> Doesn't the MRE FRHs use a Magnesium reaction liberating Hydrogen gas.

Yep. I've used HeaterMeals brand food for years, indoors and out, with their self-heating saltwater/magnesium combos. I wouldn't want to say the meal would be _hot_ if the ambient temperature is freezing or less, but you might get a warm meal out of it.

The boxes do say they aren't allowed on planes, but then my tube of toothpaste isn't allowed on, either.