Hand warmer

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A pair of air-activated disposable hand warmers, US quarter to scale

Crystallization-type hand warmer with scale showing metal disc trigger

Hand warmers are small (sometimes disposable) packets which are held in the hand and produce heat on demand to warm cold hands. They are commonly used in outdoor activities such as hiking and skiing to keep the extremities warm and assist insulated clothing. Other types of warmers are available to provide soothing heat for muscular or joint aches.

Depending on the type and the source of heat, hand warmers last between 30 minutes (recrystallisation) to 12–24 hours (platinum catalyst).

[edit] Types

Some hand warmers contain cellulose, iron, water, activated carbon, vermiculite and salt and produce heat from the exothermic oxidation of iron when exposed to air.^[1]

A second type generate heat through exothermic crystallisation of supersaturated solutions and are usually reusable. These can be recharged by boiling the warmers and allowing them to cool. Heating of these pads is triggered by snapping a small metal device buried in the pad which generates nucleation centers which initiate crystallisation. Heat is required to dissolve the salt in its own water of crystallisation and it is this heat that is released when crystallisation is initiated.^[2]

This second type typically has a shorter heat duration of 20 minutes to 2 hours.^[3]

A third type uses lighter fluid (lighter fuel) or LPG which is reacted with a platinum catalyst to release heat by oxidation reactions. These can be used on many occasions by simply refuelling.^[4]

Another type, known as HeatBands, consists of a thin band of a certain material that is worn around the wrist. It works by insulating the blood vessels that are close to the skin’s surface at the wrist and by reflecting the body's own heat back to that area. It was originally developed for sufferers of Raynaud's disease.^[5]