The typical disposable hand warmer contains iron, salt, activated carbon, water and an absorbent material. Opening the outer packet allows oxygen to contact the iron, forming iron oxide, commonly known as rust, through an exothermic reaction that produces heat. Choices for the absorbent material are polymers, sawdust or vermiculite.
The rusting reaction creates temperatures up to 135 degrees Fahrenheit. The manufacturer includes activated carbon to improve the distribution of heat from the warmer.
Producing the hand warmer requires engineering. The product should provide sufficient heat quickly, but should also continue producing heat for several hours. To meet these goals, the manufacturer sometimes increases the amount of iron in the packet. However, a second option is to vary the texture of the iron powder. Increasing the surface area of the iron, by using finer powder, increases the amount of heat the warmer produces, but increases the speed at which it rusts, reducing the time the warmer produces heat. The material the manufacturer chooses for the inner pouch affects the rate at which oxygen enters the pouch.
The manufacturer must also consider shelf life of the product. Hand warmers sometimes spend several months in the warehouse or on the store shelf. Consumers purchase hand warmers and store them before use. The outer pouch is made of polyethylene sheeting that does not allow oxygen to enter the warmer until the purchaser opens it.