| Increased interest in ultraviolet radiation as a selective ecological pressure experienced by atmospheric microorganisms and as a control against infectious bacteria suspended in indoor air, necessitates a clearer understanding of the environmental effects of relative humidity (RH) and light-activated cell repair mechanisms (termed "photoreactivation") on the UV-induced inactivation of bacteria and their spores. The response of four aerosolized bacterial pure cultures: Serratia marcescens, Bacillus subtilis vegetative cells and spores, Mycobacterium parafortuitum, and Mycobacterium bovis BCG was observed under ultraviolet germicidal irradiation (UVGI). In conjunction with culturability measurements, and a direct epifluorescent assay adapted for bioaerosol, a monoclonal antibody assay was developed to measure intercellular UV damage incorporated into the genome of airborne cells. Bench-scale experiments to determine the effects of relative humidity and photoreactivating light on the UV response of airborne cells were executed in a newly developed 0.8 m3 bioaerosol (UV-exposure) reactor. Experiments to test the feasibility and performance of full-scale UVGI systems were performed in a 90 m3 room with controlled ventilation, temperature, and RH.UV-induced inactivation rates of airborne bacteria decreased with increasing relative humidity---the magnitude of this decrease was species dependent. Aerosol reactor experiments, performed in artificial sunlight, confirmed that photoreactivation can occur to a significant degree in airborne bacteria and that the reactivation rate was maximized at high relative humidity (>80%). For the same UV dose, the amount of DNA damage incorporated into the aerosolized S. marcescens and M. parafortuitum genome increased significantly with increasing RH. These results support the hypothesis that intercellular DNA conformation, dictated by the RH level to which it is exposed, is responsible for the type and extent of UV-induced photochemical damage an airborne cell experiences. The RH influence on DNA conformation, and thus subsequent genetic damage is, in part, responsible for the greater UV-induced inactivation rates observed at lower RH (20%--50%), and the higher photoreactivation rates observed at high RH levels (>80%). Full-scale results were complimentary to bench-scale observations and suggest that the application of UV irradiation in the upper level of rooms may decrease airborne concentrations of viable bacteria by 90% under well-mixed conditions. |
