A critical review of in-duct ultraviolet germicidal irradiation systems: Mechanisms, challenges, and future directions

Ultraviolet germicidal irradiation (UVGI) is a well-established technology for mitigating microbial risks, with in-duct UVGI systems being a promising type that enables coexistence with occupants. However, there exists a disparity between the application demand and research advancement, which may be attributed to the misconceptions on in-duct UVGI systems and the shortage of strategies that significantly improve inactivation efficiency. This work systematically reviewed and analyzed the mechanisms and challenges (including existing misconceptions, factors affecting inactivation performance, and research methods) of in-duct UVGI systems. Furthermore, several future research directions were discussed, including optimizing the location of UVC lamps, developing multi-band synergistic strategies, assessing risks related to ozone and mercury exposure, refining research methods, and establishing experimental test databases. Finally, the necessity and feasibility of two perspectives were provided: 1) the development of anti-fouling materials applicable to lamps and duct walls; 2) the establishment of fault prognostic systems. In summary, the information provided here is expected to help improve the inactivation performance and system resilience.