Sunburn, tanning and other changes
SUNBURN
Yhe absorption of UV radiation by skin chromophores, particularly DNA, and the consequent damage, lead to the cascade of events known as sunburn, through which the body largely repairs the molecular defects induced. It is thought that the DNA damage itself provides the trigger for this process, inducing the release of special chemicals that bring about the typical pain, warmth, redness and swelling, which we all experience some hours after too much sun. The visible aspects are, however, largely the result of blood vessel swelling. In fact, sunburn can vary from a mild tingling and pinkness to severe blistering and loss of skin; in the latter case, you may also feel generally unwell, sometimes severely.
UVB is many times more effective than UVA at causing DNA damage, mostly in the epidermis, and is therefore also much more likely to set off the sunburn response. Further, the greater the UV radiation exposure, the worse the sunburn, but just how much damage is done depends on your skin. Thus, people with darker skins take longer to burn than those who are fair (see below), although everybody is susceptible to some extent.
SKIN TANNING AND THICKENING
Skin tanning
What we see as a suntan is the colour produced by brown UV radiation-absorbing melanin in the epidermis; this is manufactured and passed into the epidermal keratinocytes by melanocytes of the epidermal basal layer within hours of UV damage to their DNA. Such melanin is partly transferred to the surrounding basal keratinocytes, clumping protectively above their important cell nuclei, and partly to the keratinocytes above, which then move upwards as usual to be shed. Tanning in this way provides some two to four times greater protection than before against subsequent UV exposure, until the tan fades as the superficial cells are discarded over the ensuing weeks. Nevertheless, the tan is always the result of UV radiation-induced DNA damage and is maintained only as long as such damage continues to occur, leaving a permanent long-term legacy.
Clearly, therefore, there is no such thing as a safe tan other than a fake one or the one that we are born with. Thus, genetically black skin provides around 10 to 20 times better protection against the sun than white, and brown skin about five times better.
Skin thickening
Our skin also becomes thicker within days of sun exposure, and this so-called hyperplasia may last for weeks to months as well. Again this appears to be a response to UV radiation-induced DNA injury, this time in basal layer keratinocytes. Within a day or two, once DNA repair has taken place, these cells begin to divide and multiply very rapidly, resulting in a severalfold thickening of the epidermis, particularly of the protective stratum corneum, while the dermis also thickens a little. These extra layers then provide five to ten times better protection against future effects of the sun, especially for the vulnerable basal epidermal layer. Such thickening alone is usually more effective than the accompanying tan, especially in fair-skinned people. The combined effects of these can give up to 10 to 40 times the protection, albeit always at the expense of permanent skin damage.
WHO IS AT RISK OF SUNBURN?
Fair-skinned individuals, especially those with freckles and red hair, burn particularly easily in the sun, because of their relative genetic lack of melanin, and some may not tan at all, whereas darker-skinned people do so with ease. How well a tan develops is thus built in from birth and depends on our individual arbitrarily defined so-called skin types (see box). People with types I, II and III tend to burn easily and tan poorly, being at greatest risk from the adverse long-term skin effects of sunlight – namely photoageing and cancer; those with types IV, V and VI are relatively protected, except for a tendency to photo-ageing. Your skin type can be readily determined from the way you skin responds to the first significant exposure to midday summer sun each year.
