Introduction

Certain types of ultraviolet (UV) light are strongly carcinogenic. Yet complete avoidance of exposure is almost impossible. Further, ultraviolet light is indispensable, permitting the skin to synthesize vitamin D, complementing that derived from dietary sources.

Spectrum of ultraviolet light

The UV portion of the electromagnetic spectrum occupies the wavelengths between 200 nm and 400 nm. It is customary to distinguish between UV-C (200–280 nm), UV-B (280–320 nm) and UV-A (320–400 nm), each having different effects on the skin and eye. The shorter the wavelength, the more destructive the radiation.

Ultraviolet light below 295 nm is totally absorbed by the oxygen and ozone of the atmosphere, that between 295 and 329 nm being partially absorbed. The more oblique the sun's rays, the greater thickness of atmosphere which has to be traversed and hence the shorter rays are diffused or absorbed. Considerable amounts of ultraviolet light may be reflected, thus the combination of snow and blue sky may expose the skin to as much UV-B (310 nm) as comes from the sky directly.

Effects

The amount of UV light reaching the melanocyte varies by wavelength. UV-A penetrates into the dermis, between 1 to 10% reaching the basal layer, a depth practically never reached by UV-C. Heavy skin pigmentation is, of course, protective. Sunburn is maximal about 24 hours after exposure to UV-B, occurring more rapidly than for UV-A.