I interpreted the active shield as being a dispersed set of nanobots which would exist throughout the ecosystem and act as an immune system. Any harmful (or perhaps just unapproved) nanobot would be targetted and destroyed. You could also have these in your house and even in your body, although the hope would be to eliminate threatening nanotech before it got that close.
There is a good fictional interpretation of this in Neal Stephenson's Diamond Age. There is a constant warfare between good and bad nanotech going on all around. The air is full of "soot", which is composed of the destroyed bodies of the broken nanotech. Apparently the good guys always win as nobody melts down in the book, as I recall.