Germicidal UV-C light in the short wavelength range of 100–280 nm acts on thymine, one of the four base nucleotides in DNA. When a germicidal UV photon is absorbed by a thymine molecule that is adjacent to another thymine within the DNA strand, a covalent bond or dimer between the molecules is created. This thymine dimer prevents enzymes from "reading" the DNA and copying it, thus neutering the microbe.
UV-C light with wavelengths of 100–280 nm has germicidal properties; it interacts with thymine, one of the four base nucleotides in DNA. When a UV photon is absorbed by a thymine molecule adjacent to another within the DNA strand, a covalent bond or dimer forms between them. This prevents enzymes from reading and copying the DNA, effectively neutralizing the microbe.