Modeling ultraviolet dose-response of Bacillus spore clusters in air

This article presents a new way to model the narrow band ultraviolet (UV-C, 254.7 nm) radiation dose-response behavior of Bacillus spores in clusters based on knowledge of the UV-C dose-response behavior of the single spores. The approach is demonstrated using experimentally obtained survival rates for Bacillus atrophaeus var. globigii (BG) spores and BG spore clusters of several sizes exposed to UV-C fluence when aerosolized and when deposited on dry surfaces. These results are modeled to derive predicted survival rates for spores in clusters of arbitrary cluster diameter under similar conditions. The essential feature of the approach is accurate accounting for attenuation of incident UV fluence within the spore cluster to derive the fluence incident on individual spores within the cluster. The results suggest that UV fluence attenuation by bacterial spores may increase with increasing fluence, a phenomenon that has not been previously reported for bacterial spores. The results are of utility in estimating dispersed biological hazards and evaluating the effectiveness of ultraviolet germicidal irradiation (UVGI) systems.