Heat rash typically goes away within a few days, according to WebMD. A rash that lasts longer than three to four days should be looked at by a doctor.
Heat rash occurs when perspiration becomes trapped in the sweat ducts, according to the Mayo Clinic. Trapped sweat can cause several symptoms, such as blisters, red lumps and intense itching. Most of the time, the symptoms get better on their own. Mild cases of heat rash can be treated by reducing sweating and wearing lightweight clothing that allows skin to breath. In more severe cases, calamine lotion and topical steroids can soothe the skin.
Mayo Clinic advises to see a doctor if a fever develops, pus drains from the rash or lymph nodes in the armpit, neck or groin swell.