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Heat rash, also known as prickly heat, summer rash, or wildfire rash, is common and can be uncomfortable.
The medical name for heat rash is miliaria. It happens when sweat becomes trapped due to a blockage in sweat glands in the deeper layers of skin.
Inflammation, redness, and blister-like lesions can result. Sometimes, an infection can develop.
People with overweight or obesity and people who sweat easily are more likely to get prickly heat. Also, babies and children are more prone to it because their sweat glands are still developing.
- small bumps or spots, called papules.
- an itching or prickling sensation
- mild swelling
On white skin, the spots are red.
On darker skin, they can be harder to see, but if a doctor uses dermoscopy — a kind of lighted microscope for examining the skin — the spots may show up as white globules under the skin with darker halos surrounding them.
Heat rash often affects areas where sweating is more likely, including the:
- under the breasts
- under the scrotum
It can also appear in skin folds and areas where skin rubs against clothing, such as the back, chest, and stomach.
If bacteria enter the plugged sweat glands, it can lead to inflammation and infection.
Heat rash often goes away on its own within about 24 hours.
To help it resolve, move to a cool area with less humidity, if possible, and remove any clothing and other items that may increase sweating.
Other tips include:
- Wear light, loose cotton clothing.
- When exercising, choose a cool place or a cooler time of the day.
- Use showers, fans, and air conditioning to reduce the body’s temperature.
- Avoid any irritants that make symptoms worse, such as some synthetic fabrics.
- Avoid staying in wet clothing, such as after swimming.
- Apply a cool compress, such as a damp cloth or an ice pack wrapped in a towel, to the rash for up to 20 minutes at a time.
- Use light bedding.
- Drink plenty of fluids, preferably water, to prevent dehydration.
- If the rash is itchy, tap or pat it instead of scratching it.
Some over-the-counter preparations can help soothe and resolve persistent heat rash. They include:
- Topical preparations — such as calamine, menthol, and camphor-based creams or ointments — can help ease the itching. Use an emollient with calamine, however, as it can dry the skin.
- Steroid creams can reduce itching and inflammation in people aged over 10 years.
- Antibacterial products can help manage or prevent an infection.
There are three types of heat rash, or miliaria:
Miliaria crystalline: This is the most common form. It causes small, clear or white bumps filled with sweat to form on the skin’s surface. The bumps measure 1–2 millimeters across. It causes no itching or pain and is more common in babies than adults.
Miliaria rubra: This type is more commonly known as prickly heat, and it causes larger bumps, inflammation, and a lack of sweat in the affected area. It occurs in deeper layers of skin and is more uncomfortable. If the bumps fill with pus, the medical name becomes miliaria pustulosa.
Miliaria profunda: This is the least common type of heat rash. It forms in the deepest layer of skin, and it can recur and become chronic. It causes relatively large, tough, flesh-colored bumps.
Heat rash, or miliaria, happens when sweat gland ducts become blocked.
This may be due to:
- sweat glands still developing, as in newborns
- a hot and humid environment
- physical activity
- a fever
- wearing synthetic fabrics close to the skin
- wearing a nonporous bandage
- prolonged bed rest
- the use of some medications, especially those that reduce sweating
- radiation therapy
- some health conditions, such as toxic epidermal necrolysis
Heat rash usually disappears without treatment. However, see a healthcare provider if:
- the rash persists or becomes more severe
- there are signs of an infection, such as open blisters or pustular lesions
- there are signs of heat exhaustion and an inability to sweat
- there are other symptoms, such as a fever
Many illnesses cause rashes, which may look similar to heat rash. A doctor can determine the underlying cause.
Heat rashes are not often dangerous, but if symptoms last longer than a few days or signs of an infection appear, see a healthcare provider.
They will examine the rash, possibly using dermoscopy for a closer inspection.
If necessary, they may also take a skin punch biopsy or use imaging technology to identify the cause of the rash.
Skin changes are a common symptom of many conditions. Heat rash can resemble other health issues, including:
- viral infections, such as chickenpox or measles
- bacterial infections, such as impetigo
- hives, due to an allergic reaction
- fungal skin infections, such as candidiasis
- insect bites
- folliculitis, due to a blockage in hair follicles
- acute HIV
- a response to HIV treatment
If any of the following symptoms occur, they may indicate that the cause of the rash is more serious:
To reduce the risk of prickly heat or heat rash, try to:
- Avoid activities or locations that increase sweating.
- If possible, use air conditioning or a fan.
- Wearing light clothing made from natural fibers, such as cotton.
- When possible, minimize exposure to hot and humid weather.
- Gently exfoliate the skin to remove dead skin cells and sebum that may clog the sweat glands.
- Take cool showers frequently and be sure to pat the skin completely dry.
Heat rash is common, especially among babies and anyone in a hot, humid climate.
It usually goes away without treatment, although home remedies can help ease the rash and relieve any discomfort.
If heat rash seems to be involving deeper layers of skin, if there are signs of infection, such as blisters, or if it just lasts for more than a few days, seek medical attention.
Many health issues can cause rashes that resemble heat rash, so if a person has other symptoms, such as a fever, it is important to receive a diagnosis.