Originally Posted by
brdad
I remember finding some that were stored in my father's closet that had failed, but anything is possible. So, I did some extensive research (Ok, I just googled and read the summaries of the first page), and "Six year shelf life" was mentioned. So, they can apparently fail in time, and one could assume that extreme heat, cold, or other factors could affect that predicted shelf life.
Now, if you put two sets of hand warmers into an ammo box, and activate one of them, maybe the activated ones would keep the unactivated ones warm enough to maintain that shelf life during the Winter. :)