Heat Factory's large size warmers can be used in pockets, stadium cushions, sleeping bags and many other Heat Factory accessories. Each warmer provides 20 hours of reliable, even heat. Warmers are safe to use, odorless and non-toxic. The Heat Factory warmer is a soft, lightweight pouch containing a biodegradable mixture of iron powder, charcoal, salt, sawdust and vermiculite. When exposed to the air, an oxidation process takes place that generates heat. Heat Factory warmers are specially engineered to provide heat that is long-lasting, comfortable and even. Heat Factory continually tests its products to ensure that you receive only the highest quality warmers that will consistently deliver warmth as advertised. Once activated, Heat Factory warmers can even be saved for later use (preferably within a week) if placed in an airtight container. Unopened, the Heat Factory warmer has a shelf life of three years or more.