Ultraviolet light (UV) with wavelength shorter than 300 nanometers is extremely effective in killing microorganisms. The most effective sterilizing range of UV is within the C bandwidth (UVC - 253.7nm). This range is called germicidal UV bandwidth or UVC. Germicidal UV has been used in water disinfection systems for many years. As stated by the American Water Works Association, "... UV light disinfection process does not use chemicals. Microorganisms, including bacteria, viruses, and algae, are inactivated within seconds of UV light disinfection... UV light is effective in inactivating Cryptosporidium, while at the same time decreasing chlorinated disinfection by-products... " Germicidal ultraviolet (UVC) light kills cells by damaging their DNA. The light initiates a reaction between two molecules of thymine, one of the bases that make up DNA. UV light at this wavelength (shortwave UV or UVC) causes adjacent thymine molecules on DNA to dimerize. The resulting thymine dimer is very stable. If enough of these defects accumulate on a microorganism's DNA its replication is inhibited, thereby rendering it harmless.