Air activated hand warmers work by reacting
iron with air to form iron oxide, which you may
know as rust. The reaction is exothermic which
means that it releases heat.
You may be
wondering why these hand warmers work even though
under normal conditions iron does not rust simply
when exposed to air.Usually it takes water, air,
and iron to form rust and even then the rust forms
slowly. The answer is that the hand warmers
contain water so that they can form rust and also
a catalyst which speeds up the reaction so it
happens fast enough to keep you warm.
is a website with a more detailed