How Do Hand Warmers Work?

Hand warmers work by utilizing an exothermic chemical reaction to generate heat, providing warmth to cold hands. Disposable hand warmers accomplish this through the use of iron powder, salt, and water. When exposed to oxygen in the air, iron undergoes oxidation. The reaction, which is facilitated by the presence of salt and water, culminates in the release of energy in the form of heat.

Disposable hand warmers also contain vermiculite and activated charcoal. Vermiculite helps extend the duration of heat production by absorbing and regulating the release of water. Meanwhile, activated charcoal acts to control the speed of oxidation and evenly distributes heat through the hand warmer.

Reusable hand warmers, by contrast, typically contain a supersaturated solution of sodium acetate. When the metal disk inside the warmer is triggered, the sodium acetate crystallizes, releasing heat in the process. Placing the warmer in boiling water dissolves the crystals, returning the solution to a liquid state and thereby readying it for the next use.