Why Do Hand Warmers Heat Up When Exposed to Air?

When a person wanted to warm his or her hands in the past it was necessary to stick them in coat pockets or wear a pair of good gloves. At some point, a creative inventor introduced battery-powered electric socks and hand warmers that provided temporary relief.

In recent years, the task of warming hands has turned to chemical processes or other methods. But just how do these newest hand warmers work? First and foremost, it’s necessary to view these as small packets that are held in the hand or attached in some way to keep the hands warm during outdoor activities in cold weather. They don’t necessarily remain warm all the time and may not be a substitute for staying indoors or warming the hands in a different way.

Many of these hand-warming packets have some sort of adhesive that can attach them to clothing. But they are not meant to be held directly onto the skin because of the high level of heat generated. Most of them will warm the hands for at least 30 seconds. Some of the more advanced and more expensive items maintain warmth for 12 hours or more.

There are a few distinct types of hand warmer. The list includes air-activated designs that use a combination of ingredients such as activated carbon, iron, water, cellulose, salt and vermiculite. The process by which they produce heat when exposed to air is called exothermic oxidation. (Wikipedia) Basically, this type generates heat when the iron is oxidized. The carbon helps speed up the reaction while salt acts as a catalyst.

There are other processes for generating sufficient heat to warm the hands. But those that become warm when exposed to air rely on a very basic natural process. Technically, this method is known as oxidation. In truth, the heat is generated when the iron ingredient rusts. When air penetrates into the package the oxygen in our air begins to react with the iron. As the iron oxidizes (rusts) heat is generated. Carbon particles and other ingredients help spread and maintain the heat through this self-contained package. There are also ingredients that control the heat so that the level remains comfortable enough for warming the hands. When all the iron particles in one package are oxidized, the process stops. The newer air-activated designs maintain heat for a period after the oxidation is completed.
Heating pads for other uses employ a similar process. Removing the covering from one of these pads allows air to start the oxidation process in strips of iron material. In most cases, the air-activated hand warmer cannot be reused.

Warming processes also include crystallization and the use of a platinum catalyst. The crystallization type of hand warmer gets heat from crystals of “supersaturated solutions. These can be reused and recharged. A small, metal triggering device that is in the pad is snapped. This begins the process of crystallization.