The doctors in the Graz University dermatology department realized that over about a 10-year span, they had treated 8 ultramarathon runners with malignant melanoma. The medical team discussed potential triggers of the cancer in these patients and UV exposure, immunosuppression due to long-term intensive exercise, or both surfaced as possibilities. This piqued the curiosity of the doctors and they decided to look closer at the risk factors for malignant melanoma in marathon runners. To do so, they examined medical history, genetic and environmental influences, sun-related and clinical variables in 210 athletes and compared them with those of an age- and sex-matched control group.
What they discovered was that although the control subjects exhibited higher sun sensitivity and more common skins spots and moles, the marathon runners exhibited more atypical spots, moles, and lesions suggestive of nonmelanoma skin cancer.
And interestingly enough, of the marathon runners, the ones who had trained more had more of these atypical moles. Findings also revealed that about 96.7% wore shorts (not too surprising) and 98.6% wore shirts that only partially covered their backs and extremities (not too surprising either). Only 56.2% of the runners used sunscreen on a regular basis.
End result? Basically the marathon group presented with an increased risk for malignant melanoma and nonmelanoma skin cancer. It's kind of like one of those "duhh" moments. But sometimes it takes seeing the numbers to have it hit home.
Most likely that act of running has nothing to do with the increased risk, but the time in the sun does. If a similar study was done with construction workers or landscape workers, you'd probably find similar results. However, that fact that runners typically have on less clothing and have more exposed skin probably would still make them more inclined to a higher risks of skin cancer.
So what's a distance runner to do? Easy! Cover up! Actually the study recommends doing your runs during times of low-sun exposure, wearing adequate clothing, and regularly using water-resistant sunscreen.