What is Heat Rash?

The causes and symptoms of heat rash--otherwise known as prickly heat--as well as the best ways to treat it

baby in baby pool PhotoAlto/ Matton

Simply put, heat rash is the skin's response to too much heat. Also referred to as prickly heat or miliaria, heat rash can affect anyone, but it's incredibly common in newborns and infants. Here's what's happening in the body: Normally, when we start to sweat, perspiration travels up through the sweat ducts to the skin's surface and eventually evaporates. But babies are born with immature sweat ducts that can easily rupture. "Heat rash develops when the sweat becomes blocked and is trapped under the skin," says Parents advisor Jody A. Levine, M.D., director of dermatology at Plastic Surgery & Dermatology of NYC.

Types of Heat Rash

There are three different types of heat rash. Miliaria crystalline, the mildest form, affects the outermost later of skin, and is very common in infants. Miliaria rubra focuses deeper in the epidermis layer and can be associated with bed rest, which is why newborns can develop it in the first few weeks of life. The third type, miliaria profunda, targets the layer below the epidermis and is the least common of the three, often occurring as a result of multiple bouts of miliaria rubra.

What Causes Heat Rash?

"The combination of being in a warm environment and your baby being over-clothed eventually causes prickly heat," says Parents advisor Lawrence F. Eichenfield, M.D., professor of pediatrics and dermatology at University of California, San Diego, and chief of pediatric and adolescent dermatology at Rady Children's Hospital-San Diego. And a warm environment can take on many forms. "A hot summer day and a non-air-conditioned home can bring on a heat rash," says Dr. Eichenfield. "And if you overheat your home in the winter and keep the baby bundled up, that creates a high risk, too." Once your baby is officially overheated, she'll try to sweat, unsuccessfully, and develop a heat rash instead. To avoid this, you need to find the right balance between what she's wearing and where she's wearing it.

    Symptoms of Heat Rash

    Heat rash generally occurs on clothed parts of the body, where a baby is the warmest, such as the groin, abdomen, and armpits, says Dr. Levine. It appears as tiny bumps surrounded by redness on the skin. Its other name, prickly heat, refers to another possible symptom. Not that your baby will be able to tell you, but sometimes the rash can be accompanied by a pins-and-needles sensation, which could lead to scratching.

      How to Treat Heat Rash

      First, make the cause part of the treatment. Keep your baby out of excessively warm environments and resist the urge to over-dress. Choose lightweight, loose-fitting clothes that keep the skin cool and dry and allow the affected areas to breathe. If she's scratching, calamine lotion or hydrocortisone cream may be used with your pediatrician or dermatologist's approval, says Dr. Levine. Moisturizer might also be helpful, if there's a significant amount of itching or dry skin, but be sure to clear it with your doctor first; some lotions and creams can also block sweat and further irritate the skin.

      When to Call the Doctor

      Usually heat rash clears up within a few days, but occasionally the red bumps are surrounded by deeper, redder areas and blisters. "It won't look like regular prickly heat," says Dr. Eichenfield. "If it extends outside the clothed areas, then go back to your pediatrician or dermatologist."

      How to Prevent Prickly Heat

      "A cool, dry environment and loose-fitting clothes are essential in keeping prickly heat at bay," says Dr. Levine. And although it might be tempting to layer your little one up, consider what's right for the temperature, inside and out. This means lightweight, cotton clothes in the summer and the appropriate amount of clothes in the winter. Use yourself as a barometer: If you're too hot, there's a good chance she is, too.

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