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Causes
By Mayo Clinic staffYour skin has two types of sweat glands: eccrine and apocrine. Eccrine glands occur over most of your body and open directly onto the surface of your skin. Apocrine glands develop in areas abundant in hair follicles, such as your scalp, armpits and groin.
When your core temperature rises, your autonomic nervous system stimulates the eccrine glands to secrete perspiration. The perspiration travels through ducts to the surface of your skin, where it cools your body as it evaporates. Heat rash develops when some of the eccrine sweat ducts become plugged. Instead of evaporating, perspiration remains trapped beneath the skin, causing inflammation and rash.
It's not always clear why the sweat ducts become blocked, but certain factors seem to play a role, including:
- Immature sweat ducts. Because a newborn's sweat ducts aren't fully developed, they rupture easily, trapping perspiration beneath the skin. This happens most often in hot weather, but it can occur anytime infants are dressed too warmly. Newborns who have high fevers or are in incubators can also develop blocked sweat ducts.
- Tropical climates. Hot, humid weather is particularly conducive to miliaria, especially when you first move to the tropics from a temperate region. Once your body becomes acclimated — often over a period of several months — the problem usually disappears.
- Physical activity. Intense exercise, hard work or any activity that causes you to perspire extensively can lead to heat rash.
- Certain fabrics. You may develop heat rash if you consistently wear clothing that doesn't allow perspiration to evaporate normally.
- Medications. Certain prescription medications have been linked to heat rash, including bethanechol, which treats bladder problems; clonidine (Catapres), a high blood pressure drug sometimes used to treat attention-deficit/hyperactivity disorder (ADHD); and the acne medication isotretinoin (Accutane, Amnesteem).
- Bacteria. Some bacteria normally found on the skin, such as Staphylococcus epidermidis, secrete a sticky substance that may block sweat ducts.
- Other factors. Overheating in general — bundling up too much in winter, sleeping under an electric blanket — can lead to heat rash. So can using heavy creams and ointments, which block the sweat ducts. Heat rash can also occur in people who are confined to a hospital bed for long periods.
