Not all types of hand warmers heat up when exposed to air, but the ones that do derive their heat from a process called oxidation, perhaps better known as rusting. Packaged warmers contain iron particles, salt water, carbon, and other chemicals that act as insulators. When the protective packaging is unsealed, outside air penetrates the chemicals inside, and the oxygen reacts chemically with the iron, causing the iron to rust or oxidize. This oxidation process is aided by the salt solution, which acts as a form of catalyst.
One of the results of this oxidation process is the production of heat, or in chemistry terms an exothermic reaction. The carbon particles help to spread this heat throughout the entire package. Other chemicals help to keep the exothermic reaction slow enough to provide long-lasting heat, not just a quick flare-up. Once all of the iron has been converted to iron oxide or rust, the exothermic reaction is over and the hand warmer can no longer provide heat.
The principle that give these products the ability to warm hands is the same principle that powers self-adhesive heating pads. Once the protective strip is removed from the pad, oxygen from the outside air reacts with the thin layer of iron threads in the pad and the result is an exothermic reaction that can last for hours.
There are other types of hand warmers which use a completely different chemical process to create similar results. These devices contain a supersaturated solution of either sodium acetate or calcium nitrate. Supersaturation means a solution has been superheated in order to allow more of a selected chemical to dissolve in it. When the solution cools, only a small speck of material would have to be introduced in order to cause the entire structure to crystallize and solidify.
Some hand warmers use this principle of supersaturation to create heat. A supersaturated solution of sodium acetate or calcium nitrate is sealed in a bag with a strip of roughened stainless steel. When the bag is manipulated back and forth, a tiny piece of metal should break off from the strip. This tiny fleck is enough to cause a salt crystal to fall out of solution and become solid. Within seconds, the solution crystallizes into a solid. As it does, an exothermic reaction occurs and usable heat is generated for at least 30 minutes. Unlike the oxidizing warmers, however, this type can be reused by reheating the salt solution until it becomes supersaturated and unstable again.