A new study confirms what many dermatologists have suspected all along: Sun exposure while driving may account for more skin cancer on the left side than on the right.

Sunscreen, then, isn’t just for sun worshipers, gardeners and other outdoor types. It’s important even when driving.

The ultraviolet radiation that passes through window glass, sunroofs or convertible tops may contribute to melanoma and other skin cancers.

In this study—appearing in the Journal of the American Academy of Dermatology—left-sided skin cancer was more common in men.

Senior researcher Scott Fosko, M.D., head of dermatology at Saint Louis University, says the damage builds up over time. “You can see many effects with chronic exposure.”

Researchers reviewed the charts of skin cancer patients at the university in 2004. Of 890 patients with skin cancer, 52.6 percent had skin cancer on the left side of the face or body. They found there were significantly more skin cancers on the left side of the body in men—especially on exposed areas of the head and neck.

The researchers speculated that perhaps older generations of women—the average age of those in the study was 68—tended to be passengers sitting on the right side and men tended to drive more.

What about people in countries who drive on the left side of the road? An older Australian study found that precancerous sun spots were more common on the right side of the faces of Australian men, who typically drove, and the left side of the faces of Australian women, who were usually passengers. The study backs up the theory that exposure while driving explains the difference.

Alan Lewis, M.D., director of dermatology surgery at Tulane University in New Orleans, says the latest study was a good one. “When you’re going outdoors for any reason, you should definitely think about applying a sunscreen with an SPF 30 or higher,” he says. Check the label to make sure it blocks both UVA and UVB rays.

Even while indoors near a window, wearing sunscreen is a good idea, says Sarah Tuttleton Arron, M.D., director of the skin cancer unit at the University of California at San Francisco.

“A little bit of UV exposure is like a little bit of smoking, so I recommend sun protection all the time,” she says, adding that tinted car windows with UV-protection can also contribute to your safety.

Susan Kreimer writes about health and business.