Why do Hand Warmers Heat up When Exposed to Air?

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  • Written By: Michael Pollick
  • Edited By: Bronwyn Harris
  • Last Modified Date: 18 September 2018
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Not all types of hand warmers heat up when exposed to air, but the ones that do derive their heat from a process called oxidation, perhaps better known as rusting. Packaged warmers contain iron particles, salt water, carbon, and other chemicals that act as insulators. When the protective packaging is unsealed, outside air penetrates the chemicals inside, and the oxygen reacts chemically with the iron, causing the iron to rust or oxidize. This oxidation process is aided by the salt solution, which acts as a form of catalyst.

One of the results of this oxidation process is the production of heat, or in chemistry terms an exothermic reaction. The carbon particles help to spread this heat throughout the entire package. Other chemicals help to keep the exothermic reaction slow enough to provide long-lasting heat, not just a quick flare-up. Once all of the iron has been converted to iron oxide or rust, the exothermic reaction is over and the hand warmer can no longer provide heat.

The principle that give these products the ability to warm hands is the same principle that powers self-adhesive heating pads. Once the protective strip is removed from the pad, oxygen from the outside air reacts with the thin layer of iron threads in the pad and the result is an exothermic reaction that can last for hours.


There are other types of hand warmers which use a completely different chemical process to create similar results. These devices contain a supersaturated solution of either sodium acetate or calcium nitrate. Supersaturation means a solution has been superheated in order to allow more of a selected chemical to dissolve in it. When the solution cools, only a small speck of material would have to be introduced in order to cause the entire structure to crystallize and solidify.

Some hand warmers use this principle of supersaturation to create heat. A supersaturated solution of sodium acetate or calcium nitrate is sealed in a bag with a strip of roughened stainless steel. When the bag is manipulated back and forth, a tiny piece of metal should break off from the strip. This tiny fleck is enough to cause a salt crystal to fall out of solution and become solid. Within seconds, the solution crystallizes into a solid. As it does, an exothermic reaction occurs and usable heat is generated for at least 30 minutes. Unlike the oxidizing warmers, however, this type can be reused by reheating the salt solution until it becomes supersaturated and unstable again.


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Post 14

Is there such a thing as microwavable hand warmers? I would think putting something in the microwave would get it good and hot. This way you could use it over and over again instead of throwing it away after just one time.

Post 13

I think hand warmers are a must have for anyone spending long periods of time outside in the cold. My husband uses these when he goes hunting. He may spend hours sitting in one spot waiting for something to come by, and the hard warmers make a huge difference for him.

You can find these almost anywhere, and many times they are right up front by the cash registers. At the beginning of the season I buy a bunch of them so we always have some on hand. I have noticed they have an expiration date on them. I don't know how long they will work past the expiration date. I have used ones that are a year old and they still work just fine.

Post 12

My dad has poor circulation in his hands and will use hand warmers whenever he has to work outside shoveling snow. Since these keep his hands and fingers warm, it is easier for him to get the job done quicker. The hand warmers he uses are only good for one time, but they are inexpensive and nothing else works as good for him.

Post 11

@alisha -- I would be in trouble without glove hand warmers when I go skiing. I put these inside my gloves and boots and they stay warm all day long.

With the hand warmers I just take them out of the package, leave them exposed to the air for a little bit and put them inside my gloves. When I stop for a break I take them out until I am ready to hit the slopes again. I am always surprised they stay warm all day long, but they do.

I also buy the self-adhesive warmers to put on the bottom of my feet. These work the same way, but they don't seem to get nearly as warm as the hand warmers. This is probably because they aren't exposed as much, but they are still better than using nothing at all.

Post 10

Since disposable hand and feet warmers require air to be activated, will they work if you place it in a shoe or inside gloves?

I guess what I'm wondering is how much air do they need? Is it enough for it to get air momentarily or does it need it constantly for the chemical reaction to continue? Because a warmer isn't going to get much air inside boots.

Post 9

@turquoise-- That's the same thing as the re-usable hand warmers the article talked about in the last two paragraphs.

It's actually a really simple mechanism. The solution inside the packet is going from liquid form to solid form and creating heat in the process. The trigger that causes it to go into solid form (crystallization) is the metal disc. But none of this would be possible if the sodium acetate inside the solution didn't have the capability to cool down without crystallizing.

These re-usable hand warmers are pretty cool, but they don't get as hot as the one-time hand warmers and the heat doesn't last as long either.

Post 8

My brother has a hand warmer that he uses on camping trips. It's just a small packet with liquid inside. When you knead it, it starts producing heat. To reuse it, he just puts in hot water and then lets it sit for a while.

How does it work?! It's one of the coolest things I have ever seen!

Post 7

What kind of iron? Iron(II) or Iron (III)?

Post 3

wow that's pretty cool how hand warmers work.

Post 2

anyone know is a special packaging material is used to prevent air from penetrating while on the shelf?

Post 1

The Clicker in the reusable type of hand warmers operate by causing a partial vacuum called cavitation where the crystals can form and therefore crystallize the whole solution heating the hand warmer up. If a metal fleck was left from the clicker the hand warmer would be only be able to be used once since the fleck would serve as a crystalization point when the hand warmer was reset by boiling in water.

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