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!
1) How many hours?
2) Which gas?
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.