Is there an optimal wavelength for germicidal ultraviolet air disinfection?

Germicidal UV (GUV) disinfection is effective against airborne pathogens, but it has been recently reported to increase indoor air pollution. Conventional GUV at 254 nm is applied in the upper room only due to skin/eye safety limits, while “Far UVC” (e.g. at 222 nm) is applied across the whole room due to less restrictive safety limits, enabling simpler installation and disinfection. We investigate GUV between 185 and 310 nm by modeling, in search of an optimal wavelength with both high disinfection and safety. For a specific fluence rate, GUV-induced air pollution health risks are at least ~20 times larger below 245 nm than above it. This is mainly due to O3 production through O2 photolysis below 245 nm, with a contribution from particulate matter formation from enhanced volatile organic compound oxidation. When normalized to a constant CDC-recommended disinfection rate of 5 equivalent air changes per hour (eACH), pollution risk below 245 nm is also at least ~20 times that above 245 nm. At very high disinfection rates such as 20 eACH, the difference between the ratios below and above 245 nm is smaller, but still a factor of ~20. Our results show a clear advantage of upper-room GUV vs. Far UVC for indoor air quality. These results appear robust despite substantial uncertainties in absolute disinfection efficiencies, which are a critical limitation for widespread GUV application. Thus, there is no optimal GUV wavelength across all important criteria (exposure limits, disinfection efficiency, indoor air quality, and logistic requirements), and these tradeoffs should be considered in different situations to maximize the overall benefit. Use of Far UVC may require simultaneous deployment of air cleaning for pollution. As new practical UV light sources at wavelengths other than 222 and 254 nm keep being developed, this study provides guidance for evaluating and selecting wavelength(s) for GUV air disinfection.