Slide show: Common skin rashes

Heat rash (miliaria) occurs when the flow of sweat is obstructed, usually due to hot, humid weather or overdressing.

Prickly heat (miliaria rubra) (A) is a type of heat rash that appears as clusters of small, red bumps that produce a pricking or stinging sensation. Miliaria crystalline (B) appears as clear, fluid-filled bumps that generally produce no other signs or symptoms.

Heat rash isn't serious and usually resolves when the affected area cools. Cool compresses or a cool bath might help. You can prevent heat rash by wearing loose, lightweight clothing and avoiding excessive heat and humidity.