 The heat is generated from a chemical reaction, iron oxidation, when the pads are unsealed and exposed to air.Iron oxidation is also involved in the rusting of metal, and the transportation of oxygen in blood.The pads' ingredients include activated carbon, iron powder, sodium chloride, sodium thiosulfate, sodium polyacrylate and water, according to the Themacare FAQ

A portable heat wrap for treatment of back pain was developed in 1997, that didn't use counter-irritants such as menthol or capsaicin (The ThermaCare® HeatWrap™; Procter
and Gamble, Cincinnati, OH).