How Disposable Hand Warmers Work

If you work outside in the winter, or just enjoy winter outdoor activities like skiing, you might have used disposable hand warmers in your gloves. These come in the form of little packets that you expose to the air, which then causes a warming reaction that keeps your hands at a comfortable warm bath-like temperature. But what, exactly, is happening when the hand warmers do their magic?

Though the idea of using portable hand warmers is quite old — centuries ago, the Japanese used hot stones to warm their hands, according to Chemical and Engineering News — the modern hand warmer dates back to 1923, when a Japanese man named Niichi Matoba patented the process of using an iron-powder hand warmer (per the Weather Network). That's right — the primary ingredient in these hand warmers is iron. The hand warmer works by speeding up the process of rust, causing the iron to heat up through an exothermic reaction.

According to Chemical and Engineering News, a hand warmer packet usually contains a mix of iron powder, salt, water, activated charcoal, and vermiculite or another absorbent and insulating material to contain the water. The packet itself is made from a cloth-like paper covering that lets air in. When you free the packet from its air-tight wrapper, the oxygen reacts with the salt and water to begin the process of forming iron oxide — that is, rust.

The makers of these packets can adjust how fast they heat up, or how long they last, by adjusting the amount of iron powder in the packets, the surface area of the iron powder, and the permeability of the packet's material. Toe warmers, for instance, use a material that lets in more oxygen because shoes generally are oxygen-deprived spaces. The heat usually lasts about eight to 10 hours, according to the Weather Network. But if you don't need a hand warmer for quite that long, Outdoors suggests putting the disposable hand warmer in a Ziploc bag, which cuts off the oxygen needed to fuel the exothermic reaction. When you're ready to reuse it, the hand warmer packet can be taken out, and the rusting process will resume.