Marathon running is an increasingly popular sport. In 2005, almost four-hundred-thousand marathon finishing times were recorded in the United States alone. And although the speed and ability levels of marathon runners vary drastically, recreational runners, Tour De France legend Lance Armstrong and professional marathoners all bear something in common: Many hours of training runs, and lots of exposure to the sun’s rays as a result.

Now a study published in Archives of Dermatology finds marathon runners may be at greater risk for skin cancer than peers who don’t participate in the twenty-six-point-two mile-long event.

Researchers studied one-hundred-sixty-six men and forty-four women runners ages nineteen to seventy-one and assessed skin cancer risk factors. The runners were asked questions about their training, including what type of clothing they typically wore and whether they used sunscreen. A group of two-hundred-ten non-runners, matched to the runners by age and sex, were recruited as well.

In comparison, the marathon runners had more abnormal moles and more small lesions, higher numbers of which indicate a greater risk for malignant melanoma. These features were more pronounced in those with more intense training regimens. Twenty-four individuals in the marathon running group and fourteen in the control group were referred to dermatologists for skin lesions suggestive of non-melanoma skin cancer.

The ultimate message? Serious runners and even weekend warriors should cover as much skin as comfortably possible while working out outdoors, and slather the sunscreen onto areas of skin left exposed.