Heat Rash or Sunburn: Here’s How to Tell the Difference

Updated: May 27, 2020

Both of the skin conditions can occur after a hot, sunny day, but here’s how you can tell the two apart.

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As you glance in the mirror after a day at the beach, you find a red patch of skin on your body. You may assume that it’s a mild sunburn, but it could actually be a heat rash. We spoke with dermatologists to help you distinguish the two skin conditions, which are actually quite different.

How to tell the difference between a heat rash and a sunburn

Despite the fact that both skin conditions can occur in hot, sunny environments and are red in color, they look like complete opposites. An acute sunburn has brighter hues of pink and red and tends to be a connected patch of redness that occurs after your skin is exposed to the sun’s harmful ultraviolet rays. “Sunburn is going to be a flat red that will turn white when you press it,” says Kathleen Cook Suozzi, MD, assistant professor in the department of dermatology at Yale School of Medicine. “If the patient is wearing a T-shirt, the sunburn is going to respect those boundaries.” Unfortunately, your back and shoulders are just two of the worst places on your body to get a sunburn.

On the other hand, a heat rash can occur anywhere on the skin—most often in places where moisture gets trapped, such as underneath tight-fitting clothes like spandex or jeans. “When we sweat, that moisture is evaporating and helping us cool ourselves,” says Anthony M. Rossi, assistant attending at Memorial Sloan Kettering Cancer Center and New York-Presbyterian Hospital. “If you’re not allowing the body to cool and evaporate that sweat, you can cause the sweat glands to get blocked and cause heat rash, also known as miliaria.” It can even occur inside when you’re bed-ridden due to a fever or other illness since there’s no airflow to certain parts of the body when you’re confined to a bed.

There are three different types of heat rash that can cause a range of symptoms such as fluid-filled blisters or itchy red bumps that can mimic a prickling sensation. Itchy red bumps are a sign of the most common type of heat rash: miliaria rubra. Babies are more susceptible to heat rash because they don’t yet have the ability to sweat properly, but it can still affect adults.

Compared to a mildly itchy heat rash, sunburns are much more painful. As for the onset of symptoms, a sunburn will typically appear before signs of a heat rash do. A sunburn can occur mere hours after being out in the sun, but a heat rash is much more delayed—it can take hours, days, or even weeks for a heat rash to emerge.

sun-burn-heat-rashShutterstock (2)How to treat sunburn vs heat rash

Plus, treatment options for heat rash and sunburn are on opposite ends of the spectrum. “For a sunburn, I’ll have a patient liberally apply a moisturizer like Aquaphor or Vaseline to help with blistering or peeling,” says Dr. Suozzi. “But I wouldn’t want to over-moisturize someone with a heat rash because the ointment can act as an occlusive agent and further clog the sweat glands.” The key to treating heat rash is to avoid applying topical agents to the skin and stay in a cool environment like an air-conditioned home. “Removing [someone] from the hot environment and stopping them from sweating should alleviate the symptoms pretty quickly,” says Dr. Suozzi.

Other remedies that Dr. Rossi recommends include applying a cool pack to the affected area of the skin, soaking in a cool bath with mild soap, and wearing loose clothing. For itchiness, pop an antihistamine instead of grabbing a bottle of moisturizer that could clog your skin even more. And bear in mind that heat rashes can recur so it’s best to stay out of the heat and limit strenuous physical activity for at least a week. “If the patient re-exerts themselves, a heat rash can flare again,” says Dr. Suozzi. “And that cycle of flaring could happen again for a longer period of time.” In addition, it can take anywhere from days to weeks for the heat rash to disappear depending on how long it takes for your sweat glands to re-open again. Fortunately, a heat rash will usually heal on its own without any problems, but the damage done by just one sunburn can change your body in several ways.

The next time you go out on a hot, sunny day with friends or family, make sure you liberally apply sunscreen every two hours and wear loose-fitting clothes and/or stay in the shade to avoid both heat rash and sunburn. Now that you know the difference between a heat rash and sunburn, here are the differences between melasma, sun spots, and other weird skin spots.