Causes of sunburn

Many mechanisms and factors play role in causing sunburn. The most important factor in causation of sunburn after acute exposure to direct sunlight is the degree of melanin pigmentation (the number of melanocytes in skin) in skin. The darker the skin, the more is the number of melanocytes and the lesser effect of sun on the skin. The reason why blacks do not get sunburn or have any effect (if any effect also very minor and insignificant) of sun is presence of very large number of melanocytes with large amount of melanin pigment.

The UV spectrum of sunlight:

Ultra-violet (UV) spectrum of sunlight is the main culprit in causation of sunburn, although other wavelengths may also have some minor effects. The UV spectrum reaching the earth is arbitrarily divided into two major segments, comprising the wavelengths from 290–400 nm into UV-B (Ultra-violet-B) and UV-A (Ultra-violet). UV-B consists of wavelengths between 290 and 320 nm and UV-A represents those wavelengths between 320 and 400 nm. Wavelengths between 290 and 320 nm is the most efficient in producing redness or erythema in human skin and hence is sometimes known as the “sunburn spectrum”.

Photons in the UV-B are more than 100 times (fold) more effective in producing redness of sunburn than photons of UV-A. But UV-A may contribute to sunburn redness at midday when much more UV-A is present in the solar spectrum than UV-B.

Inflammation as cause of sunburn:

Sunburn is due to dilatation of the blood vessels of dermis (the inner layer of skin). There is a time lag of approximately 4 to 12 hours time in between skin exposure to sunlight and the development of visible redness, which suggest there is delayed production and/or release of vasoactive mediator or inflammatory mediators, or cytokines, that diffuse to the dermal vasculature to evoke dilatation of blood vessels. NSAIDs (nonsteroidal anti-inflammatory drugs) like Ibuprofen can reduce sunburn erythema, which proves that inflammation plays important role in causation of sunburn.

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