heat rash :

Heat rash — also known as prickly heat and miliaria — isn't just for babies. Though it's common in infants, heat rash can affect adults too, especially during hot, humid weather.

read more : What Exactly is Heat Rash?

Heat rash develops when your sweat ducts become blocked and perspiration is trapped under your skin. Symptoms range from superficial blisters to deep, red lumps. Some forms of heat rash can be intensely itchy or prickly feeling.

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Heat rash usually goes away 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.