I would like to mention that there WAS safety film prior to WW 2. Many films (movies) intended for home and school use were printed on early "safety" film, however it was not as satisfactory as Nitrate as far as stability and wear for professional use. Motion picture projectors up well into the 1950's were equipped with elaborate "safety" gear to limit fire. There were enclosed magazines for the reels of film, with "fire" traps, which where a series of overlapping rollers the film traveled thru to get to the projector. This was to limit any fire that started in the projector from getting to the reels of film. In addition, the projector and sound head had doors that closed over the mechanisms, thus isolating the film from the outside room. In addition, all reels of film not being projected were stored in metal cabinets that had a slot for each reel, and each reel had its own compartment with a door. Even the electric rewind machine was completely enclosed, such that you put the film in, shut the door and turned it on. It rewound the film and turned itself off. In addition, chemical fire extinguishers were often mounted above the projectors. I believe it was in 1950 that the movie industry was forced to make the switch to all safety film. So in a properly equipped and operated projection booth, there was never any film left casually out to create any problem from a cigarette ash or spark. And yes, projectionists (I was one) could smoke in the projection booth, which in itself was a fireproof room. I am not old enough to have worked during the Nitrate period, but I have worked in many projection booths that were still in use, that were designed and equipped during the Nitrate period. In addition, films at least thru the 1980's were shipped in very heavy steel cases that originated during the Nitrate era.
I would not feel uncomfortable projecting a well preserved Nitrate film today, if I personally inspected it first foot by foot, and the projection system was still fully nitrate compliant. In fact I have done this in a theater that I owned at one time. If the film is not starting to deteriorate, then normal projection with a Nitrate skilled projectionist with well running equipment is not a particular hazard.
I wouldn't dream of running a Nitrate film in a modern multiplex, because in modern projectors, they have done away with any form of "containment" for any projector fire, as all modern film does not burn.