Experts detail what causes heat rashes, what they look like, and what to do about them.
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What is Heat Rash
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Sure, cold winter air can dry out your complexion, but summer presents its fair share of skin concerns. There's sunburn — it can happen, despite attempts at sun protection breakouts, bug bites, and of course, those weird rashes that crop up on hot days. That last issue is referring to heat rash, a condition that's common in hot, humid environments. Here, top dermatologists explain what a heat rash looks like, what causes the issue, and what you can do to address it.

What is heat rash and what causes it?

"A heat rash, also known as 'prickly heat' or by the medical term miliaria, develops when irritation occurs due to sweat staying on the skin, rather than evaporating or being washed off," explains Shadi Kourosh, M.D., director of community health for the Department of Dermatology at Mass General Brigham. This happens when you sweat, but something (often clothing) occludes or blocks your sweat ducts. This traps the sweat in your dermis and epidermis (the two outermost layers of your skin) resulting in a rash, says Sandy Skotnicki M.D., assistant professor at the University of Toronto Department of Medicine in the Divisions of Dermatology and Occupational and Environmental Health and the author of Beyond Soap. That's why heat rash is also referred to as sweat rash. (Related: 6 Weird Things You Didn't Know About Sweating.)

What does a heat rash look like?

A few defining features to look for include tiny bumps or blisters that may also feel itchy. But depending on the extent of the sweat blockage, heat rash can take on several different forms, explains Hadley King, M.D., a board-certified dermatologist in New York. Miliaria crystallina is characterized by small, clear, very superficial blisters. A more severe form, miliaria rubra is characterized by deeper, itchy red bumps and blisters along with general background redness, accompanied by the sensation of burning or tingling, she explains. In either instance, the blisters and bumps are very small, only about one to two millimeters in diameter, according to Dr. Skotnicki.

Heat rashes can pop up anywhere on your body. However, areas where there isn't much airflow, such as the skin folds around your groin or breasts, as well as spots that are often covered by clothing, such as your back and chest, are most commonly affected, notes Dr. Kourosh.

Heat Rash
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Who is likely to develop a heat rash?

A heat rash results from external factors (i.e. whatever is causing the sweat to become trapped), rather than a particular skin type or biological predisposition, according to the dermatologists interviewed for this story. (That being said, those with sensitive skin may experience even more general irritation from the salt content in the sweat, notes Dr. King.) Heat rashes often occur in newborns and bedridden patients because they're lying on their backs for long periods of time, as well as people who live and/or exercise in hot, humid climates, says Dr. Skotnicki. (Related: Sweat Pimples Aren't Entirely Caused By Sweat — Here's What You Should Know.)

What is the best way to treat a heat rash?

As with many things, prevention is your best bet. "Making sure the skin is aerated, wearing breathable clothing, washing sweat off immediately after perspiring, and changing positions to prevent long periods of occlusion of the skin are all good steps," says Dr. Kourosh. It bears mentioning that exactly how long the sweat needs to be trapped on the skin before a heat rash occurs is completely dependent on the individual, though it could be as short as a few minutes. Point being, it's imperative to wash sweat off your skin ASAP.

If a heat rash still pops up, you should be able to easily deal with the issue at home. Seek a cool environment and avoid applying any type of thick or occlusive products (think: heavy moisturizers), suggests Dr. King. Cleanse the rash with cool water, apply an over-the-counter hydrocortisone cream if it feels inflamed, irritated, or itchy, and fan the area, she says. However, if you're not noticing an improvement after several days or your symptoms get worse, it's time to pay a visit to your dermatologist.