Heat Rash: Symptoms and How to Treat It - The New York Times

This itchy, bumpy condition is often confused for other ailments, but it's always an indication that you should get out of the sun.

With many parts of the country experiencing dangerous levels of heat, some may have noticed itchy, red bumps cropping up on certain parts of the body. Excessive heat and humidity can make you more prone to heat rash, a common condition that develops on the neck, shoulders, chest and folds of the skin. But what is heat rash, exactly, and what should you do if you get it? We spoke to a few experts to find out.

What is heat rash?

"Heat rash" is not exactly a term that health care providers use to make an official diagnosis, said Dr. Angela Lamb, a board-certified dermatologist at Mount Sinai in New York City.

Miliaria, as it's technically called, occurs when the sweat glands and ducts in your skin get blocked because of high heat and humidity. "Then, when your sweat gets trapped beneath the skin, you get these little fine pimples or blisters," Dr. Lamb said. This rash, also sometimes referred to as prickly heat or sweat rash, is often itchy or can sting.

There are three types of miliaria people can get, depending on the kind of blockage that occurs. Miliaria crystallina, the mildest form, most commonly occurs in newborns and happens when blockages form in the openings of sweat ducts on the surface of the skin. This type of heat rash looks like little beads of sweat trapped under raised skin, is not inflamed or itchy and looks less red than other types of heat rash. In fact, sometimes it's not even red at all.

Miliaria rubra, the most common type, can also affect newborns and up to 30 percent of adults who live in hot, humid regions. It "tends to be really red, itchy, bumpy," said Dr. Rajani Katta, a board-certified dermatologist in private practice in Houston. This type of heat rash occurs when sweat gets blocked in the mid-epidermis, a slightly deeper layer of the skin.

Miliaria profunda, the least common form, results when sweat glands get blocked in the dermis, the deepest layer of the skin that miliaria can occur in. It's not as itchy as miliaria rubra, but the bumps are often firmer, redder and more painful.

All forms of heat rash usually develop on parts of the body where skin folds over itself, like the underarms, groin, neck, abdomen and under the breasts. Infants typically get heat rash on the neck, shoulders and chest. Tight clothing can also make it more likely for one to develop heat rash.

Newborns, who have less-developed sweat ducts that can easily become blocked and more rolls of skin, are more susceptible to heat rash than older kids and adults, Dr. Katta said, "especially if you're bundling them up tightly when it's hot out."

How can you distinguish it from other skin conditions?

Heat rash is often confused for other skin ailments, Dr. Lamb said, like eczema, "which can flare with extreme weather in the summer when it's humid." Eczema is an inflammatory skin condition that can cause itchiness, dry skin and rashes, as well as blisters and skin infections. If you have mild eczema, you may not notice it when the weather is mild, Dr. Lamb said. But when it gets hot, you may experience an eczema flare that might be mistaken for heat rash.

"They can both be red and itchy and sometimes they can occur in the same areas," Dr. Katta added. "One difference is that eczema tends to look more patchy and scaly." The itchy areas tend to be flatter and ill-defined at the edges, whereas heat rash tends to look and feel more like distinct bumps.

Another condition often mistaken for heat rash is polymorphous light eruption, which is an itching or burning rash of tiny, inflamed bumps or slightly raised patches of skin. "It's a very unique condition that people notice when they take a sunny vacation," for the first time in a while. It can happen any time, but often occurs when people travel in the spring or early summer and are exposed to much more sun than their bodies are used to, especially after the winter months, Dr. Lamb explained. This can cause them to "break out in a rash, usually pretty exclusively on the sun exposed areas."

What's unique about polymorphous light eruption though, she said, is that "unlike heat rash, it isn't really due to temperature or humidity — it's really due to sunlight."

How do you treat and prevent it?

Experts say that anyone can get heat rash. If you develop it, or even if you have heat-induced eczema, the first thing you should do is move to a cooler location, Dr. Lamb said. Get out of the heat and into the shade, and avoid peak heat and humidity hours.

"Remove any tight fitting clothing" and wash off any thick lotions, like sunscreen or moisturizers, which might prevent your skin from breathing, Dr. Nadine Kaskas, a board-certified dermatologist at Mount Sinai, wrote in an email. "Take a cool shower or apply cool compresses with a clean washcloth." She added that you can get over-the-counter ointments like calamine lotion to help soothe any itchiness — though if it becomes especially bothersome, you should consult with your doctor, as you may need a prescription for a topical steroid cream.

When left untreated, heat rash usually calms down on its own once you've gotten away from any hot and humid environments, Dr. Lamb said. Though there's a slim chance of developing an infection if blisters break open and you do not keep your skin clean.

If you must be outside, Dr. Kaskas recommended taking steps to prevent your body from overheating, which could make you more susceptible to heat rash. Seek shade, take breaks from activity, avoid overexertion and stay hydrated. Dr. Lamb is a fan of those portable fans that you can hang around your neck.

It's also important to wear clothing that is breathable, keeps you cool and allows "sweat to evaporate from your skin," Dr. Katta said. That means wearing garments that are loose and lightweight.

The last important thing to note, Dr. Katta added, is that "heat rash is a warning sign" that your sweat glands or ducts are not functioning as they should, and could possibly make you more vulnerable to heat exhaustion or heat stroke. If you notice any irritation on your skin while out in the heat, take steps to cool off.