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The bactericidal effect of sunilight was demonstrated as early as 1878 by Downes and Blunt who reported that culture media exposed to sunlight for several hours remained sterile, wlrhile corresponding portions of the same media which were allowed to stand in shaded portions of the same room became contaminated. Tizzoni and Cattani (1891) and Wesbrook (1896) reported the killing of Clostridium tetani spores by sunlight, while Arloing (1885a, b, c) and Buchner (1892) observed that microorganiisms in general were killed by sunlight. Ward (1892) was onie of the first inivestigators to suggest that a specific region of the solar spectrum was responsible for the lethal effects of sunlight. He found that bactericidal activity was a function of the shorter wave lengths, that is, the blue and violet end of the spectrum, and that this activity remained evident even after screening out the yellow, orange, and red bands with suitable filters. Much recent work in this field has been concerned with the bactericidal effects of the mercury vapor arc lamp emission which is characterized by a highly bactericidal band at 2537 A. This wave length is much shorter than any of the solar radiations reaching the surface of the earth. A few investigators, Buchbinder (1942), Buchbinder et al. (1941), Blum and Mathews (1952), and Hollaender (1943) among them, have reported on the action and effects of natural sunlight on agar and aqueous suspensions of a variety of bacterial species. Whisler (1939), working with various sources of artificial light, all of which emitted strong bactericidal bands of wave lengths shorter than 302 m,u, observed that bacteria suspended in air were killed at rates proportional to the intensity of the light, but that the killing effect was modified by, and dependent upon, the per cent relative humidity of the aerosol. Preliminary work at this laboratory suggested a fairly vigorous bactericidal activity in the solar ultraviolet spectrum (3000 to 4000 A), and the presently discussed series of experiments was designed to furnish some bases for evaluating the effects of sunlight on bacterial aerosols. Inasmuch as the organisms employed for this work, Pasteurella pestis and Pasteurella tularensis, were both highly virulent, cloud studies were carried out in the laboratory where suitable conditions of safety could be maintainied. This restriction necessitated the substitution of artificial simulated sunlight for natural sunlight. While the use of artificial light allowed accurate control of intensity, the compositioii of the light was somewhat different from that of sunilight, especially in the visible portions of the spectrum. However, the use of filters in combination with the light source resulted in a moderately close approximation to natural sunlight, particularly with regard to bactericidal efficiency. Humidity, a determining factor that appears to be closely linked to the bactericidal action of sunlight, was readily controlled.

Response of Air-Borne Species of Pasteurella to Artificial Radiation Simulating Sunlight Under Different Conditions of Relative Humidity