So how are they going to work? Are they going to treat casualites while in a hazardous situation? (Hazardous as in chemical, biological and so on.)
Yeah that's typically how they work.
Do they replace the Fire Brigade in the role of deconamination? I've not really researched H.A.R.T. units much.
No, they don't "replace" them as such, although they do perform that role.