Ultraviolet Light Bulb for the Series 400 air purifier by E. L, Foust Co., produces no ozone. It produces ultraviolet light at the specific wavelength of 254nm (nanometers), which is most effective for neutralizing airborne micro-organisms. Recommended to be replaced annually if used 24/7. The bulb may still light up after the specified time, but the wavelength changes and it becomes less effective.