A healthy suntan – or is it?
Why do people have different coloured skins? Is it a sign of inequality? Yes, but only in one single issue. Dark coloured, highly pigmented, people are much more resistant to the ultraviolet radiation emitted by the sun. In fact, too lightly skinned people became extinct in Africa, because they were less fit to survive in those conditions. The process may have taken tens of thousands of years, but it certainly started as soon as the human being lost its body hair typical of the tropical apes. The unsuitability of light-skinned people for areas of burning sunshine is clearly seen in modern-day Australia, where inhabitants of European origin have the highest incidence of skin cancer in the world. Only partial relief is provided by clothing and the present lifestyle of spending about 90% of the time indoors.
By far the most important source of ultraviolet light is the sun. The sun emits a wide spectrum of radiations at different wavelengths; the erythema-causing types of irradiation are mostly (80%) short-wave UV-B irradiation (wavelength 280–320 nanometres) and to some extent (20%) longer wave UV-A irradiation (wavelength 320–400 nm). The most intensive but poorly penetrating UV-C irradiation (less than 280 nm) is absorbed in the atmosphere. The UV-lights used in laboratories to kill germs are especially dangerous to the eyes since they emit UV-C irradiation.
You might ask then why is everybody not black-skinned? If black-skinned humans developed over time in Africa, and some of them moved to other continents, why did they lose their pigmentation within 30,000 to 40,000 years? The reason is that UV-light is needed for vitamin D synthesis in the skin. If the light protection is too good, too little irradiation will penetrate to the cells producing vitamin D in the skin, and the result will be rickets. The curved legs of rickets are no good for hunting big game or running away from predators. Therefore life in different areas confers different requirements on the appropriate skin colour for survival.
The consequences of too much UV light in light skinned people are erythema of the skin and inflammation of sclera and conjunctiva of the eyes. The long term risks are increase in skin cancer and premature aging of the skin. In the eyes, UV light will cause cataract. Pigment formation and thickening of the skin are ways to protect from excessive radiation.
There are increases in both malignant melanoma and skin carcinomas (spinous cell and basal cell cancer). UV radiation from the sun is the most important factor causing skin cancer accounting for as many as 80% of all cases. Cancer risk has been well established both in population studies and in experimental animals. The mechanisms are also fairly well known. The International Agency for Research on Cancer of the World Health Organisation has classified sunlight as an established carcinogenic factor in humans.
All types of skin cancer increase, but the increase of melanoma starting from the 1950s is of the greatest concern. New melanoma cases have increased in all European countries. In the Nordic countries, the increases have been tenfold or more. Population studies clearly implicate the tan-seeking lifestyle. Particularly high risks are caused by single large UV doses to light-coloured skin, causing erythema and sunburns. In addition, some personal properties such as red hair, freckles and certain types of birthmarks on the skin increase the risk. Children are particularly vulnerable, and there is conclusive evidence that suffering sunburn during childhood or adolescence can increase the risk of developing melanoma at a relatively young age. There were more than 21,000 new cases of melanoma in people under the age of 55 years diagnosed in Europe in 2002.
But there are benefits
Since UV light produces vitamin D, avoiding the sun altogether is not healthy either. In addition to the well established effect in preventing rickets, vitamin D may be of importance in combatting some types of cancer such as breast cancer, prostate cancer, and intestinal cancers. Only moderate sunlight exposures are needed for the health benefits and one should start sunbathing slowly and prevent sunburns. During the winter, in northern areas vitamin D supplementation is needed.
Effect of ozone layer
The thinning of stratospheric ozone layer increases especially the amount of UV-B penetrating down to the surface of the globe. It is calculated that UV-irradiation has increased by about 10 to 15%, but it has not been possible to demonstrate any elevation in cancer incidence as yet. In principle, the phasing-out of ozone-depleting substances will benefit children in particular since they have less possibilities of using personal protection.
The UV radiation of the sun has both beneficial and adverse effects, and the adverse effects are quite significant. Therefore sunbathing requires both skill and moderation.
Notes and references
- Nanometre (nm) is 0.000,000,001 metres or 0.000,001 millimetres)
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