The primary ingredients in HotHands are iron powder and activated charcoal, which react chemically with air to create rust and heat. The other ingredients in HotHands are there to control the chemical reaction and store the heat and to prevent the device from heating up too quickly and potentially burning someone.
The iron powder in HotHands is kept away from air while being packaged to prevent oxidation from occurring. The powder is then mixed with activated charcoal, which holds the water needed for oxidation. These ingredients are placed inside a microporous pouch that allows limited exposure to air once the plastic package is opened. Pouches with more holes heat up faster and are usually used for shoes, while pouches with fewer holes heat up more slowly and are typically used for hand warmers.
One of the remaining ingredients is salt, which acts as a catalyst. When the package is opened, the salt mixes with the water and speeds up the chemical reaction, creating more heat. The final ingredient in HotHands is a hydrated magnesium aluminum silicate called vermiculite. This material, which expands when heated, is light, non-reactive, absorbent, fire-resistant, and odorless. It serves as an insulator, preventing the heat from burning people and allowing HotHands to store heat for longer.