Skin safety of 233 nm far UV-C ex vivo and in vivo – Pilot study for evaluating different populations and multiple exposures

Nosocomial infections remain a major healthcare challenge, underlining the demand for antimicrobial technologies. Far UV-C (200-235 nm) has emerged as a safer alternative to traditional 254 nm UV-C for microbial reduction on skin and wounds but also in occupied spaces due to its strong germicidal properties and minimal skin penetration. However, studies on humans remain limited. This study aimed at evaluating the skin safety of 233 nm UV-C for potential applications in antisepsis and public area decontamination. Ex vivo experiments first assessed age-dependent DNA damage before proceeding to in vivo studies. Healthy volunteers of varying skin types and ages underwent single exposures to evaluate the effects of age and pigmentation. Additionally, young and light-skinned volunteers received multiple exposures. Biopsies were collected to assess DNA damage and repair. A biocidal dose of 233 nm induced superficial DNA damage, showing lower damage than that induced by suberythemal UV-B exposure, a recognized safe dose for human skin. Older and dark-skinned participants exhibited higher residual DNA damage 24 h-post-exposure compared to younger and lighter-skinned individuals, though nearly complete repair was observed after seven days. Repeated exposures, up to cumulative doses of 120 mJ/cm² and 240 mJ/cm², did not induce significant changes on skin pigmentation, antioxidant activity, or immune response. However, DNA damage accumulation suggested limitations in repair mechanisms over 24 h. These results confirm the safety of a single exposure to 233 nm, but highlight potential risks with repeated applications, emphasizing the need for careful dose regulation and further in vivo studies, including well-established treatments at 222 nm.