Influence of relative humidity on particle size and UV sensitivity of Serratia marcescens and Mycobacterium bovis BCG aerosols

Setting: A study of Serratia marcescens and BCG aerosols. Objective, To evaluate the effect of relative humidity (RH) on (1) the particle size and (2) sensitivity of 254 nm germicidal ultraviolet (UV) irradiation. Methods: We built a RH controlled experimental chamber into which bacteria were aerosolized, exposed to varying amounts of UV irradiance over measured time periods, and quantitatively evaluated for viability. Aerosolized Serratia marcescens and bacille Calmette-Guerin (BCG) were subject to UV doses ranging from 57-829 muW.sec/cm(2), and sampled with a six-stage Andersen culture plate impactor at RHs ranging from 25-95%. Results: Percent survival for both organisms was inversely related to UV dose. Serratia marcescens was more susceptible to UV than BCG under all conditions. More than 95% of the bacterial aerosol particles were 1.1-4.7 mum in aerodynamic diameter, and particles sizes increased from low (25-36%) to high (85-95%) RH. The count median diameter ranged from 1.9-2.6 mum for Serratia marcescens and from 2.2-2.7 mum for BCG as RH increased. for both Serratia marcescens and BCG, resistance to UV increased as RH increased. The UV resistance of both Serratia marcescens and BCG aerosols dramatically increased at RH higher than 85%. Conclusions: Our results indicate that differences in UV dose, kinds of microorganisms, airborne particle size and RH affect UV susceptibility. (C) 2000 Harcourt Publishers Ltd.