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.

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Heat rash

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.

There are three types of miliaria, which are classified according to where the sweat ducts are blocked. Signs and symptoms for each type vary considerably:

• Miliaria crystallina. The mildest form of heat rash, this affects the sweat ducts in the stratum corneum, the topmost layer of skin. Miliaria crystallina is marked by tiny, clear, superficial blisters and bumps (papules) that break easily but aren't itchy or painful. It usually clears on its own in a few days but can come back if hot, humid weather persists. And though it's most common in newborns, adults can develop it, too, especially if they've recently moved from a temperate climate to the tropics.
• Miliaria rubra. Occurring deeper in the outer layer of skin (epidermis), miliaria rubra causes red bumps that are intensely itchy or prickly, giving rise to its common name, prickly heat. There is often little or no sweating in the affected areas (anhidrosis). Adults can develop miliaria rubra shortly after they're exposed to hot weather, but the rash more often appears after several months of exposure. Infants usually develop this type of heat rash between the first and third weeks of life.
• Miliaria profunda. A less common form of heat rash, miliaria profunda occurs mainly in adults who have had repeat bouts of miliaria rubra. It affects the dermis, a deeper layer of skin, and appears soon after exercise or any activity that causes sweating. The lesions are firm and flesh-colored, much like goose bumps. Though it's not uncomfortable, miliaria profunda can cause a widespread lack of perspiration, leading to symptoms of heat exhaustion, such as dizziness, nausea and a rapid pulse.

Your 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.

Though heat rash can affect anyone, newborns are most susceptible. Other factors that make you more prone to heat rash include:

• Climate. People living in the tropics are far more likely to have heat rash than are people in temperate climates. You're especially susceptible when you first move to a hot, humid locale.
• Intense physical activity. Anything that makes you sweat profusely, especially if you're not wearing moisture-wicking clothing, can trigger heat rash.

Heat rash usually heals on its own and doesn't require medical care. See your doctor if you or your child has symptoms that last longer than three or four days, the rash seems to be getting worse or there are signs of infection such as:

• Increased pain, swelling, redness or warmth around the affected area
• Pus draining from the lesions
• Swollen lymph nodes in the armpit, neck or groin
• A fever or chills

No tests are needed to diagnose heat rash; your doctor can determine the problem with a visual exam.

Heat rash usually heals without problems, but complications sometimes occur, such as:

• Infection. Occasionally, heat rash becomes infected with bacteria, causing inflamed and itchy pustules. This is especially common in children in diapers but shouldn't be confused with diaper rash, which results from irritation to tender skin, not from blocked pores.
• Heat exhaustion. In hot weather, people with miliaria profunda are at risk of heat exhaustion, which can cause low blood pressure, nausea, headache and a rapid pulse. Untreated heat exhaustion can lead to heatstroke, a life-threatening condition.

The best treatment for any form of heat rash is to reduce sweating by staying in air-conditioned buildings or, when that's not possible, using fans to circulate the air, wearing lightweight clothing made of fabrics that "breathe" and limiting physical activity. Once skin is cool, heat rash tends to clear quickly.

Mild heat rash doesn't require any other treatment, but more severe forms may need topical therapies to relieve discomfort and prevent complications. Topical treatments may include:

• Calamine lotion to soothe itching
• Anhydrous lanolin, which may help prevent duct blockage and stop new lesions from forming
• Topical steroids in the most serious cases

Some anecdotal evidence supports the use of oral vitamin C supplements to help relieve heat rash.

To help protect yourself or your child from heat rash:

• In summer, dress in soft, lightweight, cotton clothing. Avoid overdressing in winter — children should wear the same amount of clothing that an adult would wear to stay comfortable.
• Avoid tightfitting clothes that can irritate tender skin.
• When it's hot, stay in the shade or in an air-conditioned building or place a fan at a safe distance to gently circulate the air.
• Keep your sleeping area cool and well ventilated.
• Bathe in cool water with a nondrying soap that doesn't contain fragrances or dyes. Many companies make gentle, natural products especially for infants and children.
• Avoid using powders, creams or ointments — they don't prevent heat rash and can block pores.

• In hot weather, dress in loose, lightweight clothing that wicks moisture away from your skin.
• Spend as much time as possible in air-conditioned buildings.
• After bathing, let your skin air-dry, instead of toweling off.
• Use calamine lotion or cool compresses to calm itchy, irritated skin.
• Avoid using creams and ointments, which can block pores further.