiny -- about the size of several molecules. How Chelation Works Here is a mechanical explanation of the chelating process -- and a comparison of intravenous chelation therapy with oral chelation. The "harm" caused by metals is because they act as "multipliers" of free radicals.

So, a tiny molecular amount of metal can be "stuck" in any organ, lung, liver, heart, flesh, etc.