Our 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.
Our material is so good at this task that we 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. Our warmers regularly are found tucked into drawers and used after fifteen years or so. We do prefer our products to under-promise and over-deliver!