The skin's job is to protect the inside of the body from the outside world, and acts as a preventive barrier against intruders (for example, infection, chemicals, or ultraviolet light). It also plays an important role in the body's temperature control. One way that the body cools itself is by sweating, and allowing that sweat or perspiration to evaporate. Sweat is manufactured in sweat glands that line the entire body (except for a few small spots like fingers, toenails, and the ear canal).
Sweat glands are located in the dermis or deep layer of the skin, and are regulated by the temperature control centers in the brain. Sweat from the gland gets to the surface of the skin by a duct.
A heat rash occurs when sweat ducts become clogged and the sweat can't get to the surface of the skin. Instead, it becomes trapped beneath the skin's surface causing a mild inflammation or rash.
Heat rash is also called prickly heat or miliaria.
Picture of the layers of the skin including the sweat glands
What are the causes of heat rash?
It is uncertain why some people get heat rashes and others don't.
The sweat gland ducts can get blocked if excessive sweating occurs, and that sweat is not allowed to evaporate from a specific area. Some examples of how blockage may occur include the following:
* Creases in the skin like the neck, armpit, or groin have skin touching adjacent skin, which makes it difficult for air to circulate, preventing sweat evaporation.
* Tight clothing that prevents sweat evaporation.
* Bundling up in heavy clothing or sheets. This may occur when a person tries to keep warm in wintertime or when chilled because of an illness and fever.
* Heavy creams or lotions can clog sweat ducts.
Babies have immature sweat glands that aren't able to get rid of all the sweat they produce, which can cause a heat rash if they are exposed to warm weather, are overdressed, excessively bundled, or have a fever.
Heat rash may occur as a side effect of some medications (for example, isotretinoin [Accutane] or clonidine [Catapres].