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

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

Heat rash isn't serious and usually resolves on its own. You can prevent heat rash by wearing loose, lightweight clothing and avoiding excessive heat and humidity as much as possible.