David Stephen, of King's High School, asks :-

How does ultraviolet radiation cause skin cancer?

Mark Elwood, of the Hugh Adam Cancer Unit at the Otago Medical School, responded.

Ultraviolet radiation is part of the solar spectrum of electromagnetic radiation, which ranges from very short wavelength radiation such as cosmic rays and x-rays up to very long wavelength radiation such as radio waves. In general, the shorter the wavelength the more the radiation affects biological organisms, and particularly DNA in biological systems. Thus visible light and longer wavelength radiation such as radio waves have no important effects, whereas x-rays and cosmic rays can cause DNA damage in tissues exposed to them. One of the consequences of such DNA damage is the production of cancer.

Ultraviolet radiation is of shorter wavelength than visible light and can produce such damage. Indeed, within the ultraviolet wavelength spectrum, the shortest type, sometimes called UVC, is quite damaging in this regard and is used as a method of killing micro-organisms and sterilising objects. There is no UVC received naturally at ground level because it is all absorbed by upper atmospheric ozone. The next wavelength band, UVB, does in part get down to ground level, and it is primarily this type of radiation which is increased by depletion of atmospheric ozone, and which varies greatly with time of the day, season and so on. Ultraviolet B is the main cause of sunburn. The radiation acts as a direct carcinogen, with the potential of causing DNA damage of various forms, which can upset the normal regulatory systems of cell growth and produce cancerous tissue. The next longer wavelength band UVA does not show such effects as strongly, and whether it is relevant to the production of cancer is still in dispute. The situation is a little more complicated because as well as acting as a primary biological damage agent and producing cancer in that way, there is evidence that ultraviolet radiation can act at later stages of cancer development as a promoting agent, and may also have some effect in diminishing some immunological responses which could also remove protection against the development of cancers.

There are two major types of cancer produced, one type which arises from the ordinary epithelial cells of the skin, and the other which arises from the pigment cells of the skin, the melanocytes. The latter type, melanoma, is a much more severe cancer because it can spread throughout the body easily, although it is much less common than the epithelial type of skin cancer. One of the most intriguing facts which is still being explored by current research is that it is fairly clear that the way in which ultraviolet radiation causes these two different types of skin cancer is in fact different. Epithelial types of skin cancer seem likely to be related to the total lifetime dose of ultraviolet radiation which has been received, and are therefore most common in exposed skin in elderly people. However the production of melanoma seems related more to severe acute episodes of extensive ultraviolet radiation, such as those which accompany sunburn, which may explain why melanoma occurs at a more varied body site distribution in younger people. There is even some evidence that long continued regular exposure to ultraviolet radiation through sunshine may in fact protect against melanoma, because the protective skin thickening and tan produced is sufficient to overcome the carcinogenic action.