I am an archivist in a repository with an extensive historic photograph collection.
There are two general types of old film negatives that are potentially dangerous: cellulose nitrate based film and acetate based films. Not only are old negative toxic, in many cases they are EXTREAMLY flammable. I would not let my baby chew on them, and I woudl not even have them in the house if they can be scanned at a very high resolution, stored in a stable digital format, and the original film disposed of as hazardous waste.
See this for more details: http://www.nedcc.org/resources/leaflets/5Photographs/01ShortGuide.php
Nitrate film is highly flammable, it releases hazardous gases when it deteriorates into a substance similar to nitroglycerine. Large quantities of nitrate film has caused several disastrous fires. Due to the instability of cellulose nitrate, much of our photographic legacy from this period is disappearing. A photographic collection that contains any flexible, transparent film negatives from the time period of 1890-1950 has nitrate film in it. These negatives need special attention and should immediately be separated from other negatives. Deteriorated nitrate negatives are easy to identify, but nitrate negatives in good condition are almost indistinguishable from other types of transparent films. There are four ways to identify nitrate negatives. See the above link for details. We are a regional storage facility for our government agency, and store our nitrate negatives in special scientific freezers, in a fire-vault room. We disposed of deterioratied nitrate negatives as hazardous waste, and work with the county fire department to ensure safe disposal.
Nitrate film was, and is, highly flammable. It releases hazardous gases, when it decomposes naturally. Beginning in the mid 1920's, it was slowly replaced with cellulose acetate film base (cellulose diacetate, cellulose acetate propiarate, cellulose acetate butyrate and cellulose triacetate.) It became known as "Safety" film. However, the cellulose acetates do have stability problems. The deterioration of cellulose acetate is autocatalytic, like that of cellulose nitrate; once deterioration has begun the degradation products induce further deterioration. It affects the plastic support of acetate film, causing it to become acidic, to shrink, and to give off an odor of acetic acid (vinegar).
A useful tool in helping determine the amount of acid vapor present, and gain an overview of the condition of acid-vapors in an entire collection are "A-D Strips" (acid-detecting strips) from the Image Permanence Institute at the Rochester Institute of Technology in Rochester, NY. They are acid-base indicator papers which turn from blue to green to yellow in the presence of acid, and measure the extent of the acetate base support deterioration.
As with nitrate negatives, deteriorated acetate negatives are easy to identify, but in good condition are almost undistinguishable from other types of plastic films. There are also four ways to identify acetate film base negatives.
Again, see the link above for identification details.