Organic molecules absorb energy primarily in the 200-to-300 nm range, and it is this range that evidences the most disinfectant or germicidal properties. The bacterial disinfection is greatest at 260 nm, although other wavelengths also affect the cells. A given individual microorganism is almost transparent to UV, absorbing only a small fraction of the incident UV irradiance. Thus, germicidal effectiveness is directly proportional to absorbance of the organism, which often peaks around 260 nm accounting for the maximum germicidal effectiveness at that wavelength.
UV light is absorbed by proteins, RNA and DNA in a given microorganism. Absorption of UV by proteins in membranes at high doses ultimately leads to the disruption of the cell membranes and hence death of the cell. However, at much lower UV doses, absorption of UV by DNA can disrupt the ability of the microorganism to replicate, referred to as inactivation. A cell that cannot replicate cannot infect.
Generally, it has been found that the more complex the microorganism, the more sensitive it is to UV inactivation. Thus, viruses are the least sensitive, then bacterial spores and finally bacteria are very sensitive. Until recently protozoa, such as Crytosporidium parvum and Giardia lambia, appeared to go against this trend as it was thought they were very insensitive to UV because of difficulty in penetrating the shell in their cyst or oocyst state. However, recent work has demonstrated that these organisms are in fact quite sensitive to UV.3 This means that UV disinfection can now be safely extended to cover almost all pathogens at very moderate UV doses.