According to James Reilly, in the IPI Storage Guide for Acetate Film (Rochester, NY:IPI, 1993), all film-based negative deterioration is dependent on exposure to high humidity and temperature. Once conditions are right for deterioration, chemical changes become cumulative and actually rapidly increase.
The characteristics of the deterioration vary depending on the type of negative. Of great concern are nitrate negatives that are not only fragile but also dangerous.
If you discover you have nitrate film, have the negatives copied at a reputable photo conservation lab in your area. Federal regulations imposed by the United States Department of Transportation restrict shipping
nitrate because it is considered a fire hazard. Once you have the copies, ask your local fire department how to dispose of the negatives. Most communities mention the disposal of nitrate in their fire codes.
Steps for identifying nitrate film and other useful information on handling such film can be found in Preserving Your Family Photographs.