There are five main ingredients inside a hand warmer:
To become hot, oxygen reacts with the iron powder, water and salt in the packet, which oxidizes the iron. (Oxidization is also known as rust.)
But when your car’s fender or your garden shovel rusts, it does not feel hot. That’s because that process happens very slowly. A sped-up exothermic reaction (the one that creates heat) in hand warmers makes it so that we notice the warmth quickly.
Vermiculite is a mineral that absorbs water. It helps keep the amount of water inside the packet in check so that the oxidization process can continue. Activated charcoal helps disperse the heat evenly so you don’t have any hot spots against your skin, and it controls the pace of the reaction.
The vermiculite and activated charcoal work together with the oxidization so the hand warmer can last for hours. Some companies add more iron so the hand warmer lasts longer.
Others choose to change the iron powder. “If you vary the raw materials in the warmer, you can change how quickly the reaction happens or how much of the warmer is reacted at one time,” Vergona says.