I went to school for chemistry and chemical engineering. This is pretty elementary environmental chemistry, no PhD required.

There are an enormous number of popular myths about the toxicity, non-toxicity, and comparative toxicity of many elements and chemicals. Understanding the actual risks are part of a chemist's job. Most people lack the chemistry education required to put claims about particular chemicals into context, so their intuitions about the relative health risks of chemicals are often wildly incorrect. Many ubiquitous household items are much more of a health risk than DU, both chemically and as a radionuclide, but no one worries about it.

When the DU shell explodes upon impact or fired through a barrel, it oxidizes and if inhaled can pose a severe health risk. There's lots of news articles on illnesses caused by DU.