Human ocular-discomfort-free thresholds for 222 nm Far-UVC irradiation: A prospective interventional study

The purpose of this study is to examine irradiation conditions of 222 nm Far-UVC that do not cause ocular discomfort to participants, considering both the irradiation intensity (ODF-IT; ocular-discomfort-free irradiance threshold) and irradiation dose (ODF-DT; ocular-discomfort-free dose threshold). A prospective, single-blind interventional study was conducted with 36 healthy adults exposed to 222 nm Far-UVC under five different irradiation conditions varying in intensity and total dose, along with a visible-light control condition. The irradiation device, placed under the TV monitor, delivered 222 nm Far-UVC or visible light to participants viewing the video. Ocular discomforts were recorded at multiple time points up to 24 h. Ophthalmic examinations-including slit lamp assessment, corneal fluorescein staining, and corneal endothelial cell density-were performed before and after irradiation. No adverse changes were observed in spherical equivalent refractive error, or corneal endothelial cell density under any condition. Corneal fluorescein staining occurred in some cases but showed no consistent correlation with ocular discomfort. Discomfort was significantly associated with radiant exposure dose, video viewing time and time after video viewing. The ODF-IT and ODF-DT values of 222 nm Far-UVC determined by the present study were around ≤1.0 μW/cm² and approximately 5.5 mJ/cm² respectively.