How Mycoal Warm Packs work.
When iron rusts it gives off warmth called oxidising heat, but you cannot feel this heat because its oxidising rate is too slow. If this is speeded up, more heat is produced.
This is the principle behind MYCOAL WARM PACKS.

The warmers are activated by removing them from their protective airtight packaging to expose them to air. The blended ingredients of iron, salt and water, combined with the oxygen from the air, causes the rapid oxidisation of the iron particles. This reaction produces heat. The heating temperature and duration are controlled by the amount of air ventilation, which passes through the many tiny holes on the inner packets. The duration of the reaction is finite and the warmer should be disposed of after it has cooled.

How Mycoal warm packs different from other brands?
Mycoal packs are manufactured in Japan by the Mycoal factory, they have been making these products for many many years and have a total obsession with quality. Every single aspect of the process is rigorously controlled from the particle size to the number of holes in the inner pack. This results in a very reliable and consistent product. Obviously, the raw materials are very carefully chosen to provide the consistent duration and heat output required. This raw material is subjected to quality testing before usage.

At the factory one of the interesting aspects is actually the packaging material, this is a vital component of the disposable warmer. Its function is quite critical to exclude unwanted oxygen, to preserve the warmer until its needed.

The material is so good at this task that they have a four year expiry date from manufacturing. But, in common with all statistics given, this is a minimum! If your warmer has past its expiry date by a few years, just go right ahead and use it. It may not give quite at as much heat as a fresh one, but it’ll probably work. Mycaol warmers regularly are found tucked into drawers and used after fifteen years or so. Mycoal do prefer their products to under-promise and over-deliver!

Why don’t handwarmers work inside my shoes?
Handwarmers are engineered to work with a richer oxygen content, closed in shoes tend to restrict available oxygen. The opposite is also true of course! If you put footwarmers inside gloves you will find that they get incredibly hot, potentially being hazardous. Please use the warmers as recommended.