Butt warmers in cars may ease the frigid winter commutes, but dermatologists warn that extended exposures to seat heaters can lead to a skin condition called "toasted skin syndrome."

Two reports in the Archives of Dermatology describe cases of the rash apparently caused by pressing the back of the legs to warmed-up seats for prolonged periods.

"Sometimes people don't initially make the connection between the rash and the exposure, but based on the pattern you can figure out that it's probably from heat," said Dr. Jennifer Stein, a professor of dermatology at New York University Langone Medical Center, who was not involved in the studies.

Toasted skin syndrome, known as erythema ab igne or EAI by doctors, is caused by exposure to heat, although it is not a burn.

There have been a few cases of people getting burned by car seats that malfunctioned, but that was not the situation with the patients in these reports.

In one case, a 67-year-old woman developed a web of red lines traversing the backs of her legs.

Her dermatologist, Dr. Brian Adams at the University of Cincinnati College of Medicine in Ohio, determined that the pattern of markings -- which he described in his report as "rusty brown reticulated patches" -- matched the parts of her legs that contacted her car seat.

Pictures of the woman's legs even show that her left leg, which remained pressed against the seat, had more discoloration than her right leg, which was extended to operate the pedals.

During the winter, the woman had her car seat heater turned on during 130 trips lasting 45 minutes each and another 10 trips lasting two hours each.

"I would caution people to avoid prolonged, tight juxtapositioning of their lower legs on the hottest setting of the heated seats," Adams told Reuters Health by email. "Turn down the setting."

In another case, a 40 year old woman, who drove an hour a day for four months using a seat heater, also showed up to the doctor's office with similar markings on the backs of her thighs.

Dr. Eliot Mostow at Northeastern Ohio Universities College of Medicine in Rootstown, who reported the case, determined that the butt warmer was to blame.

Toasted skin syndrome might be ugly, but it's not serious.

"The biggest issue for most people is purely cosmetic," Mostow told Reuters Health.

Adams said that because toasted skin syndrome can mimic other conditions, it could lead to unnecessary testing if it's not diagnosed accurately.

Toasted skin syndrome "is interesting because I think it reflects the changes in technology over time," Stein told Reuters Health, adding that people used to get the rash from sitting close to fires.

In recent years, she's seen cases arising from people using space heaters under desks or having laptops on their laps.

The treatment is simply to avoid contact with the heat source that caused it. Eventually it will fade, but the discoloration could last for months.

Mostow said that if people intend to show off their legs during spring break vacations, they would be wise to avoid toasted leg syndrome and use their seat heaters in moderation.

"If you're doing it for long periods of time...and you see those color changes happening, maybe you should moderate things," said Mostow.