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Anne Rammelsberg, a chemistry professor at Millikin University, offers this explanation:

Germicidal ultraviolet (UVC) light kills cells by damaging their DNA. The light initiates a reaction between two molecules of thymine, one of the bases that make up DNA. The resulting thymine dimer is very stable, but repair of this kind of DNA damage--usually by excising or removing the two bases and filling in the gaps with new nucleotides--is fairly efficient. Even so, it breaks down when the damage is extensive.
The longer the exposure to UVC light, the more thymine dimers are formed in the DNA and the greater the risk of an incorrect repair or a "missed" dimer. If cellular processes are disrupted because of an incorrect repair or remaining damage, the cell cannot carry out its normal functions. If the damage is extensive and widespread, the cell will die.

SPECTRUM of light ranges from the infrared at wavelengths longer than visible light to the ultraviolet at wavelengths shorter than visible light.

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