Man holding chemical heat pad to warm his hands. Heat pads are a mixture of chemicals in a pack that produce a chemical reaction that creates heat for up to 8 hours. They are often used for hand warming in cold temperatures. A pouch contains a mixture of iron powder, charcoal, salt, sawdust, and vermiculite or zeolite. When removed from the airtight plastic envelope and exposed to oxygen, an oxidation reaction takes place generating heat by an exothermic reaction.