Photoprotection

The two major defences of the skin against radiation injury are the presence of melanin pigment and the thickness of the stratum corneum. There was a time when the role of melanin in photoprotection was subordinated to that of the stratum corneum but, in the recent past, the burden of evidence has declared melanin to be the natural sunscreen par excellence. The arguments in favour of the superior photoprotective properties of melanin have been convincingly set out by Pathak & Fitzpatrick (1974) and they are based on clinical, epidemiological and experimental findings.

Skin cancer

The most obvious clue to the photoprotective role of melanin resides in the prevalence of skin cancer, which is by far the commonest of the cancers. As already noted, it is associated with intense and long-term exposure to UV-B and it therefore occurs more frequently on the chronically exposed body areas, such as the head and neck (see Fig. 9.4). There is a relative infrequency of skin cancer in Negroids and other pigmented peoples (Amerindians, Asians), even at the equator where UV is strongest. Susceptibility to skin cancer (including malignant melanoma) is enhanced in fair-skinned, light-haired Caucasoids who sunburn easily and tan poorly.

Persons of Celtic background appear to be significantly over-represented in the skin cancer statistics (Urbach, 1969). The Republic of Ireland has the third-highest death rate from skin cancer (next to Australia and South Africa), even though it is located between 52° N and 54° N and receives a relatively low annual influx of UV-B. It may be that the individual with Celtic skin and red hair has a genetic inability to resist the deleterious effects of UV.