They're warm and cozy and on a cold winter's day, there's nothing more soothing than to sit back and bask in their toasty embrace. But used too much, heated car seats can bring on erythema ab igne (aka toasted skin syndrome), a skin condition that can leave one's backside with lesions, pain, itchiness and more.

Two cases of the netlike dermatosis brought on by normally-functioning heated car seats were presented in the latest issue of Archives of Dermatology, one involving a 40-year-old woman who reported using her heated car seat an hour a day during Ohio's four-month winter season and another, involving a 67-year-old woman who drove approximately 120 hours during the winter months with the heated seats activated.

"I've seen plenty of cases of erythema ab igne over the years but this is the only case I've seen that's stemmed from a heated car seat," says Dr. Eliot Mostow, an Akron, Ohio dermatologist, who wrote about the case involving the 40-year-old woman. "Most cases I've seen have been from heating pads. People use them for back pain for a few months and do it every night. It's a combination of duration and repeated use, long-term use."

Unfortunately, repeated exposure to even moderate heat -- and car seats can reach an upper limit of 109 degrees -- can cause a net-like mottled pigment change in some people's skin.

"It's probably related to some blood vessel changes and changes in the pigment cells," says Mostow. "You get more severe cases with longer exposure, But to be honest, everybody's different. Some people will get it sooner with the exact same exposure. They're more sensitive."

Heated car seats and heating pads aren't the only culprits when it comes to toasted skin syndrome.

According to the case study, hot water bottles, heat packs and heating pads can cause erythema ab igne (or EAI) on the abdomen, lower back or other sites. Open fires, electric space heaters and steam radiators can cause damage to inner thighs or shins. And laptop computers can cause toasted skin syndrome to the anterior thighs and breasts. There have even been reported cases of furniture-induced EAI caused by heated chairs.

While the 40-year-old woman Mostow treated experienced only subtle color change and a bit of itching (both of which went away after discontinuing use of her car's heated seat warmers), other cases of EAI can be much more serious.

"It's usually a cosmetic issue but it occasionally causes more problems such as pain and ulceration," says Mostow. "I've seen cases with heating pads that are really severe."

In the case involving the 67-year-old woman, the back of both of her legs were covered with rusty brown reticulated patches extending from her mid thigh to just below the hollow of her knee. The left leg had more lesions than the right leg, presumably because the woman's right leg would sometimes be away from the seat, engaging the gas pedal (when her car was on cruise control, both legs were snug against the heated seat).

"I suspect this woman will have permanent color changes in her leg," says Mostow of the 67-year-old patient, who was treated by another dermatologist.

What can you do if you're worried about contracting toasted skin syndrome from your heated car seat, heating pad or laptop computer?

"It comes down to what our mothers taught us," says Mostow. "Everything in moderation. If I had a recommendation, it would be to use the car seat heater to warm up the seat, then turn it off. When it gets cold, turn it on again. But don't use it for a whole six-hour drive."

Mostow says the same thing goes for laptop use or continued use of any heated item.

"I think this is a good one for people to be aware of," he says. "Be conscious that heat can cause changes to the skin. I'm sure there are people who say, 'I don't care,' but I think there's an argument for taking a look. I think a lot of people will be looking at the back of their legs and their butt for changes in their skin."

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