The ultraviolet (UV) rays have wavelengths ranging from about 10 nm to about 400 nm. They are that portion of the electromagnetic spectrum immediately below the visible range and extending into the X-ray frequencies. UV radiation is of three types according to wavelength: ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC).
UVA is an ultraviolet radiation with wavelengths ranging from about 315 to 400 nm.1 UVA rays are not absorbed by the ozone layer. Thus, they reach the surface of the earth. UVA rays account for about the 95% UV radiation that reach the earth's surface. Compared with UVB and UVC rays, UVA rays have the longest wavelengths and are able to penetrate the deeper layers of the skin.
Exposure to UVA rays is associated with premature ageing, wrinkling, and sun spot formation. It causes skin tanning but is very weakly sunburn-producing and carcinogenic. The amount of UV rays that reach the earth is highest between 10 o'clock in the morning and 3 o'clock in the afternoon. Thus, it is recommended to wear proper protection against sunlight during those hours especially when the skies are clear.
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1 Space Environment Technologies. (n.d.) "ISO 21348 Definitions of Solar Irradiance Spectral Categories" (PDF)". Retrieved from [].