Heat Packs hand warmers last 7+ hours each and are ideally-sized for use in gloves, mittens or pockets. Safe, odorless and non-toxic. Heat Packs hand warmers contain an inert mixture of iron powder, charcoal, salt, sawdust and vermiculite. When exposed to the air, an oxidation process takes place that generates heat. Heat Packs warmers have a shelf life of three years or more if left unopened. 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.