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Heat rash also known as prickly heat and miliaria (isn’t just for babies). Though it’s most common in infants, heat rash affects up to a third of adults living in tropical climates and can plague anyone during hot, humid weather.

Heat rash develops when your sweat ducts become blocked and perspiration is trapped under your skin. Symptoms range from superficial blisters to deep, red lumps. Some forms of heat rash can be intensely itchy or prickly.

Most often, heat rash goes away on its own. Severe forms of heat rash may need medical care, but the best way to relieve symptoms is to cool your skin and prevent sweating.

Symptoms

Adults usually develop heat rash in skin folds and wherever clothing causes friction. In infants, the rash is mainly found on the head, neck, shoulders, chest and back, but it can also occur in the armpits and groin.

Causes

• 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.