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What is UV light

Close to a century ago, scientists first identified that part of the electromagnetic spectrum responsible for the bactericidal effect of sunlight---Ultraviolet (UV) light.
The full spectrum includes, in order increasing energy, radio waves, infrared, visible light, ultraviolet, x-rays, gamma rays and cosmic rays.
Since UV is not visible, it is technically not "light", but use of the term "ultraviolet light" is so widespread that, it will be used here. Most sources of light generate some UV. For air disinfection, UV is generated by electric lamps that resemble ordinary fluorescent lamps.
Ultraviolet rays have shorter wavelengths than visible light. A wavelength, the distance between the crests of two waves, is often measured in units called nanometers. A nanometer (nm) is a billionth of a meter, or about 1/25,000,000 inch. Wavelengths of visible lights range from about 400 to 700 nm. Ultraviolet wavelengths range from about 1 to 400 nm and are beyond the range of visible light.
UV exposure can be very harmful, or harmless, depending on the type of UV, the type of exposure, the duration of exposure, and individual differences in response to UV. The UV region of the electromagnetic spectrum encompasses a range from 400 nm (nanometers) through 100 nm (1 nm=10-9 m=10) and is further sub-divided into four smaller regions:
UV-A (315 to 400 nm): Long wave UV, also known as "black light ", the major type of UV in sunlight, responsible for skin tanning, generally not harmful, used in medicine to treat certain skin disorders.
UV-B (280 to 315 nm): Medium-wave UV, a small, but dangerous part of sunlight. Most solar UV-B is absorbed by the diminishing atmospheric ozone layer. Prolonged exposure is responsible for some type of skin cancer, skin aging, and cataracts (clouding of the lens of the eye).

UV-C (200 to 280 nm): Also known as "shortwave" UV, includes germicidal (253.7nm wavelength) UV used for air disinfection. Unintentional overexposure causes transient redness and eye irritation, but does NOT cause skin cancer or cataracts.
Vacuum UV (100 to 200 nm)

Why is UV-B harmful while UV-C (germicidal UV) is not? - The difference has to do with the ability of UV rays to penetrate body surfaces. UVC has an extremely low penetrating ability. It is nearly completely absorbed by the outer, dead layer of the skin (stratum corneum) where it does little harm. It does reach the most superficial layer of the eye where overexposure can cause irritation, but it does not penetrate to the top of the lens of the eye and can not cause cataracts. UVC is completely stopped by the ordinary eye glasses and by ordinary clothing.