Drivers who spend a lot of time behind the wheel increase their risk of skin cancer, US work suggests.
Cancer risk went up with the number of hours at the wheel
Experts say repeated sun exposure through the car's side windows is to blame, and drivers who roll down the window are at even greater risk.
Most glass used for windows blocks UVB rays that cause sunburn but not deeper penetrating UVA rays.
The Saint Louis University School of Medicine team presented their work to the American Academy of Dermatology.
They looked at 898 patients (559 men and 339 women) with skin cancers occurring on either side of their body.
Among the men, the rate of cancers directly correlated to the areas of the body most often exposed to UV radiation while driving - which in the US is the left-hand side of the body.
Many of these tumours were cancers that develop over time and are linked to cumulative sun exposure rather than intense, intermittent sun exposure.
They affected sun-exposed areas like the head, neck, arms and hands.
Lead author Dr Scott Fosko said: "This finding supports our theory that drivers who regularly spend more time in the car over the course of several years are more likely to develop skin cancers on the left side of the body, particularly skin cancers that develop gradually over time."
Dr Fosko's team is now starting to gather detailed information on the driving habits of the skin cancer patients they see in their clinic.
Initial data shows that those who spend the most time per week driving a car are more likely to develop left-sided cancers.
"We're also finding that all drivers who occasionally drive with the windows open had a higher incidence of left-sided cancers," Dr Fosko said.
He added: "Since there are more cars on the road than ever before, it is likely that this trend will continue. And with more women driving...higher reports of left-sided skin cancers in women in the future."
Most windscreens, unlike the side windows of a car, are made of laminated glass that can filter both UVB and UVA.
Dr Fosko suggested tinting the side window glass or using UV filters on windows might help reduce a driver's risk of harmful exposure.
Cancer Research UK advises: "Although glass greatly reduces the risk of sunburn, it does not prevent long term damage from UVA.
"So if you are driving long distances or sitting in your conservatory every day for long periods of time, with the sun beaming in on you, then you are putting yourself at risk."
But Josephine Querido, cancer information officer for the charity, stressed this did not necessarily mean that you would get skin cancer.
"In terms of the amount of UV sunlight people are exposed to during the course of a year, the amount received through car windows is likely to be a very small percentage.
"It is important to take care whenever you are exposed to the sun, to know your skin type, and, above all, never burn as this can double your risk of getting skin cancer," she said.