Quantitative Analysis of Effects of UV Exposure and Spore Cluster Size on Deposition and Inhalation Hazards of<i>Bacillus</i>Spores

Abstract

Reviews of the effects of solar UV radiation on the survival rate of aerosolized biological material found that the current understanding of environmental viability degradation in response scenarios is insufficient to inform appropriate emergency response measures. We evaluated the effects of UV degradation, in terms of the number of viable, culture forming units as a function of spore cluster size on the downwind hazard presented by a release of a biological organism such as Bacillus anthracis into the environment. We used experimentally derived survival rates for B. atrophaeus var. globigii (BG) spores and BG spore clusters (as a surrogate for Bacillus anthracis) of various sizes exposed to UVC fluences to derive predicted survival rates for single spores and spore clusters of up to 10 µm. For the range of weather conditions encountered in hazard estimates, as characterized by Pasquill-Gifford-Turner classes, we calculated and compared the downwind inhalation and deposition hazards for single spores versus spore clusters up to 10 μm diameter based on standard plume dispersion and particle deposition models. These models can be used to predict survival rates for solar exposure taking into account differences in plume depletion due to deposition, and differences in dose–response due to particle size. The combined effects of solar degradation and size-dependent deposition resulted in clusters presenting from a few to up to 10 orders of magnitude greater hazards than single spores depending on meteorological conditions and downwind distance.