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How Hand Warmers Work

author image Maggie Lynn
Maggie Lynn has been writing about education, parenting and health topics since 2005, in addition to being an educator. She holds a Master of Science in child and family studies.
How Hand Warmers Work
Hand warmers fight the winter chill. Photo Credit Jochen Sand/Digital Vision/Getty Images

Types of Hand Warmers

When it comes to hand warmers, there are two types of chemical reactions used to generate heat: supersaturated solution and air-activated. These chemical reactions work to produce an exothermic release of heat, but do so through very different methods. These types of warmers are best implemented when placed in tight spaces, such as in a glove, shoe or pocket. They offer a lightweight, portable approach to generating heat to help keep the extremities warm during cold weather.

Air-Activated Warmers

Air-activated hand warmers generate heat through a process known as oxidation. Once you break the packaging seal, air makes its way into the pack through the tiny perforated holes in the warming bag itself. The air molecules react with the iron in the hand warmer bag and form iron oxide, otherwise known as rust. The bag is designed to trap any moisture, keeping the user’s hands dry, while an ingredient called vermiculite holds the heat in and prevents it from leaving the site of the reaction. Conversely, to disperse this heat throughout the pack material, carbon is often added as well. Salt is included for its catalyzing properties in producing instant heat. To fill the remaining space in the bag, cellulose is used as an inert ingredient, along with charcoal or sawdust.

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Supersaturated Solution Warmers

The second type of hand warmers use a chemical reaction called a supersaturated solution. These generally contain metal discs in the middle of them. In order to activate heat, the metal disc must be snapped. This causes the crystals within the warmer to precipitate. This released heat energy is held within the bag, compounding the heat to reach temperatures as high as 130 degrees Fahrenheit. These use table salt otherwise known as sodium acetate. An initial boiling of the hand warmer must be done in order to best dissolve the salt, thus creating a supersaturated solution capable of holding heat for extended periods of time. These pads are reusable; when you boil them in water for a few minutes, they're ready to use again.

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