Formation of Air Contaminants by Ultraviolet Germicidal System: Effects of Ventilation Strategy and Ventilation Rate

The use of ultraviolet germicidal irradiation (UVGI) systems has gained attention for controlling airborne infections. However, due to the photochemistry by ultraviolet C (UVC), UVGI systems can unintentionally generate air contaminants indoors, such as O₃ and secondary organic aerosols (SOA). This study investigated the formation of oxidants and SOA by two types of UVGI systems (UVC 254 nm and UVC 222nm) using computational fluid dynamics (CFD) simulations in an occupied classroom considering ventilation strategy (Mixing ventilation and displacement ventilation) and ventilation rate (1.1 h^-1 and 3 h^-1). The result shows that when an upper-room ultraviolet germicidal irradiation (UVGI) system using UVC at a wavelength of 254 nm is operating, the concentration of O3 decreases slightly for all cases, because of ozone photolysis decomposing O₃ into OH. In contrast, the use of UVC 222 nm, which does produce O3, leads to an increase in indoor O₃ concentration by approximately 5 to 6 ppb. Regarding the formation of SOA, when UVC 254 nm is operating, the increase in SOA concentration is less than 10% compared to the UVGI off case. On the other hand, the use of UVC 222 nm leads to a noticeable increase in SOA concentration, up to 70%, compared to the UVGI off case. The impact of UVC 222 nm on SOA concentration is diminished as the ventilation rate increases from 1.1 h^-1 to 3 h^-1. In addition, the breathing zone SOA concentration under displacement is around 40% lower than under mixing ventilation.