Fluence rate and ozone production from far-UVC lamps - A perspective

Filtered Far UV-C lamps (≈222 nm) are increasingly proposed for continuous air and surface disinfection in occupied spaces. Their performance is governed by the fluence rate within the room, which determines both airborne pathogen inactivation and photochemical by-product formation, particularly ozone. However, fluence rate is frequently confused with irradiance, inconsistently measured, and rarely linked quantitatively to ozone production. This prospective clarifies the radiometric foundations of fluence rate, distinguishes it from irradiance, and discusses the implications for modeling, measurement, and interpretation of Far-UVC installations. Methods for calculating room fluence rate distributions are reviewed, together with the limitations of practical measurement techniques. Photolytic ozone generation at 222 nm is examined using established atmospheric chemistry, and the concept of a normalized ozone formation coefficient is introduced to relate room-average fluence rate to ozone production. A steady-state indoor ozone model incorporating surface losses, ventilation, and occupant effects is summarized. Finally, a framework is presented linking steady-state airborne pathogen reduction and ozone concentration through their common dependence on average fluence rate. This relationship quantitatively demonstrates the trade-off between disinfection efficacy and ozone risk and provides a practical tool for realistic system design expectations and risk. Overall, this perspective emphasizes that fluence rate is the central design variable for Far-UVC air disinfection system design and that meaningful evaluation of both benefit and risk requires its accurate determination.