I'm imagining something like this:-
1) Remove the inlet filter
2) Place device in a "sealed" cabinet (safety cabinet, fume cupboard; even an old out-of-use oven, 'fridge or chest-freezer)
3) Rig up an extension cable to power the device
4) Bung a few gas pellets in there
5) Have the device run merrily away for a few hours
6) Remove device
7) Run it in fresh air for a while
... best leave the windows open, though!
But:-
1) How many hours?
2) Which gas?
Answers:-
1) As long as needed (some trial and error may be required here)
2) Would depend upon the nature of the "contamination" (that is, what it is - which spore, pathogen
etc.)
But I wonder if a CS pellet would sort it (them) out?
Actually, I suspect that something a bit more "gentle" would suffice in most cases involving NIV's of this type. Especially as:- the trouble with a gas (vapour, whatever) that kills "all known pathogens" is that it is very likely to be a bit ... err, deadly.