Heat rash -- also called prickly heat or miliaria -- is a common condition in which areas of the skin feel prickly or sting due to overheating. Heat rash looks like tiny bumps surrounded by a zone of red skin. It usually occurs on clothed parts of the body, such as the back, abdomen, neck, upper chest, groin, or armpits and usually gets better once the skin is cooled.
Heat rash occurs most often in hot, humid conditions. It's most common in infants. Active people, newborns in incubators, and bedridden patients with fever also are more likely to get heat rash.
Heat rash begins with excessive perspiration, usually in a hot, humid environment. The ducts from the sweat glands in the skin become blocked. This causes the sweat to leak into the surrounding tissue, causing irritation and redness. You may feel the prickly, or stinging, sensation that gives this condition its name.