Bactericidal Action of Ultraviolet Radiation on Air-Borne Organisms.

In the study of the bactericidal action of ultraviolet radiation on air-borne organisms, it is difficult to determine the number of bacteria per unit volume floating in the air and to measure the lethal radiation to which the organisms were exposed. In studying this action, Fair and Wells (1935) atomized a water suspension of Escherichia coli into a room and measured the normal rate at which the bacteria disappeared by sampling the air at regular intervals using a Wells air centrifuge. The process was then repeated, except that between the first and second samplings the air was irradiated for a short time with ultraviolet light. The lethal action due to the radiation superimposed upon the normal die-away was thus determined. From the results obtained, the conclusion was reached that "In relatively dry air the lethal power of ultraviolet light is from ten to one hundred times greater than is found when organisms are exposed on the surfaces of agar plates or are irradiated in water or other denser media." To permit a better control of air motion thereby making possible more acculrate measurements, Whisler (1936) atomized the bacteria into air drawn through a specially constructed duct. The air-borne bacteria passing along the duct were exposed as they approached an ultraviolet light source. The air was sampled at different places along the duct with an air centrifuge. In analyzing the action of the ultraviolet upon the bacteria it was assumed that "the quantum theory of light applies to the bactericidal action of ultraviolet radiation; that a single photon in the lethal wave band absorbed by a vital part of a bacterium is sufficient to kill it; and that all organisms of one species are identical." Based upon these assumptions and the results of the air samples taken along the duct, Wells (1940) concludes that "an organism suspended in dry air is perhaps twenty times as vulnerable to a given wave length as when suspended in water." Whisler (1936) (1940), Wells and Wells (1936), and Koller (1939) find that air-borne bacteria are about ten times as resistant to radiation at high relative humidity as when floating in air at low humidity. Whisler attributes this to a "physical rather than a biological factor." According to Koller "It seems more reasonable to think that the effect is a physiological one." Theoretically it is important to determine whether there is a greater action of ultraviolet on air-borne bacteria than on the same organisms seeded on agar, and whether there is any effect due to humidity. In the practical use of ultraviolet radiation as a bactericidal agent for air sterilization this information is essential.