How does UVC disinfection work?

Short-wave UVC radiation in the 260-nanometer range has a strongly germicidal effect. At this wavelength, there is a high photo-chemical reaction in the exposed cell. When two thymines inside the DNA lie alongside to each other, they react and dimerice. In cases of repeated damage, the cell is no longer able to regenerate the damaged generic information. The cell loses the capability of splitting and multiplication and gets inactivated.  

Technically, the required wavelength can be produced with mercury vapour discharge lamps. The mercury low-pressure discharge is the most common and most economic technical implementation to generate germicidal UVC rays at the present time.