Heat allergy is an allergic reaction of the skin to heat that causes a rash or hives to erupt on the skin. Hives related to heat allergy are usually localized to the area of heat exposure, but may sometimes be generalized all over the body. The generalized form of heat allergy-related hives may take years to disappear completely.

Skin reactions to heat are similar to skin reactions to sun. There are two common types of heat reactions:

• Prickly heat. A nonallergic skin rash from heat often experienced by infants in the diaper area or in areas covered by clothing. It is caused by clogging within the narrow sweat ducts located just under the skin. It often results from sweat forming under hot skin and friction from clothing. Prickly heat will usually clear up by keeping the skin cool and dry.
  
  
• Heat urticaria. An allergic skin reaction that causes hives to appear on skin exposed to heat. Heat urticaria may require desensitization through controlled exposure to heat as a treatment. This type of condition is very rare.

While extremely rare, a heat allergy can cause the potentially fatal condition known as anaphylactic shock. This condition requires emergency medical treatment. The signs and symptoms of anaphylactic shock include:

• Difficulty breathing
• Low blood pressure
• Dizziness