Efficacy of ultraviolet germicidal irradiation of upper-room air in inactivating airborne bacterial spores and mycobacteria in full-scale studies

The efficacy of ultraviolet germicidal irradiation (UVGI) for inactivating airborne bacterial spores and vegetative mycobacteria cells was evaluated under full-scale conditions. Airborne bacteria inactivation experiments were conducted in a test room (87 m3), fitted with a modern UVGI system (216 W all lamps operating, average upper zone UV irradiance 42±19 μW cm−2) and maintained at 25°C and 50% relative humidity, at two ventilation rates (0 and 6 air changes per hour). Bacillus subtilis (spores), Mycobacterium parafortuitum, and Mycobacterium bovis BCG cells were aerosolized continuously into the room such that their numbers and physiologic state were comparable both with and without the UVGI and ventilation system operating. Air samples were collected using glass impingers (9 breathing-zone locations) and multi-stage impactors, and collected bacteria were quantified using direct microscopy and standard culturing assays. UVGI reduced the room-average concentration of culturable airborne bacteria between 46% and 80% for B. subtilis spores, between 83% and 98% for M. parafortuitum, and 96–97% for M. bovis BCG cells, depending on the ventilation rate. An additional set of experiments, in which M. parafortuitum was aerosolized into the test room and then allowed to decay under varying UVGI and ventilation rates, yielded an inactivation rate of 16±1.2 h−1 for the UVGI system, all lamps operating. The Z value (inactivation rate normalized to UVGI irradiance) was estimated to be 1.2±0.15×10−3 cm2 μW−1 s−1 for aerosolized M. parafortuitum at 50% relative humidity.