Like reusable hand warmers, disposable hand warmers rely on chemical activation instead of lighter fluid or lithium batteries. Each pouch typically includes salt, iron powder, activated carbon, and a little water. When the pouch is taken out of its airtight seal and exposed to oxygen, the iron powder causes an exothermic reaction, generating heat. The carbon helps evenly disperse the heat, and an absorbent material like wood or a polymer helps retain it throughout the day.