Heat Factory foot warmer insoles are designed to heat in the restricted air environment of a shoe or boot. Ideal for daily use in shoes, ski boots, or whenever feet are cold. Peel back the adhesive backing and press warmer to outside of sock under foot for comfortable warmth. Provides soothing, gentle heat for up to 7 hours. Not intended for use in open air. 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.