UV radiation is generally divided into the areas of UVA, UVB, UVC, and vacuum UVC. UV radiation has a shorter wavelength and more energy than visible light. While UVA and UVB from the solar spectrum partially penetrate the earth’s atmosphere, UVC (100-300nm) is filtered out completely in the stratosphere. That is why life, which uses a UVC unstable molecule (DNA) for coding, was able to develop on Earth.

In general, all forms of life on Earth are organized based on DNA or RNA which is similarly constructed. This means any organism can be damaged by UVC radiation. The simpler the form of organization and the smaller the cell volume of the various organisms, the more effective is the kill rate through UVC. The dose at which 90% of the organisms of a species is killed, it is called LD90 dose. To kill simple bacteria generally requires between 1.5 and 6 mJ/cm2, for larger yeasts with nucleus, between 6-10 mJ/cm2 and for fungal spores that are made for survival in extreme environments it may take even 20 to a maximum of 120 mJ/cm2 to kill 90%.

Within the groups of organisms there are also differences because of dyes in the cell walls and the type of DNA packaging. Extensive tables and literature exist listing the LD90 doses of different species for the calculation of disinfection doses. The presentation and the density of the microorganisms is also important for the design of disinfection systems, because shading, reflection, and dispersion affect the dose with which the UVC radiation can reach the DNA. Another important factor for the required LD90 dose, and thus the success of the UVC disinfection is the cell cycle of the organisms. Cells with high protein synthesis, as is necessary during the cell division, are damaged significantly faster because the DNA is extracted and thus easier to reach for the UVC radiation.