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Ultraviolet radiation (UVR) from sunlight is only about 5% of the total amount of electromagnetic radiation reaching the Earth's surface; however, it has been shown to be very harmful to the skin. Electromagnetic radiation is classified according to the wavelength (1 nm = 10 -9m), and shorter wavelengths have higher frequencies and higher energy. Of the solar spectrum, the shortest wavelengths reaching the earth's surface are in the ultraviolet region.
The dimensions of the relevant UV wavelengths begin at 290 nm and extend through 400 nm and are classified as UVB (290-320 nm) and UVA (320-400 nm).
Most of the UV radiation reaching the earth's surface is UVA. Radiation between 400-700 nm is classified as visible (light), and above 700 nm is infared (heat). Radiation below 290 nm is UVC and is highly damaging. However, UVC is screened out by the ozone layer of the atmosphere and does not reach the earth's surface.
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