There is a reaction that occurs for the heat to be produced. This reaction is called exothermic reaction which allows heat to be released to it's surroundings and it's measured in KJ.(1) This reaction can produce up to 268 KJ of heat. However, this depends on the type of hand warmer you have.(1) If you have the air activated hand warmer you can produce more heat due to the fact the you have an oxidation reaction and if you have the Super saturated solution packets those tend to let out less heat because the solution doesn't last for long periods of time.