what does a UV bulb do for a humidifer

UV Germ-Free Humidifiers use an ultraviolet (UV) light bulb which shines on the water inside the humidifier and has been proven in independent laboratory testing to kill up to 99.9% of bacteria, germs, and viruses present in the water. The UV bulb in Germ-Free humidifiers adds an extra level of germ-free protection and sanitization to your humidifier. While previously only available for warm mist humidifiers, UV Germ-Free technology has recently also become available for some cool mist humidifiers.