A heat pack is a flexible bag containing special heat-holding chemicals. Applying a heat pack to a strained muscle or a broken bone is considered heat therapy, as opposed to placing an ice pack on a sprained ankle or a head injury for cold therapy. Heat tends to allow muscle tissue to expand and loosen, which is why a coach or physical therapist may use a heat pack immediately following an injury or wait until the initial swelling has lessened.

A heat pack may be used to treat stiff muscles as they recover from overexertion. A patient suffering from Bell's Palsy or other facial ailments may also benefit from a heat pack. Other typical uses of a heat pack are personal hand warming, preparation for scuba diving and hypothermia treatment. A heat pack may also be found in food containers designed to transport casserole dishes or other hot meals.

Perhaps the most common form of heat pack uses a blue-colored silica gel wrapped in a puncture-resistant plastic bag. The gel can be placed in a microwave oven and heated to several hundred degrees, although human skin shouldn't be exposed to temperatures much above 140 degrees Fahrenheit. After the gel has been heated, the heat pack can be applied carefully to the affected area. The heat pack can be reheated as necessary and a protective towel should be placed between the patient and the heat pack itself. This form of heat pack can also be frozen and used for cold therapy.

Another form of heat pack uses science in the form of a supersaturated solution. The heat pack itself contains a solution of sodium acetate, a special form of salt. The salt solution is supersaturated, meaning it actually has more salt molecules than it can keep in a stable liquid form. The user presses a 'trigger' made from stainless steel, causing one additional sodium molecule to start a chain reaction. As the solution begins to solidify, heat is generated.

Specifically, this type of heat pack can reach 130 degrees Fahrenheit during the reaction, but not much higher. After a few hours, the reaction stops and the heat dissipates. Boiling the heat pack for several minutes will return the solution back to its supersaturated state so the entire process can begin again.

A new form of heat pack relies on oxygen and the rusting process for heat generation. The heat pack is kept in a sealed package until required. The user opens the package and removes a protective strip on the adhesive side of the heat pack. After applying the patch directly to a stiff muscle, the user should feel a pleasant warming sensation.

The heat pack itself contains a special blend of iron filings and other chemicals. As the oxygen from the outside air penetrates the heat pack, the iron begins to oxidize or rust. One side effect of oxidation is heat energy. This heat energy from the slow rusting process is the basis for the warming properties of the heat pack. Eventually the oxygen and iron will stop reacting, but the result is a steady heat without the need for electricity or other external sources.