Abstract: A heating device having at least one package surrounding a hygroscopic salt and a heater mix having at least a metal reactant. The heating device may include a source of moisture positioned in the package, the source of moisture initially generating an atmosphere having nearly 100% relative humidity inside the package. The hygroscopic salt is spatially separated from the source of moisture within the at least one package and positioned so as to absorb moisture out of the atmosphere to form an electrolyte, and the hygroscopic salt is further positioned so that the electrolyte generated by the hygroscopic salt is in contact with the heater mix. Rather than have an internal source of moisture, the at least one package may be water vapor permeable to allow ambient moisture from the atmosphere to enter the interior of the package to combine with the hygroscopic salt to generate an electrolyte.