Prickly heat rash
Prickly heat is a very itchy red skin rash, causing a prickling or burning feeling. It is medically known as miliaria. It may occur when you sweat a lot in hot or humid weather. Dead skin cells and bacteria block the sweat glands. The skin becomes inflamed with a spotty rash that may blister.

If you are overweight or sweat easily you are more likely to have prickly heat. The rash is most commonly where there is friction with clothes also on the hands, feet, under the arms and the chest. Babies and children are also more likely to be affected than adults because of their underdeveloped sweat glands, in which case the rash is commonly found in the nappy area and in the skin folds or on the neck.
The rash usually lasts for a few days and then disappears on its own. It may however last longer if the hot and humid conditions continue.

Symptoms
The condition features many small red bumps and a constant prickling or itching sensation. In the most severe forms, salt crystals may form in the sweat-gland ducts, producing small blisters. Itching may encourage scratching, which may lead to skin infection.

Causes
Prickly heat is due the sweat glands becoming blocked when the weather is hot and humid. Dead skin cells and bacteria may block the sweat glands, leading to sweat being retained and building up on the skin leading to inflammation, which causes the rash.