The heat generating materials used for body warmers consist of iron powder, activated charcoal, water, salt and water retainer. These materials are packed in an air-permeable inner bag, which is further enclosed in the air-tight outer bag. Recently, the demand for such body warmers has been rapidly increasing because of their convenience. But when the ambient temperature at which a body warmer is used changes, the user may feel too warm or too cold. The user feels warm when entering a heated room or feels cold when going outside. This is because the temperature generated by the body warmer varies depending on the environmental temperature.This study, using quality engineering methods, attempts to develop a formula maintaining a constant temperature which is not affected by enviromental temperature changes.
