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Sterilizing radiation produced by ultraviolet light

Ultraviolet radiation with a wavelength less than 320 nm can kill microorganisms. Radiation with a wavelength around 260 nm has the highest bactericidal efficacy, and its bactericidal effect varies to some extent with the type of microorganism. Low-pressure mercury vapor discharges produce intense radiation at a wavelength of 253.7 nm, which is close to the wavelength of maximum bactericidal effect. Therefore, this type of light source is often used for sterilization. A germicidal irradiance unit (GU) is sometimes used. This unit represents the amount of irradiance that produces a bactericidal effect equivalent to 1 watt/square meter of 253.7 nm radiation.

Except for very low irradiances, under constant radiation intensity, the number of surviving microorganisms follows an exponential decay law, meaning that the number of surviving microorganisms decreases by the same factor with each successive unit of time. For example, after 15 seconds of exposure to 1 GU of radiation, the survival rate of a particular type of Escherichia coli is 1%. After another 15 seconds, the survival rate is reduced to 0.01%, and so on. Different microorganisms vary greatly in their sensitivity to germicidal radiation, and the bactericidal effect is also affected by environmental conditions (such as humidity).

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