Also called: Prickly Heat, Sweat Rash

What Is Heat Rash (Miliaria)?

Miliaria — often called heat rash — is a skin rash that happens when sweat builds up under the skin behind blocked sweat ducts. Sweat ducts are small tubes that carry sweat from glands under the skin to the skin’s surface.

The rash can be a little itchy, but miliaria (mill-ee-AR-ee-uh) doesn’t cause other problems and isn’t serious. Keeping skin cool to ease sweating can help the rash go away within a few days.

What Causes Heat Rash?

Sweat can collect behind blocked ducts when kids sweat a lot (for example, when being physically active in hot weather) or in warm, humid weather.

Heat rash is most common in babies but can happen at any age.

What Are the Signs & Symptoms of Heat Rash?

A heat rash causes small, itchy red or clear bumps that can look like blisters. On darker skin, the rash may be harder to see and can look gray or white rather than red. 

The rash most often is on the upper chest or back, the head, the arms, the neck, in body folds, or anywhere the skin doesn’t get enough air or rubs together or against tight clothing. Children also might have mild swelling in the area. 

Sometimes the rash can be deeper under the skin and look more irritated.

How Is Heat Rash Diagnosed?

Doctors can diagnose a child with heat rash by looking at the skin. No medical tests are needed. 

How Is Heat Rash Treated?

Home care to cool the skin will help clear up heat rash.

To help cool your child’s skin:

• Try to keep them in a cool environment, with air conditioning or a fan, if possible.
• You can put a thin, clean cloth dipped in cool water on the rash a few times a day. Take the cloth off once it is no longer cool. They also can take a cool bath or shower (but without soap until the rash is gone). Pat the skin dry gently or let it air dry.
• Dress them in lightweight, loose cotton clothing or, if possible, let their skin air out without clothing. 
• Give them plenty of cool liquids to drink.
• For a baby with a rash in the diaper area: Try cloth diapers or leaving the diaper open or off as much as possible until the rash heals.

Don’t apply thick creams or ointment to the skin (which can further block the clogged ducts) or cover the rash with bandages.

If your child’s rash is very itchy, keep their nails trimmed to help prevent scratching that could break the skin and lead to an infection. Encourage them to pat or tap on the rash to help with itchiness. For kids over 2 years old, you can put calamine lotion on the skin (but not on the face or on the genitals) 3–4 times a day to help with discomfort and itching.

What Can Help Prevent Heat Rash?

• Encourage kids to change out of clothes that are wet (like after swimming) or sweaty (like after sports, exercise, or playing).
• In hot weather, have kids wear loose, lightweight clothes that keeps moisture away from skin.
• Don't put too many layers on a baby.
• Keep bedrooms cool. If possible, use fans and air conditioners, which also keep air moving. Use lightweight sheets and blankets.
  

When Should I Call the Doctor?

Call the doctor if your child’s rash:

• oozes pus or looks infected
• gets very painful or swollen
• is very itchy
• isn’t gone after a few days
• looks red or purple (like bruises)
• is bright red or streaky

Also call the doctor right away if your child gets a fever. If your child is under 3 months old and has a rectal temperature of 100.4°F (38°C) or higher, take them to the emergency room (ER).