UV-C light waves are composed of photons, or particles of light. They are the most energetic of the optical electromagnetic spectrum (composed of UV, visible, and infrared light) and are, therefore, the most photochemically active.2 The peak germicidal effects of UV-C irradiation occur at wavelengths of 260-265 nm. UV-C light generated by low vapor pressure mercury lamps exhibit peak germicidal efficacy at 254 nm.3
How is UV-C light germicidal?
UV-C photons cause irreversible damage to the DNA and RNA of many microorganisms. When UV-C light is applied to an area, it is absorbed into the DNA/RNA of the microorganism disrupting the replication process and preventing growth and multiplication. As bacteria die off there will not be new ones to continue the cycle.
So what does this mean on a practical level? Well, that using UV-C light will inactivate microorganisms stopping the replication cycle and ensuring higher levels of disinfection.

- https://www.sciencefocus.com/science/how-much-of-the-electromagnetic-spectrum-does-the-sun-emit/
- https://www.ies.org/standards/committee-reports/ies-committee-report-cr-2-20-faqs/
- Ultraviolet Germicidal Irradiation Handbook