Question:
With the improving weather it looks like we might finally be in danger of developing a sun tan this year. Seems like a good time to ask whether anyone's been able to figure out one or two things that still aren't clear to me about tanning.
I've read loads of leaflets and watched TV programmes about exposure
to the sun - how you're supposed to swathe yourself in clothes and
hats, use a high factor sun tan lotion, avoid the midday sun etc., but
none of it seems to answer these questions:
Is there a significant danger of getting skin cancer (or
'melanoma' or whatever) if you *never* burn? In other words, is it
short-term over-exposure that is the danger, when your skin burns and
peels, or is it simply the amount of exposure, regardless of whether
you burn or not?
Answer:
Ultraviolet (UV) radiation in sunlight is the most prominent and ubiquitous
physical carcinogen in our natural environment. It is highly genotoxic but does not penetrate the body any deeper than the skin. Like all organisms regularly exposed to sunlight, the human skin is extremely well adapted to continuous UV stress. Well-pigmented skin is clearly better protected than white Caucasian skin. The sun-seeking habits of white Caucasians in developed countries are likely to have contributed strongly to the increase in skin cancer observed over the last century. Skin cancer is by far the most common type of cancer in the U.S.A. and Australia, which appears to be the result of an 'unnatural displacement' of people with sun-sensitive skin to sub-tropical regions. Although campaigns have been successful in
informing people about the risks of sun exposure, general attitudes and
behaviour do not yet appear to have changed to the extent that trends in skin cancer morbidity and the corresponding burden on public healthcare will be reversed. The relationship between skin cancer and regular sun exposure was suspected by physicians in the late 19th century, and subsequently substantiated in animal experiments in the early part of the 20th century.
UV radiation was found to be highly genotoxic, and DNA repair proved to be
crucial in fending off detrimental effects such as mutagenesis and cell
death. In fact, around 1940 it was shown that the wavelength dependence of mutagenicity paralleled the UV absorption by DNA. In the 1970s research on UV carcinogenesis received a new impetus from the arising concern about a possible future depletion of the stratospheric ozone layer: the resulting increases in ambient UV loads were expected to raise skin cancer incidences.
Epidemiological studies in the last decades of the 20th century have greatly refined our knowledge on the aetiology of skin cancers. Analyses of gene mutations in skin carcinomas have identified UV radiation as the cause. The relationship between the most fatal skin cancer, i.e. malignant melanoma and solar UV exposure is, however, still unclear and needs to be clarified to optimise preventive measures and minimise mortality from skin cancers.
