Marathoners and skin cancer risk

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The Times reports on a study indicating (unsurprisingly) that distance runners are at increased risk for skin cancer:
Sun exposure may not be the only risk factor that distance runners face. The authors write that although there is no question that regular exercise is important to good health, there is good evidence that high-intensity training and excessive exercise can lead to suppressed immune function.

This is quite well established, Dr. Ambros-Rudolph said. Many alterations in immune cell function have been noted at the cellular level in marathon runners.

For example, there is the association between excessive exercise and immunosuppression reflected in the increased incidence and severity of upper respiratory tract infections in marathon runners after races.

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.

Clinical examination by dermatologists showed that none of the participants had lesions that suggested malignant melanoma. But 24 of the marathon group and 14 of the control group were referred for surgical treatment of lesions that appeared to be basal or squamous cell carcinomas or the precancerous lesions called actinic keratoses. Follow-up reports on these patients were not available because of the limits in Austrian laws on personal privacy.

About a third of the marathoners ran up to 25 miles a week, and nearly half ran 25 to 45 miles. Almost 15 percent ran more than 45 miles a week. Those who trained the most intensively had the highest rates of skin lesions.

This isn't a particularly surprising result, especially as the researchers report (and my experience confirms), many athletes don't wear sunscreen. And there's certainly plenty of anecdotal experience that high training loads lead to immune suppression—it's one of the major outcomes of overtraining.

This next bit is weird, though:

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.

This doesn't sound right physically. Remember that the shorter the wavelength the higher the frequency (and thus the higher the energy of the photons). In general, when photons interact with some compound they shift it to lower energy, not higher; think fluorescent lights where the original emission is in the UV but it's absorbed by the coating and reemitted in the visible.

Anyway, that's not actually what's going on here. Rather, the water is changing the absorption spectrum of your skin. Here's the text from the paper:

They also showed that sweating because of physical ex- ercise may significantly contribute to UV-related skin dam- age, because it increases the photosensitivity of the skin, facilitating the risk of sunburns.14 These effects are pre- sumably due to hydration of the horny layer, which leads to a shift in the stratum corneum UV absorption spectrum, to shorter wavelengths, and to a decrease in reflection and dispersion.

That does make sense.


I will point out that, although it is irrelevant, the wavelength of light does go down when it enters a medium with an index of refraction greater than 1. The frequency remains the same, of course, but since the index of refraction of water is about 1.33, the speed of an electromagnetic wave traveling through water is 1/1.33 c and the wavelength of any wave entering the water is of necessity reduced by the same factor.

It is irrelevant because this can't possibly alter how damaging the UV rays are -- they carry the same energy regardless.

I've read a lot of these same studies on the effects of intense training on the immune system, but I, and most of the folks I run with, have had the opposite experience. I don't think I've had a cold or the flu in at least six or seven years and almost never get anything more serious than a runny nose or sore throat. I train about 50 miles a week most of the year, and many of my friends run 70 or 80 a week and are rarely if ever sick. I think there's plenty of anecdotal evidence on that side of the fence too.

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