Extreme exposure to filtered far-UVC: a case study

Far-UVC devices are being commercially sold as "safe for humans" for the inactivation of SARS-CoV-2, without supporting human safety data. We felt there was a need for rapid proof-of-concept human self-exposure, to inform future controlled research and promote informed discussion. A Fitzpatrick Skin Type II individual exposed their inner forearms to large radiant exposures from a filtered Krypton-Chloride (KrCl) far-UVC system (SafeZoneUVC, Ushio Inc., Tokyo, Japan) with peak emission at 222 nm. No visible skin changes were observed at 1,500 mJcm^-2, whereas skin yellowing that appeared immediately and resolved within 24 hours occurred with a 6,000 mJcm^-2 exposure. No erythema was observed at any time point with exposures up to 18,000 mJcm^-2. These results combined with Monte Carlo Radiative Transfer computer modelling suggest that filtering longer ultraviolet wavelengths is critical for the human skin safety of far-UVC devices. This work also contributes to growing arguments for the exploration of exposure limit expansion, which would subsequently enable faster inactivation of viruses.