Bactericidal Effects of Ultraviolet Radiation Produced by Low Pressure Mercury Vapor Lamps

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Abstract
The killing effect of 2537A radiation has been studied, both for bacteria suspended in air and bacteria and molds on surfaces. A 15-watt sterilizing lamp (giving 25 μw/cm of 2537A radiation at 1 meter from the lamp) will kill 97–99 percent of B. Coli suspended in air at a distance of 24 inches in 10 seconds. Accordingly in an air duct a 15-watt lamp can sterilize approximately 200 cu. ft. of air per minute. A 15-watt sterilizing lamp will kill B. Coli on the surface of agar at a distance of 10″ in 20 seconds. The Bunsen Roscoe reciprocity law applies to the killing of bacteria by ultraviolet radiation between the intensities of 2000 μw/cm2 and 1.3 μw/cm2. The lethal dose of 2537A radiation for B. Coli on agar is 6600 μw sec./cm2. For practical purposes the effect of temperature may be neglected. A film of grease is very effective in shielding bacteria from radiation. The lethal dose of 2537A radiation for black mold spores is about 45 times that for B. Coli. The reciprocity law applies. The sensitivity of mold spores decreases with increasing wave-length and is negligible for 3650A. Low pressure lamps in ultraviolet transmitting glass produce concentrations of ozone of the order of 0.1 part per million. From these experiments it may be concluded that low pressure mercury vapor lamps when used under the proper conditions are very powerful bactericidal agents if the bacteria are directly exposed to the radiation, but that the effectiveness is greatly reduced by even such slight shielding as is offered by a film of grease.

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