容易患上皮肤癌的原因尚未清楚，但负责研究的医生表示跑步时受伤会使身体产生某种细胞因子 (cytokines)和蛋白质，增加免疫细胞的生长和活动，结果导致免疫细胞无暇对付皮肤癌的来袭。报道引述 the Medical University of Graz in Austria 的研究人员说：

The exact mechanism is unknown, but there is evidence that trauma sustained during extreme exercise can induce the release of cytokines, proteins that can stimulate the growth and activity of various immune cells and that may limit the ability of the immune system to fight potential cancers.

研究人员又说， 在烈下日做运动比进行其他活动对皮肤造成较大的伤害，因为汗水会增加皮肤对紫外綫幅射的敏感度：

Physical exercise on sunny days can be more harmful to the skin than other kinds of sun exposure, the authors suggest, because sweating may significantly increase the sensitivity of the skin to ultraviolet radiation, making sunburn more likely. Moisture on the skin reduces the UV light to shorter wavelengths that are more easily absorbed and decreases their reflection and dispersion.

Dr. Ambros-Rudolph [lead author of the report] emphasized that the main problem, for both casual runners and extreme exercisers, is sun exposure.

“We hear a lot about sun exposure and skin cancer,” she said. “But we forget about it when participating in outdoor sports. Sunscreen alone is not the ultimate answer. It’s also important to wear reasonable gear that covers the shoulders and upper back, and to avoid training in peak sun hours.”

防晒用品不是万灵药，还要在衣着上做防预工作；但有多少跑手会涂防晒用品呢？又有多少两者都做呢！

不少跑手会选择在烈日下练习，如在下午三四时左右。今年夏季，我间或在四时前后练习，见到不少跑手兴緻勃勃地跑，颇受感染。在烈日下跑，虽然辛苦，更易令人疲倦，但就是这种艰苦造就其吸引力；但看来还是避免在烈日下跑步为佳——为了健康。

作者：Paul Cheung