The advice on the packet says to use only for a few minutes at a time and also use for hands only.
Is it safe to use them for several hours if "sandwiched" between 2 pairs of gloves?
Can they be used in an emergency to treat hypothermia?

Hypothermia treatment is complicated, and I'm not sure what the 'in field' recommendations are. It certainly wouldn't be applied to hands; back or stomach, possibly, but even then it's difficult, and could result in a drop in core temprature as cold blood is exchanged from surface to core.

I think the time limit and hand warnings are because the pads get really rather hot, and could burn. They're probably covering themselves against lawsuits. Or trying to.

Thanks captain.
I think I'll try using the pads carefully between layers of gloves or mitts for as long as they are comfortable to guard against surface burns.
I searched the Forum and found Jon's "Buyer's Guide-Survival Aids" (14/12/04)which mentions using heat pads in armpits and groin to treat hypothermia.
A search on the internet comes up with conflicting advice.Application of heat by hot water bottles and chemical pads to specific areas are recommended by some authorities but are decried by others.

Ive used both the chemical ones (ones you reset by boiling)and ones where you burn a charcoal stick inside. Ive never burnt myself using the chemical ones. The charcoal one had a small bag without which it was just to hot to hold.

The charcoal one stank and the chemical ones didnt last long enough. After which its a "dead weight" to carry.

IMHO I dont think they would be much cop as a treatment for hypothermia.

It's the ones that are self-activating once you open the packet that I'm using.There is a reaction between the iron and carbon.They lasted for about 6-7 hours when I tried them in cold conditions.Haven't found them over-hot or smelly.

I use the foot version of the chemical ones when skiing - fantastic. Stick it onto the outside of the sock and they last all day.

Toastie feet!

Maybe it was the particular make I used, but I found the chemical hand pads useless. When I used them in cold winter conditions, inside my mitts, they didn't heat up. In fact it wasn't until I got into the pub at the end of the day that they heated up.
They seemed to need a certain minimum environmental temp to kickstart the exothermic reaction, making them completely useless to my thinking.

Anyone else have this problem? Can't remember the make, but its the ones I see most commonly in most outdoor shops, and it was on more than one occassion, so unlikely a duff pad.

Alexander: what you call 'chemical' are actually physical; they are based on a state change of a super-saturated sodium acetate solution (which behaves as a super-cooled liquid), and the heat is released as a result of the formation of crystals. The pad should never rise above the melting point of sodium acetate (54 celcius); since the heat is given off by the formation of solid crystals, if the temperature rises above the melting point, crystals cannot form. This also explains why the pads are able to give off heat for a reasonable period of time; further crystal growth occurs as heat is lost from the pad, until there is no uncrystallized sodium acetate in left in solution.

Clever stuff, eh? ;-)

There are genuine, one-time warming pads that rely on a chemical reaction, and are activated by exposure to air. These do get hot due to the lack of the self-regulating mechanism, and, if handled improperly, could burn.

re: chemical pads not working.

Sometimes, the packet they are in has been punctured, and the reaction has taken place very slowly, so the pad is already dead, or partially dead.

Also, if you starve them of access to air, the reaction fails (not sure if it's based on oxygen or water). That's why they often come to life a bit if you plump them up a bit.

thanks capt, i did wonder just how hot they could get.

i think it's oxygen they need. at least a reasonable air supply or the reaction may stop.

they make great glove or boot driers too.

I bought some from Beans of Bicester at about £1 for a pack of two pads.No problems so far.They had "Hot Pads" in bold,made in China and had a tiny Hi Gear TM.It is recommended on the packet that you squeeze them gently from time to time to promote the reaction.
More comment on their possible use to treat hypothermia would be welcome!

Past threads on hypothermia:

January Trail/Trial - Have they done it again?
correct treatment for hypothermia - first aid.

Where I posted Hypothermia.org

Thanks again,captain.I've had a good trawl on the internet.
The problem for me is that on some websites the advice is not to apply direct heat at all,but on others chemical heat pads etc. applied to areas such as axillae,groin and abdomen are recommended. I appreciate your earlier point about not making things worse by applying heat to the wrong areas and forcing cold blood back to the major organs.

See every day is a school day.....;-)

Cap'n Paranoia, Do you know whats doing on in the wee disk inside that you click to start it?

Mike the feet ones sound like a great idea!

The metal disk causes a shock wave that initiates crystal formation. Once crystals start to form in a supercooled or supersaturated liquid, they can grow very quickly. You can sometimes trigger the pad by flicking it.

A good example I remember is from running the works bar. Sometimes, we'd forget to re-fill the fridge with beer, and our more discerning customers insisted on having their Peroni Nastro Azzuro chilled. So we'd chuck a couple in the freezer. And sometimes forget that we'd done so until later in the evening.

On one such occasion, I remembered in time, and served said customer with cold, liquid beer, popping the crown top off. Customer took a couple of sips of very cool beer, and put the bottle down on the bar. Then we watched as crystals grew from the top down, until the entire bottle was frozen solid, a process which took about 15 seconds.

We concluded that a combination of releasing the pressure and bubbles acting as nucleation sites caused the supercooled beer to freeze. As geeky engineers, we were all fascinated by this, but, despite numerous attempts to re-create this phenomenon, we were unable to get the temperature and timing just right.

BTW, as if by magic, the Lidl shopkeeper appeared...

Selling two re-usable hot pads for 99p. As well as ski socks, ski gloves, windproof gloves and polyester thermals. Their 'Thursday special'.

Wow! I didnt know you could superchill beer! Id try that but I dont think i could wait! In fact I can hear those little beers calling me now.........sandy,..sandy!