The warmers operate on a chemical reaction with air similar to rusting. The warmer ingredients are iron, water, cellulose, vermiculite, activated carbon and salt. The heating process takes place in this fashion:|The iron in the pouch, when exposed to oxygen, oxidizes and therefore produces heat (aka, "Air Activated"). When iron oxidizes it produces iron oxide, more commonly referred to as rust.|The salt acts as a catalyst. The carbon helps disperse the heat. The vermiculite is used as an insulator for the purpose of retaining the heat and the cellulose is added as a filler. All of these ingredients are surrounded by a polypropylene bag. Polypropylene allows air to permeate the ingredients while holding in moisture.|The item is a one time use product.