Every single year, without fail, I participate in a little winter ritual. I arrive at some snowy place, eager to strap on snowshoes or nordic skis and tromp or glide out to a charming hut or a beautiful overlook, or hopefully both. First thing I do is break out the disposable hand warmers. I tear open the little packet, remove the teabag-looking miracles, slip them into my gloves, and then wonder sets in: How the hell do these thing work? Are they even safe? If I puncture one in my glove, will my fingers be dissolved into some horrible chemical pudding?

Then my hands warm, and I ooh and aah at the wonders of science and I immediately forget all about those questions and carry on trudging through the snow to my hopefully picturesque destination.

Not today.

Today I learned how they work, and it’s pretty fascinating. Each disposable packet contains pretty much the same ingredients: salt, water, activated charcoal, and something called vermiculite. It’s all sealed in a porous pouch because the heating process is kickstarted by oxygen. Actually, the rusting process is more accurate—what you’re really doing is creating rust. The heat is just a byproduct.

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It works like this. When you free the warmer from its plastic packaging, air enters the warmer’s porous pouch and reacts with iron and water (oxidizing), creating rust. This exothermic reaction releases heat energy. The salt helps speed up the oxidation and heat generating process (you’ll note that iron sitting around outside isn’t ablaze with heat on its own—it takes a little something extra to create the heat). The little bits of charcoal are there to store the water needed for the reaction, and it also helps spread the heat. Vermiculite is a hydrated magnesium aluminum silicate (duh) that serves as an insulator, regulating the heat reaction a bit, and, very importantly, keeping the warmer from burning up all at once and eating your hand. They generate heat for as long as the oxidization lingers, usually no more than a couple hours at best. Temps as high as 100 degrees Fahrenheit are pretty normal.

To answer my earlier question—yes, they’re perfectly safe. They are in fact non-toxic too. Of course, they’re wrapped in plastic and you throw them away afterward, both of which are not ideal, environmentally speaking, but they aren’t going to burst into flames in your pocket, or leach a hideous chemical into your skin.

If you’ve ever used the bulkier, but reusable hand warmers, they work a bit differently. Usually, they’re a plastic pouch filled with water and sodium acetate, a cool chemical that is held in the pouch as a liquid that would much rather be a far warmer solid. You bend a little metal disc which agitates some sodium acetate molecules that instantly remember they’d rather not be a liquid, and a chain reaction starts as the rest of the sodium acetate heats up and hardens. Once the liquid is hard, you can reset the process by boiling the pouch until everything liquefies again.

The resuable warmers are cool, but they fit awkwardly in gloves so they’re more suitable for hiking if you don’t need to hold a pole. Or anything else, really.

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So there you have it. The magic of exothermic reactions warming your frozen hands and toes. Bet you’ll never think of rust the same way again.

So which warmers are best?

Hot Hands brand hand warmers are far and away the toastiest disposable warmers I’ve ever used. And they’re cheap. A ten pack goes for about $14.

Grabbers, which you see more of, at least in California, are nice but don’t seem to last nearly as long. They’re also even cheaper at about $9 for a ten pack.

Metal canister “pocket furnaces” are also an option. Zippo makes a liquid fueled hand warmer that lasts 12 hours. $22

Rechargeable electric warmers are another intriguing option. No mess, nothing to throw away, charge ’em up with a USB. $28