Heat Factory Foot Warmers are designed for use with Heat Factory Socks and Footbeds. When placed inside sock pocket or footbed, they will generate soothing heat for up to 6 hours. Ideal for use when fishing, hunting, skiing, or sitting around the camp fire. 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.