Ultraviolet germicidal irradiation inactivation of airborne fungal spores and bacteria in upper-room air and HVAC in-duct configurations

Abstract

The efficacy of ultraviolet germicidal irradiation (UVGI) for inactivating airborne fungal spores and bacterial vegetative cells was evaluated under three configurations — intrinsic, upper-room air, and in-duct. Correspondingly, experiments were performed in (1) a pilot-scale chamber (0.8 m3), fitted with four corner UV lamps that irradiated the entire chamber (average UV fluence rate 10.6 ± 0.8 µJ s–1cm–2); (2) a full-scale room (87 m3), fitted with a UVGI system that irradiated the top 30 cm of the room (5 fixtures, 216 W total lamp power, average upper-zone UV fluence rate 26 ± 1 µJ s–1cm–2); and (3) the supply air duct of a heating ventilation and air-conditioning (HVAC) system. Fungal spores of Aspergillus versicolor and vegetative cells of bacterium Mycobacterium parafortuitum were aerosolized continuously such that their numbers and physiologic state were comparable both with and without the UVGI lamps operating. The Z value (UVGI inactivation rate normalized to UVGI fluence rate) was estimated to be 1.2 ± 0.4 x 10–4 cm2 µJ–1 for aerosolized A. versicolor. Upper-room air UVGI inactivated culturable airborne fungal spores with a first-order rate constant of 0.4 ± 0.2 h–1. Ultraviolet lamps enclosed in ventilation system ductwork inactivated fungal spores and vegetative bacterial cells at single-pass efficiencies of 75% and 87%, respectively, at an air stream velocity of 2.2 m s–1. There was no detected inactivation of fungal spores and vegetative bacterial cells at an air stream velocity of 5.1 m s–1. Key words: bioaerosols, UVGI, air disinfection.