Extreme Exposure to Filtered Far-UVC: A Case Study†

Abstract

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 1500 mJ cm^-2 ; whereas, skin yellowing that appeared immediately and resolved within 24 h occurred with a 6000 mJ cm^-2 exposure. No erythema was observed at any time point with exposures up to 18 000 mJ cm^-2 . These results combined with Monte Carlo Radiative Transfer computer modeling 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.

Figure 1

Normalized spectral distribution of the filtered far‐UVC source used in the majority of self‐exposures (dashed) and with additional filtering (dot). For comparison, the unfiltered far‐UVC source used in the study by Woods et al. is also plotted (solid). There were no obvious visual changes in the skin observed between the filtered and additionally filtered exposures.

Figure 2

(Left) Right inner forearm following exposure to 12,000 mJ cm⁻² filtered far‐UVC, 5 h post exposure. (Right) Change in irradiation site b* from the CIELAB color space at various radiant exposures. A positive value represents a yellowing of the skin.

Figure 3

Change in irradiation site b* from the CIELAB color space. A positive value represents a yellowing of the skin. Persistent yellowing of the skin is observed even with the additional filtering displayed in Fig. 1. Tape stripping removes the yellow color from the skin.

Figure 4

Results from Monte Carlo radiative transfer (MCRT) modeling of the filtered far‐UVC source (orange). The relative spectral fluence rate at selected depths within the skin is presented. These are compared with the MCRT modeling of Barnard et al. (blue) (15). Unfilt, Unfiltered, Epi, Epidermis.