Three problems I see with these and MRE heaters (same thing, different shape), based on my usage of the MRE heaters, is as follows:

1, they produce a LOT more heat than you want by your skin, but not enough to heat much of a volume. 2nd degree burns or freezing.
2, they produce lots and lots of hydrogen. KABLAMO! Or you drown in hydrogen.
3, they must be kept utterly dry.

The basical hot-hands type heat packs have #3, to a lesser degree #1, and not #2. Keeping in mind they are just paper and rust when they are done, they are also reasonably safe. MRE heaters... not so much.
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-IronRaven

When a man dare not speak without malice for fear of giving insult, that is when truth starts to die. Truth is the truest freedom.