The survival of bacteria in dust: III. The effect of light on the survival of bacteria in dust.

Sterilization of air by ultra-violet radiation is effective when the organisms are dispersed in droplet nuclei, the smallest particles being the most vulnerable (Wells & Wells, 1936; Bourdillon & Lidwell, 1948). Airborne bacteria, however, are also carried on dust particles shaken from textiles or swept from the floor, and against these ultra-violet radiation at intensities higher than those normally used for sterilizing air is found to be ineffective (see Dumbell, Lovelock & Lowbury, 1948). It does not follow, however, that continued ultra-violet irradiation is without effect against the bacteria of settled dust, which may persist in a room for long periods (Lowbury, 1950). Garrod (1944) has shown that the floor dust of infectious wards receiving normal daylight contains fewer streptococci, even in winter, than dust in similar wards which are poorly daylit. Hollaender, du Buy, Ingraham & Wheeler (1944) have advocated ultra-violet irradiation of floors, which they claim reduces the numbers of bacteria in the air significantly more than ceiling irradiation. The sterilizing action of daylight, originally described by Dowries & Blunt (1877), has been quantitatively studied on pure cultures by Buchbinder, Solowey & Phelps (1941). Buchbinder (1942) also reports germicidal action by fluorescent lamps equal per unit of illumination to that of daylight. The purpose of our experiments, described in this paper, was to measure the effect of daylight, fluorescent and tungstenlighting, and an intensity of ultraviolet radiation comparable with that reflected from irradiated ceilings, on the death-rate of bacteria in dust. Limited studies on the effect of varying atmospheric humidity on the lethal action of ultra-violet radiation are also described.