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.
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.
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 hand 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
I don’t leave home without them! I would add that in my experience the ones that are packaged in a mylar pouch store for much longer and have less variability than the ones in a plastic pouch. Probably because plastic is more air permeable. Not a big deal for current season warmers but if you store them over the summer (like maybe you found a super closeout deal), the plastic bagged ones can be pretty unreliable.
God bless whoever came up with these things! They are a lifesaver for me. I am not a person who feels overly cold. I tend to run on the warmer side. But my hands? Forget it. They are like ice and they stay like ice long after I’ve come inside. And they HURT. Gloves seem to do very little for me. These little packets are wonderful!
Thank for the explanation and the assurance that they’re safe. I always wondered.
While they are safe and wonderful for most people, I am actually allergic to them. I remember the palms of my hands swelling, itching, and turning angry red every time I used them as a child. My discomfort was passed off as my being too sensitive to the heat from the warmers.
Later in life I came to find out the hard way that I have metal allergies. That explains my reaction to the iron filled warmers.
Never used/needed them, but would put them in the kids’ gloves if really cold. We’ve skied at -32F, actual air temperature, at Stowe and in the -20’s in NY state in the 1990’s. I ski mostly in NM now, so more of a problem hydrating than staying warm.
like Gary I learned how to ski in the east but over the ridge from stowe at smuggs. My kids when younger used the glove warmers but ditched those along with their neck gaiters once they became teens (looking cool is very important!)
-32 is brutal. I remember skiing when the high for the day was -14 and it was -22 on the mountain. we had the whole resort to ourselves that day, I used leather hestra mountaineering gloves with an extra liner. had to warm up inside every two runs.
it was too cold for the glove warmers. i put some in my jacket pockets but the cold drained them quickly. I did open up a bunch of them and put them in my snow boots back in the car and then stuffed clothing in the top of the boot. that worked real well! toasty boot at the end of the day. fun stuff!
well im a crossing guard in halifax nova scotia -and i use hand warmers on very cold days.they are a life saver for me .whoever invented these came up with a very good idea .tks
“much rather be a far warmer solid” Wouldn’t it be a cooler solid? And in the process of cooling down it would give off extra heat?
oh so this is how it works thanks for telling me this i needed this for school .
oh this is how it works thanks for telling me this i needed this for school
How do they package them without activating
Packaged in a sealed plastic bag so that oxygen is not able to activate the warmer, opening the bag exposes them to oxygen and the reaction starts.
I missed your point, now obvious, the packaging process. At a guess they are mixed and packaged in an oxygen free environment.
Permeable… a common mistake to use the term Porous. Porosity is a volumetric term, Permeability is a flow term…ie air flowing through permeable membrane. Much of your body operates on semi-permeable membrane processes. Please don’t get nit-picky about my comments here… the volumetric term of Porosity is so mis-used it drives me crazy, so don’t let my problem become your problem by getting upset at my correction efforts here. Thanks.
Thanks for that! Could you explain what volumetric means in regard to porosity? I want to understand more fully and explain to my kids. Am I correct in understanding porous means the materials have space or room within it, volume wise, to take in more liquid or solid material? And porous means the outer surface can allow gas to flow, but not necessarily absorb as a porous material can? Thanks!
Porosity or void fraction is a measure of the void (i.e. “empty”) spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%