Impact of inoculum composition on bacteria and bacteriophage UVC inactivation at 222 nm and 254 nm

Using technologies that emit ultraviolet-C (UVC) radiation to inactivate pathogens has gained increasing interest as a strategy for reducing the risk of disease transmission, but it can be difficult to predict their performance in applied settings. Directly irradiating occupied spaces with emerging far-UVC has been proposed, as its shorter wavelength (200-230 nm) is reportedly less damaging to human eyes and skin than longer wavelengths. In this study, we conducted surface tests to evaluate the impact of different inoculum compositions (with and without soil loads) and conditions (wet vs. dried droplets) to determine how these factors affect the efficacy of UVC sources with peak emissions at 254 and 222 nm against five bacteria and two bacteriophages. The presence of a soil load reduced the efficacy of 222-nm UVC more than that at 254 nm, and both UVC sources were generally less effective against microbes in dried (vs. wet) inoculum. Given that pathogens will often be exposed to UVC in the presence of proteins, salts, and other constituents depending on how they are emitted, it is important to consider these challenges when quantifying efficacy.