The Heat Factory warmer is a soft lightweight pouch containing a mixture of iron powder, charcoal, salt, sawdust, and vermiculite. When exposed to the air, an oxidation process takes place that generates heat up to 130 degrees for more than 8 hours. (Bring a heavy-duty zip lock bag to halt the process for use later.) Contents are biodegradable. Unopened, the Heat Factory warmers have a shelf life of three years. Place the heaters in the mesh pockets near the lens and battery, ideally. Toss them inside our camera cases to keep things warm when not in use. Keep your hands and feet warm, too.