You may love your car’s “butt warmer,” that built-in seat heater. But did you know that it may be toasting your skin—literally?

The heat from seat heaters can cause a rash called erythema ab igne (Latin for “redness from fire”). If that mouthful is a little hard to remember, I bet you won’t forget what doctors call it—toasted skin syndrome (TSS).

One example: Researchers recently reported on a woman who used her seat warmer an hour a day for four months and ended up with a persistent, itchy, weblike red rash on the backs of her thighs—toasted skin.

This is not necessarily a mild rash, and if someone with toasted skin syndrome continues using a seat heater, it’s possible for the skin discoloration to become permanent and for open sores to develop at the site of the rash. Just from getting comfy in the car!

HOW THE RASH SNEAKS UP ON YOU

I called the article’s author, Eliot Mostow, MD, MPH, head of dermatology at Northeast Ohio University College of Medicine in Rootstown. The truth is, there hasn’t been much research done on this topic, but Dr. Mostow shared what we know so far.

First, I asked him how long or how frequently you have to sit on a heated car seat for TSS to occur. It depends, he said, on several factors—how hot the seat gets and whether it stays that hot constantly or, as some do, automatically cycles on and off…where the hottest part of the seat happens to contact your back, legs and buttocks…how much you weigh (which affects how hard your skin presses against the heat)…and whether your clothing insulates you from some of the heat. If several of those factors conspire against you, Dr. Mostow said, it’s possible to develop TSS after several weeks or months of regular use.

How long TSS lasts also varies. In extreme cases, it may never completely fade away, but here’s one clue: Four months after discontinuing use of the seat warmer, the patient in the study had only a 50% improvement—meaning, in this case, TSS could take at least eight months to disappear.

So how can a seat warmer make your skin look like Spiderman’s mask? Dr. Mostow said that the lacey pattern of toasted skin outlines the blood vessels underneath the skin, and he thinks that the redness and itchiness might have to do with the prolonged dilation of those vessels.

Then I asked about symptoms. Dr. Mostow told me that one major problem with TSS is that you don’t feel it at first—and since it usually occurs on the backs of your legs, your buttocks, and/or your back, you’re not likely to spot it in the mirror, either. What does it look like? It starts out as a red netlike discoloration and then it turns brownish. Sometimes the skin itches, too. And while some people with severe cases feel burning pain, often there are no symptoms at all. But there’s no one-size-fits-all description, said Dr. Mostow, because your genetics and your skin thickness play a role in how your skin reacts to heat. If you have dark skin, he added, it’s harder to notice the rash early, so you should be extra vigilant.

TREAT IT AND BEAT IT

The best way to look for TSS, said Dr. Mostow, is to keep an eye on the skin on the back of your legs. (If you need help, grab a hand mirror or ask someone to look for you.) If you notice TSS, Dr. Mostow advises you to…

    • Stop using your seat heater altogether—or at least turn it off after it warms up or minimize how long you use it.
    • Use a skin moisturizer (especially after you bathe, so your skin will absorb moisture better), because dry skin is more easily irritated. Reapply whenever the skin feels dry again.
    • Take a picture to document your condition and then make an appointment with your primary care doctor or dermatologist (and bring your photo) for further examination.
    • To help prevent the problem in the future, turn off the seat heater whenever your skin feels uncomfortably hot and check the backs of your legs, your back and your buttocks every few days.