The masks contain iron particles  and when they come into contact with oxygen the iron slowly begins to warm, it’s a natural oxidation reaction. There are other natural ingredients that help this oxidising reaction to happen and to heat evenly; water, salt, charcoal and vermiculite. The Lula Eye Masks are each sealed individually because once you open the pouch and air comes in contact it will start warming. It stays at a comforting temperature for a minimum of 20 relaxing minutes, so you can drift into a daydream or a deep night’s sleep.