The older stuff would sometimes deteriorate and eventually give off a gas that would tarnish the metal parts. I don't know if the newer stuff does it as much or not. I think that with the new polymeres that issue has been taken care of. It is still a good idea to store them seperatly, away from your other knives. I am sure that any celluloid is still somewhat flammable, but to what extent the newer stuff is, I am not sure. It is basically the same material that sunglasses an such are made of. In the old factories, the roofs of the buildings were on hinges. This way, when they were producing celluloid, which at the time was a very explosive procedure,if the stuff blew up it wouldn't completely destroy the building. Just push the roof open and leave minimal damage inside. Needless to say, insurance became expensive or unobtainable which helped lead to the demise of many celluloid producing factories. Wouldn't that have been a great place to work.
Greg