Both air-activated hand warmers and supersaturated solution hand warmers work by producing an exothermic reaction. Both types work best when kept covered rather than exposed to the cold.
Air-activated hand warmers contain iron, vermiculite, carbon, salt and cellulose. When air hits the iron, the process of oxidation begins, with the creation of rust releasing heat. The other chemicals help disperse the heat evenly. Air-activated hand warmers are safe for use on airplanes and are typically disposable. Supersaturated solution hand warmers contain water, salt and a metal disc. When the user snaps the disc, the crystals of the metal precipitate, creating warmth. Supersaturated solution hand warmers are reusable if they are boiled and allowed to cool.