Air-activated hand warmers contain cellulose, iron powder, activated carbon, vermiculite (which holds water) and salt. They produce heat from the exothermic oxidation of iron when exposed to air. The oxygen molecules in the air, together with the water, react with iron, forming rust (which is a hydrated oxide of iron). Salt is usually added to catalyze the process.
The commercial product is an air-permeable fabric package containing the mixture, and supplied in a sealed plastic pouch. The reaction begins as soon as the package is removed from the pouch, thereby exposed to the air, typically in a glove or pocket of a jacket. It reaches its maximum temperature in about 20 minutes, and continues to generate a modest level of heat for many hours. The instructions warn against contact with the skin of babies or young children, as the package can reach 74 °C (165 °F). When exhausted, it can be discarded with the household garbage.