Why Sweat and Heat Make Your Skin So Sensitive

The summer heat exacts a toll on the skin. Melasma, eczema, and other conditions often rise with the temperature.

I don’t need a thermometer to tell me when the temperature has gone over 90°F. All I need is my left wrist. That’s where I wear my watch, and the instant the air temp pops past 90, I get a rash under the band. It’s a condition that goes by a lot of names—heat rash, prickly heat, eccrine miliaria—and it’s just one of the ways our skin reacts to too much heat, too much sun, and the increasingly punishing summers we’re suffering as climate change raises temperatures around the globe. If your skin has been protesting more and more in hot weather, you’re not remotely alone. Here are the most common heat-related conditions dermatologists are seeing, and what you can do about them.

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Prickly heat

Your skin is your body’s natural radiator, giving off energy when you get overheated via coil-shaped, sweat-producing glands. When the moisture reaches the surface of the skin, it evaporates, carrying warmth away from the body. The system is operating even when you don’t feel especially hot, but in high temperatures, as you start running with sweat, it goes into overdrive. That can cause problems.

“If your body is sweating faster than your pores can get it out, the sweat gets caught underneath the skin,” says Dr. Heather Rogers, a Seattle-based dermatologist on the clinical faculty of the University of Washington. “The sweat glands become blocked and you get these tiny, red, itching bumps.”

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Anywhere the skin can’t breathe—your back or lower thighs when you’re sitting on a patio chair, the waistband of a swimsuit—will readily develop prickly heat. Babies may develop rashes on their backs from lying down in the heat. “Those are all places where you’re likelier to have occlusion, or blockage of the sweat glands,” says Rogers. There is no way to avoid prickly heat entirely in hot weather, but wearing loosely fitting clothes and moving around to allow skin to breathe can help. A cool shower can also help rinse surface sweat away.

Acne

Teens and adolescents can suffer especially acutely in hot weather, as acne, which may be held in check in the cooler months, blooms in the heat. Regardless of the season, acne most commonly occurs when pores become clogged by oil, sweat, and microscopic dirt, trapping bacteria. The immune system then attacks these sites, leading to further inflammation.

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“In hot weather we see increased sweat mixing with oil glands and blocking pores,” says Dr. Danilo Del Campo, a Chicago-based dermatologist working at the Chicago Skin Clinic. “In young people especially, this can lead to increases in acne.”

“If you are prone to acne,” says Rogers, “you’re going to see it get worse in the summer.”

Here too, showers can help rinse away sweat and oil. Avoiding the worst of the heat by staying indoors in an air-conditioned environment can give your skin a break as well.

Melasma

Women who are or have been pregnant or are using hormonal birth control will sometimes develop patchy areas darker than the surrounding skin on their cheeks, forehead, nose, or upper lip. Known as melasma, the condition, which can also occur in men, results from overactive melanocytes, the skin’s pigment cells. Melasma can fade on its own, but in some people it may never go away entirely. 

Extreme heat caused not just by hot weather, but also by hot indoor environments like yoga studios, can cause melasma to flare. In the winter or when conditions are otherwise cool, the darkening can fade.

“Melasma is a very, very common thing,” says Rogers. “One in three women will suffer from it. Sun exposure and heat can bring it about, and even if you’re not in direct sun, being hot all the time will activate your melanocytes and make the melasma worse. That’s very frustrating to people. The solution is to try to stay cool, wear your sunscreen, and stay out of direct sun.”

Polymorphous light eruption

Especially common in women, but present in both sexes, polymorphous light eruption is a rash that commonly occurs in the first couple of days or weeks of summer, when the skin is newly exposed to direct heat and sunlight. It most commonly appears on the chest, forearms, and back of the hands, and is thought to be a result of the body’s immune system reacting—or overreacting—to the touch of the sun.

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Anecdotally, people are reporting more polymorphous light eruption in the wake of the COVID-19 pandemic, perhaps as a result of the immune system being sensitized by either the virus or the vaccine used to prevent the disease, says Rogers.

“After COVID and after people having been vaccinated, the autoimmune flares that we saw as dermatologists significantly increased,” she says. “The thought was that it was related to the immune system being revved up by exposure to COVID.”

“Increased sunlight does affect changes to the immune function of the skin,” says Del Campo. Dodging direct sunlight is the best way to avoid, in turn, immune-related conditions.

Eczema

Commonly caused by excessive drying of the skin, eczema can grow worse in the summer as people are jumping into and out of chlorinated pools, showering frequently, and becoming dehydrated. Lips can become chapped too for more or less the same reasons. The answer is to stay hydrated, avoid too much sun exposure, and use a skin moisturizer and lip balm. “These precautions can prevent your skin from losing too much of its oil,” says Rogers.

Skin cancer

Most sun-related skin conditions happen in real-time, in the short term. Skin cancer plays a long game—and it doesn’t take a lot of  sun exposure to cause damage. As few as two blistering sunburns can increase a person’s long-term risk of melanoma, a serious form of skin cancer that starts in the melanocytes, says Rogers.

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“If you get skin cancer, it’s not because you got sunburned that summer, but usually 10 to 30 years previously,” she says. That’s because “you collect damage from the sun that leads to mutations in skin cells,” she says. “The [cells] that are really damaged die. The ones that are kind of damaged don’t die, and then can turn into skin cancer down the road.” 

For reasons that are not entirely clear, men are more likely than women to develop melanoma by age 50, and are also more likely to die of it. White people are more likely to develop melanoma than Black people, but Black people do develop the disease, and it is often caught too late because individuals and their doctors tend not to look for it as vigilantly, according to Rogers.

The Skin Cancer Foundation recommends the everyday use of a broad spectrum sunscreen with an SPF of at least 15 to help prevent skin cancer; for extended sun exposure, the SPF should be 30 or higher. Sunscreen should be applied every two hours or, if you’re going in the water or sweating heavily, every hour.