Manx

You missed my point. There are essentially no artificial substances. If a substance doesn't exist and it is created in a lab, it is still a natural product. So to claim that because DU exists somewhere, under some conditions (deep underground, on Mars, wherever) it is not an artificial danger to the ecosystem is true. But nothing at all, man-made or not, is an artificial danger to the ecosystem. But anything (even water) can be an danger to a specific ecosystem when it is artifically added.

So essentially, your original point, the distinction of the "naturalness" of DU is not important.