HEAT rash occurs when the sweat produced by the body cannot get to the surface of the skin to evaporate. One of the most important tasks of the skin is to regulate the temperature of the body, and the body cools itself by sweating and allowing the sweat to evaporate.

The sweat is produced by the sweat glands located deep in the dermis — the second layer of the skin. The sweat gets to the surface of the skin via ducts. Heat rash occurs when the sweat ducts become blocked and the sweat cannot get to the surface of the skin. Instead it gets trapped beneath the surface of the skin, causing a mild inflammation and a rash. The rash is usually reddish and tiny blisters may form due to inflammation.

The appearance depends on where on the skin the sweat is trapped. If it is trapped in the most superficial layers of the skin, it causes a condition called Militia Crystalline. There may be no other complaints except for some sweat bubbles.

On the other hand, if the sweat is trapped deeper in the dermis, you can get a condition called Miliaria Rubra and there are blisters in the dermis. The area becomes dry and itchy, hence the name prickly heat.

The rash usually occurs in areas where there is tight clothing where the air cannot circulate. A heat rash will usually fade if the skin is allowed to cool. It can be prevented by avoiding hot, humid climates, by using fans, air conditioners, and allowing the air to circulate.

We are not sure why some people get heat rash, but it occurs in the creases of the skin, where adjacent areas of the skin touch and prevent sweat from evaporating — example in the armpits, in the groin and on the neck.

Babies can develop heat rash if they are in hot weather, if they are over-dressed and bundled up, and if they have a fever.

Those most at risk for heat rash are infants, young children, the elderly and the obese. Intense exercise which brings on sweating can also bring on a heat rash.

Treatment is with calamine lotion, hydrocortisone cream or vitamin A or C creams.

Dr Persadsingh is a skin specialist. He is the author of Acne in Black Women, The Hair in Black Women and Eczema in Kids of Colour.