How Do Disposable Hand Warmers Work?

Jul 20, 2023

When your hands — or toes — get so cold that even your best gloves and socks can't keep them toasty, those little plastic pockets of magical powder that we call hand warmers can bring big relief. Hand warmers are good to have in emergencies, and they're perfect to tuck inside your mittens during winter activities like snow skiing, ice skating or just watching your favorite football team.

But how exactly do these little packets work? Are they filled with tiny demons who warm your hands with hellfire? Or is it some horrific chemical that will turn your hands into sentient beings who want to live independently from the rest of your body?

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Luckily, the science behind those hand warmers is actually pretty simple. It turns out they use the same process that transforms iron to rust, just a lot faster.

There are five main ingredients inside a hand warmer:

The first hand warmer was invented in Japan almost 100 years ago in 1923. Today they come as individually wrapped mesh packets sealed in plastic.

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The packets are microporous, which means they have tiny holes that let in oxygen. The warmers for hands have very tiny holes, while the warmers designed for feet have larger (but still very small) holes. That's because there's less air inside your boots and shoes, so those warmers need more oxygen to activate.

To become hot, oxygen reacts with the iron powder, water and salt in the packet, which oxidizes the iron. (Oxidization is also known as rust.)

But when your car's fender or your garden shovel rusts, it does not feel hot. That's because that process happens very slowly. The exothermic reaction (the one that creates heat) is sped up in hand warmers so that we notice the warmth.

Vermiculite is a mineral that absorbs water. It helps keep the amount of water inside the packet in check so that the oxidization process can continue. Activated charcoal helps disperse the heat evenly so you don't have any hot spots against your skin, and it controls the pace of the reaction.

The vermiculite and activated charcoal work together with the oxidization so the hand warmer can last about eight to 10 hours, on average.

So it's not demons, and it's not scary chemicals and it's not even magic. But the heat in that hand warmer is toasty; those little packets can produce temperatures between 100 and 180 degrees Fahrenheit (37 and 82 degrees Celsius), which should keep your digits nice and cozy. And it's totally safe to throw used hand warmers in the garbage. No hazmat team required.

Reusable hand warmers work a little differently. They have a supersaturated sodium acetate solution inside the packet. As the solution crystalizes (sodium being salt), it releases energy as heat. To reuse these hand warmers, boil the closed packet in water to return the crystals to a liquid.

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