Hand warmers are small (mostly disposable) packets which are held in the hand and produce heat on demand to warm cold hands. They are commonly used in outdoor activities such as hiking and skiing to keep extremities warm and assist insulated clothing. Other types of warmers are available to provide soothing heat for muscular or joint aches.

On one side the packets often have adhesive strips so they can be stuck to the underwear (never directly onto the skin as this can cause burns) to keep the lower back warm, for example. There are also types that can be inserted into the shoes or stuck to the socks.

Depending on the type and the source of heat, hand warmers last between 30 minutes (recrystallisation) to 12–24 hours (platinum catalyst).

Air activated (iron)

Some hand warmers contain cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

, iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, activated carbon

Activated carbon

Activated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...

 (to speed up reaction), vermiculite

Vermiculite

Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...

 (water reservoir) and salt

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 (catalyst) and produce heat from the exothermic

Exothermic

In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...

 oxidation of iron when exposed to air. This type of heatpads normally lasts from 1 to 10 hours.

Supersaturated solution (crystallization-type)

A second type generate heat through exothermic crystallisation of supersaturated solutions (typically sodium acetate

Sodium acetate

Sodium acetate, CH3COONa, also abbreviated NaOAc, also sodium ethanoate, is the sodium salt of acetic acid. This colourless salt has a wide range of uses.-Industrial:...

) and are usually reusable. These can be recharged by boiling the heaters and allowing them to cool. Heating of these pads is triggered by flexing a small metal disk in the pad, which generates nucleation

Nucleation

Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...

 centers that initiate crystallisation. Heat is required to dissolve the salt in its own water of crystallisation and it is this heat that is released when crystallisation is initiated.

This type typically has a shorter heat duration of 20 minutes to 2 hours.

Lighter fuel

Another type uses lighter

Lighter

A lighter is a portable device used to generate a flame. It consists of a metal or plastic container filled with a flammable fluid or pressurized liquid gas, a means of ignition, and some provision for extinguishing the flame.- History :...

 fluid (lighter fuel) or LPG

Liquified petroleum gas

Liquefied petroleum gas is a flammable mixture of hydrocarbon gases used as a fuel in heating appliances and vehicles. It is increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer...

 which is reacted with a platinum

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 catalyst to release heat by oxidation reactions. These can be used on many occasions by simply refuelling.

Battery

There is also a battery operated handwarmer. In this type, electrically resistive heating devices are used to convert electrical energy in the battery to thermal energy. Some use disposable batteries, but others are rechargeable, like cellphones, and can be used for many hundreds of cycles with the same battery. This type is not disposable. Typically they can last up to 6 hours. One of the main benefits is that there are no fumes or other mess with this type.

Charcoal

One of the older and perhaps less used types of handwarmer are made hot through the burning of charcoal in a special case. These can last up to 6 hours and become comfortably hot. The cases for these usually have felt on the outside and have materials inside that do not burn, but spread the heat evenly such as metal. To activate, one or both ends of a stick of charcoal are lit and then quickly extinguished to create a hot ember

Ember

Embers are the glowing, hot coals made of greatly heated wood, coal, or other carbon-based material that remain after, or sometimes precede a fire. Embers can glow very hot, sometimes as hot as the fire which created them...

. The smoldering stick is then placed inside the case and the case is tightly shut. The charcoal sticks are available from most outdoor activity shops and are fairly inexpensive.

See also

• Instant cold pack
  Instant cold pack
  An instant cold pack is a device that consists of two bags, one containing water, which is inside a second bag containing ammonium nitrate. When the inner bag is broken by squeezing the package, the ammonium nitrate is allowed to dissolve in the water in an endothermic reaction...
  
  
• Heating pad
  Heating pad
  A heating pad is a pad used for warming of parts of the body in order to manage pain. Localized application of heat causes the blood vessels in that area to dilate, enhancing perfusion to the targeted tissue...
  
  
• Hot water bottle
  Hot water bottle
  A hot water bottle is a container filled with hot water and sealed with a stopper, used to provide warmth, typically whilst in bed, but also for the application of heat to a specific part of the body....
  
  
• Sodium acetate
  Sodium acetate
  Sodium acetate, CH3COONa, also abbreviated NaOAc, also sodium ethanoate, is the sodium salt of acetic acid. This colourless salt has a wide range of uses.-Industrial:...
  
  (commonly used in exothermic crystallisation hand warmers)