How do “Little Hotties” work?

How do “Little Hotties” work? My work outdoors this winter has been vastly more comfortable because of these magic little pouches called “Little Hotties”.  Having lived in southern California for over 40 years, I'd never heard of these things.  Nearly every store here carries them: grocery stores, sporting goods stores, gas stations, even pharmacies.  They are available in different shapes and sizes, for various purposes.  For me, the hand and foot warmers are the key to my comfort. 

I wondered, though, how they worked – and whether there were dangerous or harmful chemicals in them. It was obvious from the packaging that the process involved oxidation, as the pouches come sealed in an airtight plastic bag.  As soon as you open the bag, they start getting warm.  But how?

Well, the short answer is that they're perfectly safe – they work by oxidizing iron powder.  They contain a very fine iron powder mixed with a few other ingredients that act to regulate the rate of oxidation (and therefore the temperature).  The Little Hotties web site has a description, and I found more here.  When you're done with one of these things, all that's left is a little plastic, some rust, a bit of salt, and a teaspoon or so of expanded rock (vermiculite).  All safe and even environmentally friendly, except for the bit of plastic...