Finally, a technology I chanced upon over 20 years ago has made it to the consumer market. It is a reusable heat pack called Wonder Warmers.
Depending on the size pouch you have the heat lasts for over an hour and can reach 130° F (54° C) when new. After several cycles of use the useful heat output
diminishes to 20 minutes, as does the initial maximum temperature. 110° F (43° C) is a more realistic temperature over time, and easier on your skin.
To use it again, throw the crystallized pouch in a pot of boiling water and in about 15 minutes or less the crystals will melt, forming the original liquid that is ready to react with a click of the metal disk inside.
Okay, so it doesn’t last forever. But it lasts a heck of a lot longer than those silly single use heat packs that rely on oxygen and thus are relatively useless at altitude. Because Wonder Warmers provides all the necessary ingredients for the chemical reaction in its own sealed pouch, oxygen isn’t required and the heat output is unaffected by altitude. Being a liquid it weighs more than oxygen triggered heat pads, but again, it works more than once. Unfortunately the bulk of most Wonder Warmers pouches makes them inappropriate for keeping your toes warm in your boots, but for gloves with heat pack pockets they would be superb.
With a bit of McGyvering they would also be perfect for warming butane fuel canisters on stoves. The beauty of their reuseability is even more appropriate here since once you’ve finished with cooking you can boil one more pot of water and throw the Wonder Warmer into the boiling water where it will transform back in to its liquid state, ready for the next use.
A more popular use for these heat packs has little to do with being outdoors, except as a soothing heat pack for strained muscles at the end of an adventurous day, or maybe just to relieve a strain from a twisting fall around the house.
The biggest problem with Wonder Warmers is their ability (or propensity) to be accidentally triggered while in your pack. To avoid this, you will want to carry them in a crush proof container. Otherwise, when you need them you may find them effectively useless.