UV Dose-Response Behavior of Air-Exposed Microorganisms

Laboratory experiments were performed to demonstrate an experimental method for measuring the UV dose-response behavior among air-exposed microorganisms, and to quantify the effect of relative humidity on this behavior. These experiments included two nonpathogenic microorganisms as surrogates for airborne microbial pathogens: Bacillus subtilis spores and coliphage MS2. UV dose-response behavior of these microorganisms was measured as a function of relative humidity (RH). Microorganisms were transferred to membrane filters at less than monolayer coverage and were subjected to exposure to UV radiation delivered from a collimated beam to a planar surface in a humidity-controlled environment at two germicidally active wavelengths: 254 nm and 282 nm. Dose-response data for Bacillus subtilis spores and coliphage MS2 exposed to UV radiation were fit to the Phenotypic Persistence and External Shielding (PPES) model. B. subtilis spores showed a modest effect of RH on the UV dose-response behavior, whereas essentially no effect of RH was evident for air-exposed MS2 for their UV dose-response behavior at 254 nm or 282 nm.