Heat rash, technically known as miliaria, can occur in children, infants, and adults. It is caused by exposure to heat and humidity and develops when your sweat ducts become blocked and perspiration is trapped under your skin. There are several types of miliaria, including miliaria crystallina, miliaria rubra, and miliaria pustulosa. The different names are related to the increasing depth of the inflammation around the sweat ducts. Some forms of heat rash can be intensely itchy. Heat rash usually goes away on its own; however, severe forms of heat rash may require treatment. The best way to relieve symptoms is to cool your skin, prevent sweating and wearing loosely fitting clothing. Some phototherapy and drying lotions may also be helpful.