Heat rash — also known as prickly heat and miliaria — isn't just for babies. Heat rash affects adults, too, especially during hot, humid weather.
Heat rash develops when blocked pores (sweat ducts) trap perspiration under your skin. Symptoms range from superficial blisters to deep, red lumps. Some forms of heat rash feel prickly or intensely itchy.
Heat rash usually clears on its own. Severe forms of heat rash may need medical care, but the best way to relieve symptoms is to cool your skin and prevent sweating.