Ozone and ultra-fine particle concentrations in a hotel quarantine facility during 222 nm far-UVC air disinfection

Far-UVC (222 nm UV-C light) is a promising tool to mitigate aerosol transmission of pathogens indoors. However, recent studies have raised concerns related to ozone (O3) production and secondary chemistry. In this study, we measured indoor O3 and ultra-fine particle (UFP, 17.5-289 nm) concentrations with and without 222 nm far-UVC (average fluence rate 1.7-1.8 {micro}W/cm2) in a hotel quarantine facility in Baltimore (MD, USA). We obtained nearby outdoor O3 concentrations from the Environmental Protection Agency (EPA) website. In a sealed empty guest room, the average O3 concentrations were 3 ppb (UV off, 0.1-0.5 ACH), 16 ppb (UV on, 0.1 ACH) and 9 ppb (UV on, 0.5 ACH). In a standard guest room, the average O3 concentrations were 12 ppb (UV off, 1.4 ACH) and 14 ppb (UV on, 1.4 ACH), and correlated with outdoor concentrations ({rho} = 0.65 - 0.74, p = 2*10-12 - 2*10-29). A linear regression model, adjusted for outdoor O3, estimated that use of far-UVC lamps increased the O3 concentration by 5.7 ppb (95% confidence interval (CI) 4.9 - 6.5 ppb) in the standard hotel room. Indoor O3 concentrations increased with far-UVC usage, however, the concentrations remained 6-12 ppb lower, on average, than outdoors and well below EPA ambient limits. We did not find a clear relationship between indoor UFP concentrations and UV usage. Although our study was limited by absence of direct outdoor measurements of local O3 and UFPs, our findings do not support a major impact of far-UVC on UFP concentrations in the real-world environment that we studied.