Hand warmers generate heat through chemical reactions, with disposable and reusable options available. The body prioritizes maintaining blood flow to the core in cold temperatures, leading to cold hands and feet. Disposable hand warmers produce heat through the oxidation of iron powder, providing up to 12 hours of warmth.

Most warmers work through a simple chemical reaction similar to rusting that occurs when warmers are exposed to air. That is why keeping them under wraps until needed is a must. The warmer is a mixture of iron, water, cellulose, vermiculite, activated carbon and salt. When the iron in the warmer is exposed to oxygen in the air, it oxidizes.

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 react with iron, forming rust. Salt is often added to catalyze the process.