Another form of an under boob rash could be heat rash, also known as miliaria. Heat rash under the breasts is caused by abnormal sweat gland function

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DARE - PRICKLY HEAT Prickly heat or Miliaria, also called sweat rash, is a skin disease marked by small and itchy rashes due to sweat trapped under the skin by clogged sweat

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Heat Rash - DoveMed

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Another form of an under boob rash could be heat rash, also known as miliaria. Heat rash under the breasts is caused by abnormal sweat gland function