IMAGE No. 10/17
High wavelengths – from 760 to over 5,000 nm (IR) - may penetrate into the skin beyond the dermis, account for a considerable share of sunlight (56%), and have no tanning power. In medicine, infrared rays are used to treat several painful symptoms, to speed up scarring processes, and to manage skin circulation disorders. Lower wavelengths – from 310 to 400 nm (UVA) - penetrate down to the dermis, account for 4.8% of sunlight, are not visible, and produce a bluish light, perceived by human eyes, through secondary emanation. UVA rays cause a quick but transitional increase in skin colour by increasing the quantity of the existing melanin, however without affecting melanocytes; they are therefore ineffective for the treatment of vitiligo. They are used (and abused) in cosmetology to achieve artificial tanning: by penetrating down to the dermis, they attack fibres and are therefore responsible for skin aging (loss of skin elasticity, onset of wrinkles).

Very short wavelengths – from 200 to 260 nm (UVC) – are almost unable to penetrate into the skin, account for a negligible share of sunlight, and have no therapeutic interest.

Medium-low wavelengths – from 261 to 310 nm (UVB) – can only penetrate 2-3 hundredths of millimetre into the skin; such value corresponds to the mean thickness of the epidermis, which is thus never crossed through. They are contained in sunlight in quantities ranging from 0.02% in industrialized regions to 0.2%. Compared to UVA rays, they provide a late but permanent photobiological effect, represented by the onset of an erythema (red skin) followed by the production of new melanin as a result of the stimulating action performed on melanocytes; at skin level, this action translates into intense and long-lasting pigmentation.