The decomposition of nitrate film is an exothermic reaction. It creates its own heat. When the heat builds up to a high enough level, that's when spontaneous combustion occurs. If you don't let the film get hot enough it won't ignite. The problems are twofold. The exact temperature at which nitrate film ignites isn't well understood. (Somewhere around the 100ºF mark.) Heat builds up slowly and film usually becomes "critical" when nobody is around to notice.
The film isn't going to just burst into flames. It goes through several stages of decomposition first. It only self-ignites in the later stages. It can decompose all the way through the final stage without igniting but, once it gets past the beginning stages, it will be too far gone to be useful. By that time, you should have been able to address the problem before it becomes more dangerous.
We're photographers and we're used to handling substances that are more toxic, flammable or dangerous than ordinary people so I have less worries about telling people here what I know about nitrate film. I have handled it but have never projected because it is illegal to project nitrate in a facility that is not specifically outfitted for the purpose. When I came across it, I set it aside and I took it out to the loading dock. Later, when nobody was around, I quietly took it away and got rid of it. Let's just say the film has been destroyed and leave it at that.
(It was only a small roll, about 6 inches in dia.)
If you come across nitrate film, segregate it and put it in a safe place where, if it catches fire, it won't do major damage. (Outside, away from buildings.) Then, at your first opportunity, have it destroyed. If the images are valuable, have it copied first.
If you want to know more about nitrate film, check the following links: