Experimental investigation comparing Far-UVC (222nm) and UVC (254nm) for inactivation of bacteria on hard and fabric surfaces

Bacterial infections are very common and can result from contact with contaminated surfaces in indoor environments. Far-UVC irradiation (222 nm) is now being explored as an emerging solution for disinfecting surfaces. We conducted a comparison of the surface disinfection efficacy between two UVC sources: a krypton-chloride Far-UVC lamp at 222 nm and a traditional 254 nm mercury-type UVC lamp. We evaluated the bactericidal effects of UVC irradiation at ten doses from 0.5 mJ/cm² to 37.6 mJ/cm² for both wavelengths on methicillin-resistant Staphylococcus aureus (MRSA), Klebsiella pneumoniae (K. pneumoniae), and Pseudomonas aeruginosa (P. aeruginosa), on two commonly encountered hard surfaces: plastic and stainless steel, and MRSA was tested for cotton fabric. The results indicated that for hard surfaces two decay stages were identified as the UVC dose increased. In the first stage, the three bacteria exhibited rapid inactivation. However, as the irradiation approached 5 mJ/cm², the decay rate slowed down, indicating the occurrence of a second stage. Our study highlights the importance of cautious calculation when using a specific stage to estimate UVC dose for disinfection purposes. Specifically, we found that achieving 99.9% disinfection efficacy of MRSA on a plastic surface requires an actual Far-UVC dose of 32.80 mJ/cm². In contrast, if only the first stage decay rate constant is considered, the dose would be greatly underestimated (8.368 mJ/cm²). This shows the large discrepancy between these two estimation approaches. The Far-UVC irradiation results on cotton fabric surfaces did not exhibit a second stage, which may relate to the fabric structure.