The previous chapter discussed the variable influences of hormones on the epidermal melanin unit. The secretion of the pineal hormone, melatonin, is governed by the amount of light reaching the eye and this mechanism enables animals like the weasel and arctic fox to alter their coat colour according to the seasons. In humans light also plays a dominant role in pigmentation; not indirectly through hormones, but by the direct effect of solar ultraviolet radiation (UV) on the epidermal melanin unit. This effect induces the so-called tanning reaction, which can increase the pigmentation of sun-exposed areas markedly above the level of natural pigmentation. The contrast can be striking and, as Noel Coward observed, ‘Sunburn is very becoming – but only when it is even – one must be careful not to look like a mixed grill’!

In this century, and particularly in the Western world, the pursuit of a tan has become a passion, and there are people who will spend hours sunbathing (Fig. 3.1) or in sunbeds and suntan parlours. The achievement of a bronzed appearance is believed to signify health and beauty whereas, in fact, exposure to the sun (particularly in vulnerable, light-skinned Caucasoids) can have the very harmful effects which are discussed below.

Types of ultraviolet radiation (UV)

UV is part of the electromagnetic spectrum and it lies between the visible and X-ray regions (Fig. 3.2). Of the total radiant energy received by the earth from the sun, only 5–10 per cent is in the ultraviolet, the remainder being divided between the visible (about 40 per cent) and the infrared (about 50 per cent). Different wavebands in the UV spectrum show different capacities to cause biological injury.