How Do Disposable Hand Warmers Work?

Disposable hand warmers are a great way to keep your digits toasty in cold weather. They are small packets of magic that provide relief in emergencies and during winter activities like skiing and ice skating. But how exactly do these little packets work? The science behind disposable hand warmers is actually pretty simple.

There are five main ingredients inside a disposable hand warmer: iron powder, water, salt, activated charcoal, and some kind of water absorbent material. But they don’t get hot until you open the airtight plastic wrappers that the packets come in. Oxygen from the air flows into the packet and reacts with the iron powder, water and salt that’s already there, which oxidizes the particles of iron. This process is known as oxidation and it turns the iron powder into iron oxides, more commonly known as rust.

Water has to be present in order to break up molecules of oxygen so that the reaction can happen. The water absorbent material in the packet hangs onto the water molecules for safekeeping until you open the packet. Meanwhile, the salt helps the oxidation along because it accelerates the flow of electrons.

The process of oxidation is exothermic, meaning it gives off heat. But the reaction happens so quickly in disposable hand warmers that it produces temperatures between 100 and 180 degrees Fahrenheit (37 to 82 degrees Celsius). The material of the packet has to let in just the right amount of oxygen and they’re made with microporous materials, meaning they have tiny holes to let oxygen in at the gases equivalent of a slow drip. Warmers for hands have very tiny holes, whereas the warmers designed for feet have slightly larger ones. That’s because there’s less air inside your boots and shoes, so those warmers need more access to oxygen in order to activate. The activated charcoal serves to distribute the heat that’s created evenly throughout the packet so that you don’t wind up with any extra hot clumps.

Manufacturers can tweak how fast the reaction happens and how long it continues producing heat. By changing the size of the iron powder particles, smaller particles with more surface area will heat up faster and by increasing the amount of iron in the packet overall. These packets are designed to last anywhere from a few hours up to a full day.

So it’s not demons and it’s not scary chemicals. It’s not even magic, but the heat in that hand warmer is toasty. Disposable hand warmers are totally safe and should be disposed of in the garbage. No hazmat team required.