The warming rays of the sun must have given pleasure to humans throughout the half a million years that they have existed on planet Earth. The importance of sun is not only experienced through warmth but also through regulation of our body clock and production of vitamin D. Therefore, despite Edison’s electric light bulb, it is not surprising that we still find humans enjoying the sun today. Indeed, our need for sun is so strong that it provides the best example for evolution within the human race: fair skin may have evolved to allow adequate vitamin D production from the limited sun exposure in Northern Europe. Unfortunately, as is so often the case, it seems that you can get too much of a good thing.
Sun light reaching the earth’s surface contains visible light (colours) and invisible wavelengths: ultraviolet A and B (UVA and B) and infra red. Dangerous damage to skin cells is caused by ultraviolet light as these wavelengths cause alterations in the genetic material which controls the functioning of all cells (DNA). Most DNA damage is efficiently repaired by the cell but sometimes the damage is too great. The earliest sign of too much sun is well known: sun burn. Sun burn is caused when skin cells are so damaged by the ultraviolet light that they undergo programmed cell death which is a defence mechanism to prevent cells surviving that are no longer working properly. Initially the skin tans within hours by allowing oxidation of the pre-existing brown skin pigment (melanin) and subsequently by producing more melanin. The long term effects of sun are evident in all of us to some degree: wrinkles. Wrinkling of the skin is largely due to chronic lifetime exposure to sun and this is the reason underarms and buttocks usually stay wrinkle free.
For people who have a high sun exposure, the possibility of failure in the body’s ability to repair the DNA damage increases and may lead to skin cancer. The commonest skin cancers are: basal cell carcinoma, squamous cell carcinoma and melanoma. Melanoma is the most serious skin cancer and in severe cases is fatal in more than 50% of people. Melanoma incidence is increasing more rapidly than any other cancer and the peak age at diagnosis is in 50-60 year olds (Cancer Research UK, 2008). Risk of developing skin cancer is determined by a number of factors particularly their sensitivity to the sun. Although blonde/red hair, blue eyes and freckles are all factors that are associated with increased sensitivity, these attributes are not completely reliable (eg dark skinned Athenians with blue eyes). Most people will know if they go brown easily or if they burn easily. Those who burn easily are at higher risk than others. However, we are seeing increasing amounts of sun damage (including skin cancer) in all white skinned people. It is particularly important not to let children get burnt as this increases their risk of skin cancer later in life. If there is a family history of skin cancer or if immunosuppressive treatments are being taken, further caution in the sun is necessary. Unfortunately UV damage builds up, so people usually develop skin cancer years after sun exposure as a young person.
To ensure the skin is protected everyone should take care in the sun to limit overall UV exposure and avoid burning. This is just as important in later life as our ability to repair sun damage is less effective as we get older. The best approach is sun avoidance (shade, broad brimmed hats, long sleeves etc) with
sunscreen (>SPF25, UVA 4-5*) as a back up. When choosing a sunscreen it is important to think about when and where it will be used. Cosmetics with sunscreens in are great for day to day, but tend not to last long on the skin, so are ideal for the lunch break if applied midmorning (but not at 7am!). Usually
sunscreens are adequate but supplements containing fish oils (Cancer Detection and Prevention, 2006) have been shown to limit the damage caused by UV exposure.
Although our primitive instincts highlight our biological need for sunlight the risks of skin cancer later in life should not be forgotten.
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