Giving you a small window into emulsion making, one of the steps is 'finishing' or 'chemical sensitization'.
During this step, a sulfur compound such as thiourea is added to the emulsion and it is heated to 60 degrees C for about 1 - 2 hours. The reaction is then terminated by adding a restrainer such as benzotriazole or something like that. During this process, a given grain can gain several stops in speed.
If you overdo this process, you get less speed, and lots of fog.
The trick is to sell and use the film between the time when you have the best speed/fog ratio and before the natural warmth that film is exposed to causes the process to start over (such as in the glove compartment of your car). So, film will die a 'heat death' if kept too warm for too long. You can slow this down by keeping film cool or frozen.
Radiation will also cause the same effect, but cannot be stopped by cooling the film.
The faster the film speed, the more sensitive it is to heat and radiation. In practical terms, this defines the fastest a film can be and still be 'salable'. I believe that figure to be about ISO 3200 - 6400 for B&W. It is my understanding that the fastest film speed ever achieved was ISO 20,000, but the film would have been quickly rendered unusable due to the sentivity to heat and radiation.
Old films with ISO speeds of 100 or less apparently keep quite well, as do some printing papers such as Azo. Keep in mind as well that these were often stabilized with heavy metal salts such as mercury and cadmium which don't decompose with time, but the newer films are stabilzed mainly with organic compounds which do decompose with time. Therefore, modern films and papers are more environmentally friendly, but may keep less well than some older films and papers.
This is not a simple problem for the engineer.
PE